Tiltmeter Temperature Coefficients
Contents
1. at temperature T Scal the scale factor reported in the user s manual V the measured output voltage at temperature T This temperature compensation procedure is automatically applied in Jewell Instruments digital tiltmeters and in our TBASE II analysis software It may also be incorporated into spreadsheets and other user written programs Example Figures 3 and 4 show 10 days of data blue and yellow traces for two identical high gain tiltmeters with a resolution of 1 micro radian 0 2 arc second The plots were made using the program TBASE II The upper graph in each figure plots daily temperature oscillations at the ground surface in degrees Celsius measured by temperature sensors inside each tiltmeter The lower graph shows tilt of the ground surface in microradians during the same period The large change in tilt on the fourth day resulted from ground subsidence caused by a nearby pump test The graph of tilt in Figure 3 contains daily oscillations that directly correlate with temperature Tiltmeter temperature coefficients were input as part of the configuration options in TBASE II to compensate for the temperature induced error The temperature corrected results are shown in Figure 4 Thermoelasticity in Geotechnical Engineering Thermoelasticity is the elastic expansion and contraction of materials in response to changing temperature Soil steel and concrete structures each have their own temperature coefficient the2. effect of the thick concrete The plot shows gallery temperatures and raw uncompensated upstream downstream tilting of the dam during a two week period in early October 1993 Although temperatures do not vary the real tilt angle fluctuates by 15 microradians daily as the result of daily heating and cooling of the downstream face of the dam a few meters away How to Minimize Temperature Induced Measurement Errors without Temperature Compensation There are several ways to minimize temperature induced measurement errors that do not involve any data processing at all In many cases these methods eliminate the need for the temperature compensation procedures outlined above 1 Reduce Temperature Extremes When possible instruments should be installed underground or in shaded locations where temperature extremes are minimized If temperatures do not vary they can have no effect on your measurements If your instruments must be installed in locations exposed to direct sunlight set up a hood that keeps them shaded while maintaining good ventilation 2 Choose Light Colors When other specifications are equal light colored instruments stay cooler and are preferable to dark colored ones 3 Establish Your Accuracy Requirements Before selecting the tiltmeters for your project decide on the accuracy that is required and estimate the temperature range that the instruments will experience Then get temperature coefficients for the tiltmeters under
3. purely instrumental behavior are discussed in this article Sources of Temperature Coefficients The sensors in Jewell Instruments tiltmeters are known as electrolytic tilt sensors a type of electronic spirit level comprised of a glass case and containing a conductive liquid electrolyte an air bubble and platinum electrodes As the sensor tilts the wetted area of each excitation electrode Figure 1 increases or decreases depending on the tilt direction This change causes the electrical resistance between the central pick up electrode and each excitation electrode to rise or fall It is these resistance changes that are sensed by the tiltmeter electronics which convert them to precise measurements of the magnitude and direction of tilt Temperature fluctuations cause thermal expansion and contraction of the sensor liquid shrinking or swelling the air bubble and changing the amount of liquid in contact with each excitation electrode This process alters the scale factor gain of the sensor and can shift its zero point Small changes in sensor output in the absence of any real tilt movement are the result Experiments have shown that volumetric expansion and contraction of the liquid is the single biggest source of temperature coefficients in Jewell Instruments tiltmeters This effect is much greater than dimensional changes of the sensor s glass case which has a thermal expansion coefficient 100 times smaller than that of th
4. Tiltmeter Temperature Coefficients Source Definition and Use to Improve Accuracy Tech Note EXCITATION ELECTRODES GAS BUBBLE CONDUCTIVE FLUID GLASS CAPSULE PICK UP ELECTRODE 1 li g al i E kyd R1 R2 R1 WHEN LEVEL D R1 R2 WHEN Figure 1 Electrolytic tilt sensor Movement of the bubble changes the output at the pick up electrode when an AC voltage is applied across the excitation electrodes Introduction Environmental temperature changes alter the mechanical and electrical characteristics of all instrumentation Metals expand and contract and electrical properties such as resistance and capacitance rise and fall These effects change instrument output and lessen the accuracy of the measured variable pressure flow tilt strain etc This technical note describes the source of temperature dependency in one type of instrumentation Jewell Instruments tiltmeters and explains how to remove this effect to maximize accuracy The principles presented here also apply to many other instrument types Just as instrumentation exhibits temperature dependent behavior so too do natural and engineered structures including slopes embankments concrete and steel construction Thermal expansion and contraction in response to daily and seasonal temperature fluctuations generate real movements that are detected by tiltmeters and other sensors The magnitude of this effect and ways of differentiating it from
5. coefficient of thermal expansion a which is expressed in units of strain microinches per inch or microns per meter per unit change in temperature Thermoelasticity is a major source of structural movement and precision tiltmeters easily measure this behavior Thermoelastic deformation typically produces tilts that exceed the temperature induced output changes of properly designed tiltmeters The following example illustrates how large thermoelastic movements can be Tiltmeters are commonly installed on bridge piers and columns to detect early signs of settlement and riverbed scour Figure 5a shows a bridge with one span Let us assume that the span is fixