"user manual"
Contents
1. 1 TD Tsen Teal 1 2 5 This correction is rarely applied because TD is typically small Apart from the sensor s own thermal resistance also contact resistances between sensor and surrounding material are demanding special attention Essentially any air gaps add to the sensor thermal resistance at the same time increasing the deflection error in an unpredictable way In all cases the contact between sensor and surrounding material should be as well and as stable as possible so that it is not influencing the measurement It should be noted that the conductivity of air is approximately 0 02 W m K ten times smaller than that of the heat flux sensor It follows that air gaps form major contact resistances and that avoiding the occurrence of significant air gaps should be a priority whenever heat flux sensors are installed HFPO1 HFPO3 manual version 0612 page 10 35 Hukseflux Thermal Sensors 2 Application in meteorology In meteorological applications the primary purpose is to measure the part of the energy balance that goes into the soil This soil heat flux in itself is in most cases of limited interest However knowing this quantity it is possible to close the balance In other words apply the law of conservation of energy to check the quality of the other convective and evaporative flux measurements For more information on meteorological measurement of heat flux see the appendix Users should be aware of th2. Outer sheet preferred polyurethane for good stability in outdoor applications Connection Either solder the new cable core and shield to the original sensor cable and make a waterproof connection using cable shrink or use gold plated waterproof connectors Preferably the shield should also be extended Table 11 1 1 Specifications for cable extension of HFPO1 HFPO1 HFPO3 manual version 0612 page 27 35 Hukseflux Thermal Sensors 11 2 Appendix on trouble shooting This paragraph contains information that can be used to make a diagnosis whenever the sensor does not function The sensor does not give any signal 1 Measure the impedance across the sensor wires This check can be done even when the sensor is buried The resistance should be around 2 ohms sensor resistance plus cable resistance typically 0 1 ohm m If it is closer to zero there isa short circuit check the wiring If it is infinite there is a broken contact check the wiring 2 Check if the sensor reacts to an enforced heat flux In order enforce a flux it is suggested to mount the sensor on a piece of metal create a thermal connection with some thermal paste that is used in electronics if not available toothpaste will also do and to use a lamp as a thermal source A 100 Watt lamp mounted at 10 cm distance should give a definite reaction 3 Check the data acquisition by applying a mV source to it in the 1 mV range
3. The sensor signal is un realistically high or low 1 Check if the right calibration factor is entered into the algorithm Please note that each sensor has its own individual calibration factor 2 Check if the voltage reading is divided by the calibration factor by review of the algorithm 3 Check if the mounting of the sensor still is in good order 4 Check the condition of the leads at the logger 5 Check the cabling condition looking for cable breaks 6 Check the range of the data logger heat flux can be negative this could be out of range or the amplitude could be out of range 7 Check the data acquisition by applying a mV source to it in the 1 mV range The sensor signal shows unexpected variations 1 Check the presence of strong sources of electromagnetic radiation radar radio etc 2 Check the condition of the shielding 3 Check the condition of the sensor cable Table 11 2 1 Trouble shooting for HFPO1 HFPO1 HFPO3 manual version 0612 page 28 35 Hukseflux Thermal Sensors 11 3 Appendix on heat flux sensor calibration The sensitivity Esen Of a Heat Flux Sensor is defined as the output Vsen for each Watt per sguare meter heat flowing through it in a stationary transversal heat flow The method that is generally applied during production is described below 1 Yo A 2D 99 Figure 12 3 1 The calibration method for HFPO1 a heat flux sensor 2
4. is calibrated by mounting it on a metal heat sink 1 with a constant temperature A film heater 3 is used to generate a well known heat flux If the thermal conductivity of the sensor is 0 8 W m K at a heat flux of 300 W m and a sensor thickness is 5 mm the temperature raise at the heater is 2 degrees For an un insulated sensor this would result in an error of about 20 W m2 because of radiative and convective losses For this reason the heater is again insulated using foam insulating material Now 99 of the heat flux passes through the sensor As a result the accuracy is about 1 The method has to be validated for application by comparison to a reference calibration as described above The heater calibration is traceable to the guarded hot plate of National Physical Laboratory NPL of the UK Applicable standards are ISO 8302 and ASTM C177 The calibration reference conditions for HFPO1 calibration at Hukseflux are Temperature 20 C Medium thermal conductivity 0 W mK Heat Flux 300 W m HFPO1 HFPO3 manual version 0612 page 29 35 Hukseflux Thermal Sensors 11 4 Appendix on heat transfer in meteorology Note All units used in this appendix are clarified in the text Not all units in this appendix are mentioned in the list of symbols Heat is transferred by radiation convection and conduction In most meteorological experiments the main source of heat during the daytime is the solar radiation The maximum
5. 801 2 1984 8kV air discharge RF IEC 808 3 1984 3 V m 27 500 MHz EFT IEC 801 4 1988 1 kV mains 500V other Delft January 2006 HFPO1 HFPO3 manual version 0612 page 35 35 1
6. power of the sun on a horizontal surface is about 1500 W m in case of a bright sun at noon The solar radiation is absorbed by the soil and the resulting heat is used for evaporation of water heating the air and heating the soil At night when the sun is no longer there again radiation plays a role this time the dominant energy flow is from the soil emitting far infra red radiation to the sky The maximum power is about minus 150 W m in case of a clear blue sky Other sources than the radiation are usually negligible The resulting energy flows through the soil at 5 cm are usually between 100 and 300 W m For various practical and theoretical reasons the heat flux plate cannot be installed directly at the surface The main reason is that it would distort the flow of moisture and be no longer representative of the surrounding soil both from a moisture and from a thermal spectral point of view Also in case of installation close to the surface the sensor would be more vulnerable and the stability of the installation becomes an uncertain factor For these reasons the flux at the soil surface surr is typically estimated from the flux measured by the heat flux sensor psen plus the change of energy that is stored in the layer above it over a certain time S Psurf Psen S 11 4 1 The parameter S is called the storage term The storage term is calculated using an averaged soil temperature measurement combined with an es
7. sensors are used per measurement location in order to promote spatial averaging and to have some redundancy for improved quality assurance Sensors are typically several meters apart Location with exposure to direct solar radiation should be avoided as much as possible In the northern hemisphere north facing walls are preferred Typically locations exposed to solar radiation are avoided in particular when thermal resistance or H value measurements of a building component are made When installing on the wall surface in case of exposure to strong radiation for instance direct beam solar radiation the spectral properties of the sensor surface must be adapted to match those of the wall This can be done by covering the sensor with paint or sheet material of the same colour The more heat flux the better strongly cooled or strongly heated rooms are ideal measurement locations It can be considered to temporarily activate heaters or air conditioning for a perfect measurement The least ideal is a situation in which the heat flux is constantly changing direction This often goes together with relatively small fluxes and strong effects of loading For detailed analysis of one building element it can be beneficial to install one heat flux sensor on one side and the other on the other side Measuring in such a way will allow seeing the thermal response time of a system in more detail The location of instal
8. 02 and ASTM C177 Recalibration interval Dependent on application if possible every 2 years see appendix OPTI ONS Extended cable Additional cable length x metres add to 5m AC100 amplifier LI 18 hand held readout extended temperature range Table 4 1 List of HFPO1 specifications started on previous 2 pages HFPO1 HFPO3 manual version 0612 page 18 35 Hukseflux Thermal Sensors 5 Short user guide Preferably one should read the introduction and first chapters to get familiarised with the heat flux measurement and the related error sources The sensor should be installed following the directions of the next paragraphs Essentially this requires a data logger and control system capable of readout of voltages and capability to perform division of the measurement by the sensitivity The first step that is described in paragraph 6 is and indoor test The purpose of this test is to see if the sensor works The second step is to make a final system set up This is strongly application dependent but it usually involves permanent installation of the sensor and connection to the measurement system Directions for this can be found in paragraphs 7 to 11 HFPO1 HFPO3 manual version 0612 page 19 35 1 Hukseflux Thermal Sensors 6 Putting HFPO1 into operation It is recommended to test the sensor functionality by checking the impedance of the sensor and by checking if the sensor works a
9. 0612 page 15 35 Hukseflux Thermal Sensors 4 Specifications of HFPO1 HFPO1 is a heat flux sensor that measures the local heat flux perpendicular to the sensor surface in the medium in which it is incorporated or the object on which it is mounted It can only be used in combination with a suitable measurement and control system The HFPO1 specifications except size resistance sensitivity and weight are also applicable to sensor type HFPO3 see appendix HFPO1 GENERAL SPECIFICATIONS Specified Heat flux in W m perpendicular to the measurements sensor surface Installation See the product manual for recommendations Temperature range 30 to 70 degrees C Recommended number Meteorological two for each of sensors measurement station Building Physics typically 1 or 2 sensors per measurement location depending on building and wall properties CE requirements HFPO1 complies with CE directives Series connection HFPO1 sensors can be put electrically in series to create a sensor with higher sensitivity of better spatial resolution using only one single readout channel The sensitivity then is the average of the two sensitivities Thermal conductivity 0 07 m K W nominal value dependence E Aca 0 Temperature lt 0 1 C dependence TD Table 4 1 List of HFPO1 specifications continued on next 2 pages HFPO1 HFPO3 manual version 0612 page 16 35 Hukseflux Thermal S
10. HFPO1 amp HFPO3 Heat Flux Plate Heat Flux Sensor USER MANUAL HFPO1 HFPO3 manual version 0612 Edited amp Copyright by Hukseflux Thermal Sensors http www hukseflux com e mail info hukseflux com Hukseflux Thermal Sensors Hukseflux Thermal Sensors Contents List of symbols Introduction OOUR 1 General Theory 1 1 General heat flux sensor theory 1 2 Detailed description of the measurement resistance error contact resistance deflection error and temperature dependence 8 2 Application in meteorology 11 3 Application in building physics 14 4 Specifications of HFPO1 16 5 Short user guide 19 6 Putting HFPO1 into operation 20 7 Installation of HFPO1 in meteorology 21 8 Installation of HFPO1 in building physics 22 9 Maintenance of HFPO1 24 10 Electrical connection of HFPO1 26 11 Appendices 27 11 1 Appendix on cable extension for HFPO1 27 11 2 Appendix on trouble shooting 28 11 3 Appendix on heat flux sensor calibration 29 11 4 Appendix on heat transfer in meteorology 30 11 5 Appendix on heat transfer in building physics 32 11 6 Appendix on HFPO3 33 11 7 CE declaration of conformity 35 HFPO1 HFPO3 manual version 0612 page 2 35 Hukseflux Thermal Sensors List of symbols Heat flux p W m Thermal conductivity of the surrounding medium or object on which the sensor is mounted W mK Voltage output V V HFPO1 sensitivity Esen pV Wm Thermal conductivity dependence of Esen E mK W Time t S Surface
11. Thermal Sensors Reguirements for data acguisition amplification Capability to measure Preferably 5 microvolt accuracy microvolt signals Minimum requirement 50 microvolt accuracy both across the entire expected temperature range of the acquisition amplification equipment In case of low amplifier accuracy it should be considered either to put two sensors in series to purchase a pre amplifier or to use model HFPO3 Capability for the data logger To store data and to perform or the software division by the sensitivity to calculate the heat flux Table 9 1 Requirements for data acquisition and amplification equipment HFPO1 HFPO3 manual version 0612 page 25 35 f Hukseflux Thermal Sensors 10 Electrical connection of HFPO1 In order to operate HFPO1 should be connected to a measurement and system as described above A typical connection is shown in table 11 1 HFPO1 is a passive sensor that does not need any power Cables generally act as a source of distortion by picking up capacitive noise It is a general recommendation to keep the distance between data logger or amplifier and sensor as short as possible For cable extension see the appendix on this subject Wire Colour Measurement system Sensor output White Voltage input Sensor output Green Voltage input or Analogue ground Shield Analogue ground or Voltage input Table 10 1 The electrical
12. age HFPO1 HFPO3 manual version 0612 page 23 35 f Hukseflux Thermal Sensors 9 Maintenance of HFPO1 Once installed HFPO1 is essentially maintenance free Usually errors in functionality will appear as unreasonably large or small measured values In case 2 sensors are mounted on one location the ratio of measurement resuls could be monitored over time this will give a Clue if there is any unstability Typicaly long term comparison of 2 sensors can serve as an alternative for re calibration at the factory As a general rule this means that a critical review of the measured data is the best form of maintenance At regular intervals the quality of the cables can be checked Theoretically it is a possibiliy to send sensors back to the factory for re caliration The reality os that in mist applications this is not possible Recalibration of HFPO1 often is not possible in particular if sensors are dug in or permanently glued to a surface If the intercomparison between 2 sensor on one location is judged to be insufficient use of model HFPO1SC self calibrating heat flux sensor could be considered In case of use in building physics re calibration can be done in the field by monting a second reference HFPO1 on top of the field sensor and by measuring the ratio of the outputs over a longer time By measuring 10 minute averages one can create a correlation HFPO1 HFPO3 manual version 0612 page 24 35 Hukseflux
13. al conductivity of the surrounding medium differs from the sensor thermal conductivity causes the heat flux to deflect The resulting error is called the deflection error The deflection error is determined in media of different thermal conductivity by experiments or using theoretical approximations The result of these experiments is laid down as the so called thermal conductivity dependence E The order of magnitude of E is constant for one sensor type For HFPO1 E is given in the list of specifications Esen E sen cal 1 E Acal Amea 1 2 3 Note this correction can only be applied when there is a substantial amount of at least 40 mm medium on both sides of the sensor In soils Amea Usually is not known The value of Acai typically is zero HFPO1 HFPO3 manual version 0612 page 9 35 Hukseflux Thermal Sensors Figure 1 2 3 The deflection error The heat flux 1 is deflected in particular at the edges of the sensor As a result the measurement will contain an error the so called deflection error The magnitude of this error depends on the medium thermal conductivity sensor thermal properties as well as sensor design In addition the sensitivity of heat flux sensors is temperature dependent The temperatre dependence TD reflects the fact that the sensitivity changes with temperature Esen E sen cal 1 TD Taai a Tsen 1 2 4 Combining 1 2 3 and 1 2 4 Esen E sen cal 1 E Acal Amea
14. area A m Electrical resistance Re Q Thermal resistance Rth Km W Temperature T K Temperature dependence TD K Depth of burial d m Subscripts Property of the sensor sen Property of air air Property during calibration cal Property of the object on which HFPO1 is mounted obj Property at the soil surface surf HFPO1 HFPO3 manual version 0612 page 3 35 f Hukseflux Thermal Sensors I ntroduction HFPO1 is the world s most popular sensor for heat flux measurement in the soil and through walls and building envelopes By using a ceramics plastic composite body the total thermal resistance is kept small HFPO1 serves to measure the heat that flows through the object in which it is incorporated or on which it is mounted The actual sensor in HFPO1 is a thermopile This thermopile measures the differential temperature across the ceramics plastic composite body of HFPO1 Working completely passive HFPO1 generates a small output voltage proportional to the local heat flux Using HFPO1 is easy For readout one only needs an accurate voltmeter that works in the millivolt range To calculate the heat flux the voltage must be divided by the sensitivity a constant that is supplied with each individual instrument HFPO1 can be used for in situ measurement of building envelope thermal resistance R value and thermal transmittance H value according to ISO 9869 ASTM C1046 and ASTM 1155 standards Traceability of calibration is to th
15. ccording to the following table 5 minutes estimated time needed Warning during this part of the test please put the sensor in a thermally quiet surrounding because a sensor that generates a significant signal will disturb the measurement Check the impedance of the sensor Use a multimeter at the 10 ohms range Measure at the sensor output first with one polarity than reverse polarity Take the average value The typical impedance of the wiring is 0 1 ohm m Typical impedance should be 1 5 ohm for the total resistance of two wires back and forth of each 5 meters plus the typical sensor impedance of 2 ohms Infinite indicates a broken circuit zero indicates a short circuit Check if the sensor reacts to heat flux Use a multimeter at the millivolt range Measure at the sensor output Generate a signal by touching the thermopile hot joints red side with your hand The thermopile should react by generating a millivolt output signal Table 6 1 Checking the functionality of the sensor The procedure offers a simple test to get a better feeling how HFPO1 works and a check if the sensor is OK The programming of data loggers is the responsibility of the user Please contact the supplier to see if directions for use with your system are available HFPO1 HFPO3 manual version 0612 page 20 35 Hukseflux Thermal Sensors 7 Installation of HFPO1 in meteorology HFPO1 is generally
16. connection of HFPO1 The heat flux plate output usually is connected to a differential voltage input Wire Colour Measurement system Sensor 1 output White To sensor 2 output Sensor 1 output Green Voltage input or Analogue ground Sensor 2 output White Voltage input Sensor 2 output Green To sensor 1 output Shield Analogue ground or Voltage input Table 10 2 The electrical connection of two sensors HFPO1 in series When using more than one sensor and having a lack of input channels it can be considered to put several sensors in series while working with the average sensitivity HFPO1 HFPO3 manual version 0612 page 26 35 Hukseflux Thermal Sensors 11 Appendices 11 1 Appendix on cable extension for HFPO1 HFPO1 is eguipped with one cable It is a general recommendation to keep the distance between data logger or amplifier and sensor as short as possible Cables generally act as a source of distortion by picking up capacitive noise HFPO1 cable can however be extended without any problem to 100 meters If done properly the sensor signal although small will not significantly degrade because the sensor impedance is very low Cable and connection specifications are summarised below Cable 2 wire shielded copper core at Hukseflux we use 3 wire shielded of which we only use 2 per cable Core 0 1 Q m or lower resistance Outer preferred 5 mm diameter
17. e guarded hot plate of National Physical Laboratory NPL of the UK according to ISO 8302 and ASTM C177 A typical measurement location is equipped with 2 sensors for good spatial averaging If necessary two sensors can be put in series creating a single output signal If measuring in soil in case a more accurate measurement is needed the model HFPO1SC should be considered In case a more sensitive measurement is required model HFPO3 should be considered In case of special requirements like high temperature limits smaller size or flexibility the PU series could offer a solution This manual can also be used for HFPO3 Differences between HFPO3 and HFPO1 are highlighted in a special appendix on HFPO3 HFPO1 HFPO3 manual version 0612 page 4 35 Hukseflux Thermal Sensors Figure 1 Drawing of HFPO1 sensor 5m 80 Figure 2 HFPO1 heat flux plate dimensions 1 sensor area 2 guard of ceramics plastic composite 3 cable standard length is 5 m All dimensions are in mm HFPO1 HFPO3 manual version 0612 page 5 35 Hukseflux Thermal Sensors 1 General Theory 1 1 General heat flux sensor theory As in most heat flux sensors the actual sensor in HFPO1 is a thermopile This thermopile measures the differential temperature across the ceramics plastic composite body of HFPO1 Working completely passive it generates a small output voltage that is proportional to the differential temperature that p
18. e fact that the soil heat flux measurement with HFPO1 in most cases is not resulting in a high accuracy result The main causes are 1 the fact that measurement at one location in the soil will have only limited validity for a larger area variability of soil surface can be very lage 2 the fact that variations in soil thermal properties over time result in significant measurement errors If measuring in soil in case a more accurate measurement is needed the model HFPO1SC should be considered Figure 2 1 Typical meteorological energy balance measurement system with HFPO1 installed under the soil HFPO1 HFPO3 manual version 0612 page 11 35 Hukseflux Thermal Sensors In a perfect environment the initial calibration accuracy of heat flux sensors is estimated to be 3 3 In field experiments it is difficult to find one location that can be considered to be representative of the whole region Also temporal effects of shading on the soil surface can give a false impression of the heat flux For this reason typically two sensors are used for each station usually at a distance of 5 meters Apart from the question of representativeness of the measurement location the main problem with heat flux measurements in meteorology is that the sensitivity of a heat flux sensor is dependent on the thermal conductivity of the surrounding medium This deflection error is described in chapter 1 In soil heat flux measure
19. ected with 1 2 2 When mounting the sensor in or on an object with limited thermal resistance the sensor thermal resistance itself might be significantly influencing the undisturbed heat flux One part of the resulting error is called the resistance error reflecting a change of the total thermal resistance of the object om _ Figure 1 2 1 The resistance error a heat flux sensor 2 increases or decreases the total thermal resistance of the object on which it is mounted 1 or in which it is incorporated This can lead either to a larger of smaller increase of or decrease of the heat flux 3 HFPO1 HFPO3 manual version 0612 page 8 35 Hukseflux Thermal Sensors AAA eee Figure 1 2 2 The resistance error a heat flux sensor 2 increases or decreases the total thermal resistance of the object on which it is mounted or in which it is incorporated An otherwise uniform flux 1 is locally disturbed 3 In this case the measured heat flux is smaller than the actual undisturbed flux 1 A first order correction of the measurement is Rthobj Rthsen V sen E sen Rthobj 1 2 2 This correction is often applied with thin or well isolated walls Note this correction can only be determined for objects with limited finite dimensions For this reason this correction is not applicable in soils In addition to the resistance error the fact that the therm
20. ensors HFPO1 MEASUREMENT SPECIFICATIONS Initial calibration accuracy 3 3 Overall uncertainty statement according to ISO estimated to be within 5 5 based on a standard uncertainty multiplied by a coverage factor of k 2 providing a level of confidence of 95 Application related errors should be added to this error Expected typical accuracy 12 hr totals of heat flux measurement in soil Initial calibration accuracy 3 3 Added errors within most common soils clays silts organic on most walls 20 degrees C 0 7 Added typical temperature error 10 40 degrees C 2 3 Total typical value in soil rounded off within 5 15 Expected worst case Initial calibration accuracy accuracy 12 hr totals 3 3 of heat flux measurement in soil Added errors with worst case soil pure sand 20 degrees C 0 16 Added worst case temperature error 30 70 degrees C 5 5 Total worst case value in soil rounded off within 10 25 Yo Expected typical accuracy 12 hr totals of heat flux measurement on a wall Initial calibration accuracy 3 3 Added typical temperature error 10 40 degrees C 2 3 Total typical value on a wall insulating brick cement rounded off within 5 5 Low thermal resistance walls require correction for the resistance error Table 4 1 List of HFPO1 specifications started on p
21. ference in temperature between wall and air and strongly depends on the local wind speed See the appendix on heat transfer in building physics for more information It is possible that the heat flux sensor contributes significantly to the total thermal resistance of the object resistance error In this case the heat flux measurement must be corrected For the correction see chapter 1 In order to limit the resistance error in all cases the contact between sensor and surrounding material should be as well and as stable as possible so that air gaps are not influencing the measurement In a perfect environment the initial calibration accuracy of heat flux sensors is estimated to be 3 3 In case of use of HFPO1 on walls insulating as well as bricks and cements the overall expected measurement accuracy for 12 hr totals is 5 5 Yo HFPO1 HFPO3 manual version 0612 page 14 35 Hukseflux Thermal Sensors In case of analysis of thermal resistance of building envelopes the minimum recommended measurement time is 48 hours Hukseflux also offeres a complete measurement system for analysis of building envelopes TRSYS Figure 3 1 Estimation of convective radiative and conductive heat flux in building physics The heat flux sensor is simply mounted on or in the object of interest This is typically in walls but can also be in the soil e g on top of an underground heat storage tank HFPO1 HFPO3 manual version
22. he result of 5 13 Yo is rounded off to 5 15 HFPO1 HFPO3 manual version 0612 page 12 35 Hukseflux Thermal Sensors Heat flux sensors in meteorological applications are typically buried at a depth of about 5 cm below the soil surface Burial at a depth of less than 5 cm is generally not recommended In most cases a 5 cm soil layer on top of the sensor offers just sufficient mechanical consistency to guarantee long term stable installation conditions Burial at a depth of more than 8 cm is generally not recommended because time delay and amplitude become less easily traceable to surface fluxes at larger installation depths See the appendix for more details Summary In case of use of HFPO1 in meteorological applications the use of 2 sensors per station is recommended This creates redundancy and a better possiblity for judging the quality of the measurement accuracy Typically one will work with two separately measured sensor outputs the average value is the measurement result In normal soils clays silts the overall expected measurement accuracy for 12 hr totals is 5 15 In case of pure sands the overall measurement accuracy for 12 hr totals is 10 25 Yo The accuracy mainly is a function of the thermal conductivity of the surrounding medium In case of soils the moisture content plays a dominant role The wider accuracy range in sand is due to the fact that the thermal conductivity of sand varies with moistu
23. installed at the location where one wants to measure at least 4 cm depth below the surface A typical depth of installation is 5 cm Typically 2 sensors are used per measurement location in order to promote spatial averaging and to have some redundancy for improved guality assurance Sensors are typically several meters apart The more even the surface on which HFPO1 is placed the better When covering HFPO1 with soil it should be done such that the soil below and on top is the same If possible it is safest to install HFPO1 from the side Care should be taken to prevent the creation of air gaps between sensor and soil In meteorological applications permanent installation is preferred It is recommended to fix the location of the sensor by attaching a metal pin to the cable Attachment of the pin to the cable can be done using a tie wrap HFPO1 sensors can be put electrically in series to create a sensor with higher sensitivity of better spatial resolution using only one single readout channel Sensitivity is the average of the two sensitivities Table 7 1 General recommendations for installation of HFPO1 In case of exceptional applications please contact Hukseflux HFPO1 HFPO3 manual version 0612 page 21 35 1 Hukseflux Thermal Sensors 8 Installation of HFPO1 in building physics HFPO1 is generally installed on the surface of a wall or alternatively it is integrated into the wall Typically 2
24. l with 90 of the cases below 15m s A reasonable approximation of the heat transfer coefficient at moderate wind speeds is given by Ctr 5 4 Wvind 11 5 2 With Cir in W m2K and in Vwing M S HFPO1 HFPO3 manual version 0612 page 32 35 Hukseflux Thermal Sensors 11 6 Appendix on HFPO3 HFPO3 is an ultra sensitive sensor for heatflux measurement of small heat fluxes through soil walls and building envelopes HFPO3 has been built specifically for applications where one needs to detect small flux levels in the order of less than 10 W m Differences are HFPO1 HFPO3 Sensitivity 50 yV W m 500 uV W m Diameter 80 mm 172 mm Resistance 20 18 Q Weight including 5 0 2 kg 0 8 kg m cable Table 11 6 1 Differences between HFPO1 and HFPO3 HFPO1 HFPO3 manual version 0612 page 33 35 1 Hukseflux Thermal Sensors 5 0 i Figure 11 6 1 HFPO3 heat flux plate dimensions 1 sensor area 2 guard of ceramics plastic composite 3 cable standard length 5 m All dimensions are in mm HFPO1 HFPO3 manual version 0612 page 34 35 f Hukseflux Thermal Sensors 11 7 CE declaration of conformity According to EC guidelines 89 336 EEC 73 23 EEC and 93 68 EEC We Hukseflux Thermal Sensors Declare that the products HFPO1 and HFPO3 Is in conformity with the following standards Emissions Radiated EN 55022 1987 Class A Conducted EN 55022 1987 Class B Immunity ESD IEC
25. lation preferably should be a large wall section which is relatively homogeneous Areas with local thermal bridges should be avoided The more even the surface on which HFPO1 is placed the better The optimal configuration is the heater in the same plane as the surrounding surface of the object Table 8 1 General recommendations for installation of HFPO1 in application in building physics In case of exceptional applications please contact Hukseflux continued on next page HFPO1 HFPO3 manual version 0612 page 22 35 Hukseflux Thermal Sensors Any air gaps should be filled as much as possible Permanent installation is preferred It is recommended to fix the location of the sensor by gluing with silicone glue Alternatively for short term installation either toothpaste 1 2 days or DOW CORNING heat sink compound 340 can be used Typically temporary installation is fixed using tape across the guard The tape should be as far as possible to the edge Independent attachment of the cable can be done to an object that can resist strain in case of accidental force HFPO1 sensors can be put electrically in series to create a sensor with higher sensitivity of better spatial resolution using only one single readout channel Table 8 1 General recommendations for installation of HFPO1 in application in building physics In case of exceptional applications please contact Hukseflux started previous p
26. ment the accuracy of soil heat flux measurements very much suffers from the fact that the surrounding medium is both unknown to the manufacturer and changing over time A typical HFPO1 has a thermal conductivity of 0 8 W mK while soils can vary between extremes of 0 2 and 4 W mK Sand in relatively dry condition can have a thermal conductivity of 0 3 W mK perfectly dry 0 2 while the same sand when saturated with water reaches 2 5 W mK A typical HFPO1 performing a correct measurement in dry sand will make a 16 error in wet sand As in wet sand the heat tends to travel around the badly conducting sensor the flux will be underestimated by 16 This example serves to illustrate that in soils where conditions vary the so called thermal conductivity dependence leads to large deflection errors The third important error is temperature dependence Over the entire temperature range from 30 to 70 degrees C the temperature error is 5 Taking the worst case soil pure sand for the conventional heat flux measurement in meteorology the overall worst case accuracy is estimated to be 8 24 This is rounded off to 10 25 In most situations the soil will not be pure sand and in an average climate the difference between the yearly extremes might be 10 40 degrees C and a thermal conductivity range from 0 2 to 1 W mK The temperature error then is 2 3 the thermal conductivity accounts for 0 7 the calibration 3 3 T
27. owers the heat flux travelling through it heat flux is proportional to the differential temperature divided by the local thermal conductivity of the heat flux sensor Assuming that the heat flux is steady that the thermal conductivity of the body is constant and that the sensor has negligible influence on the thermal flow pattern the signal of HFPO1 is proportional to the local heat flux in Watt per square meter Using HFPO1 is easy For readout one only needs an accurate voltmeter that works in the millivolt range To convert the measured voltage Vsen to a heat flux p the voltage must be divided by the sensitivity Esen a constant that is supplied with each individual sensor P Vsen Esen 1 1 1 HFPO1 is a weatherproof sensor It complies with the CE directives HFPO1 HFPO3 manual version 0612 page 6 35 f Hukseflux Thermal Sensors AWAY Figure 1 1 General characteristics of a heat flux sensor like HFPO1 When heat 6 is flowing through the sensor the filling material 3 will act as a thermal resistance Consequently the heat flow o will go together with a temperature gradient across the sensor creating a hot side 5 and a cold side 4 The majority of heat flux sensors is based on a thermopile a number of thermocouples 1 2 connected in series A single thermocouple will generate an output voltage that is proportional to the temperature difference between the joints copper cons
28. re content from roughly 0 2 perfectly dry to 2 5 saturated With other soils and walls see chapter on building physics the variation of thermal conductivity is much less roughly from 0 1 to 1 W mK If measuring in soil in case a more accurate measurement is needed the model HFPO1SC should be considered HFPO1 HFPO3 manual version 0612 page 13 35 1 Hukseflux Thermal Sensors 3 Application in building physics HFPO1 can be used for in situ measurement of building envelope thermal resistance R value and thermal transmittance H value according to ISO 9869 ASTM C1046 and ASTM 1155 standards When studying the energy balance of buildings heat is exchanged by various mechanisms The total result is a certain heat flux The dominant mechanisms are usually radiative transfer by solar radiation and convective transport by flowing air In most applications in building physics the sensor HFPO1 is simply mounted on or in the object of interest see figure 3 1 At the sensor surface the convective heat of the air and the radiation by the sun are transformed into conductive heat In case of incorporation into the wall the conductive flux through the wall is directly measured If direct beam solar radiation is present the solar radiation is usually dominant The maximum expected solar radiation level is about 1500 W m7 In case of convective transport of heat by the air the convective transport is roughly proportional to the dif
29. rement is difficult and suffers from various errors so that the estimation of the storage term often is the main source off error in the soil energy balance measurement HFPO1 HFPO3 manual version 0612 page 31 35 1 Hukseflux Thermal Sensors 11 5 Appendix on heat transfer in building physics Note All units used in this appendix are clarified in the text Not all units in this appendix are mentioned in the list of symbols Heat is transferred by radiation convection and conduction In most studies of buildings the main sources of heat during the daytime are the solar radiation and the convective transfer from the outside air to the walls During the night only convection remains The maximum power of the sun on a horizontal surface is about 1500 W m in case of a bright sun at noon The solar radiation on a non horizontal wall is mainly determined by the direct beam contrary to the diffuse solar radiation The direct beam solar radiation is extremely variable in both intensity and direction during the day The convective transport of heat from the wall to the air air is a function of the heat transfer coefficient Cir and the temperature difference between air and sensor Tair Tsen Qair Ctr Tair Tsen 11 5 1 In buildings under indoor conditions we can expect wind speeds of 1 m s Working environments will 90 of the cases have wind speeds below 0 5 m s Under outdoor conditions up to 30 m s can be considered norma
30. revious page continued on next page HFPO1 HFPO3 manual version 0612 page 17 35 1 Hukseflux Thermal Sensors HFPO1 SENSOR SPECIFICATIONS Esen NOMinal 50 uV W m exact value on calibration certificate Acal O Teal 20 C Sensor thermal conductivity 0 8 W mK Sensor thermal resistance Rth lt 6 25 10 Km W Response time 3 min equals average soil nominal Range 2000 to 2000 W m Non stability lt 1 change per year normal meteorological building physics use Required readout 1 differential voltage channel or possibly less ideal 1 single ended voltage channel When using more than one sensor and having a lack of input channels it can be considered to put several sensors in series while working with the average sensitivity Expected voltage output Meteorology 10 to 20 mV Building physics 10 to 75 mV exposed to solar radiation Power required Zero passive sensor Resistance 2 Ohm nominal plus cable resistance Required programming Oo Vsen Esen Sensor dimensions 80 mm diameter 5 mm thickness Cable length diameter 5 meters 5 mm Weight including 5 m cable transport dim 0 2 kg transport dimensions 32x23x3 cm CALI BRATI ON Calibration traceability to the guarded hot plate of National Physical Laboratory NPL of the UK Applicable standards are ISO 83
31. tantan and constantan copper This temperature difference is provided that errors are avoided proportional to the heat flux depending only on the thickness and the average thermal conductivity of the sensor Using more thermocouples in series will enhance the output signal In the picture the joints of a copper constantan thermopile are alternatively placed on the hot and the cold side of the sensor The two different alloys are represented in different colours 1 and 2 The thermopile is embedded in a filling material usually a plastic in case of HFPO1 a special Ceramics plastic composite Each individual sensor will have its own sensitivity Esen usually expressed in Volts output Vsen per Watt per square meter heat flux g The flux is calculated Q Vsen Esen The sensitivity is determined at the manufacturer and is found on the calibration certificate that is supplied with each sensor HFPO1 HFPO3 manual version 0612 page 7 35 Hukseflux Thermal Sensors 1 2 Detailed description of the measurement resistance error contact resistance deflection error and temperature dependence As a first approximation the heat flux is expressed as P Vsen Esen 1 2 1 This paragraph offers a more detailed description of the heat flux measurement It should be noted that the following theory for correcting deflection errors and temperature dependence is not often applied Usually one will work with formula 1 2 1 possibly corr
32. timate of the volumetric heat capacity of the volume above the sensor HFPO1 HFPO3 manual version 0612 page 30 35 Hukseflux Thermal Sensors S Tn T1 T2 Cy d ti t2 11 4 2 Where S is the storage term Ti T2 is the temperature change in the measurement interval Cy the volumetric heat capacity d the depth of installation of the soil heat flux sensors ti t the length of the measurement interval At an installation depth of 4 cm the storage term typically represents up to 50 of the total flux surr When the temperature is measured closely below the surface the response time of the storage term measurement to a changing surf iS in the order of magnitude of 20 minutes while the heat flux sensor sen buried at twice the depth is a factor 4 slower Square of the depth This implies that a correct measurement of the storage term is essential to a correct measurement of surf with a high time resolution Usually the volumetric heat capacity Cy is estimated from the heat capacity of dry soil Cs the bulk density of the dry soil r g the water content on mass basis g m and Cy the heat capacity of water G ra CatqmCy 11 4 3 The heat capacity of water is known but the other parameters of the equation are much more difficult to determine and are dependent on location and time For determining bulk density and heat capacity one has to take local samples and to perform careful analysis The soil moisture content measu
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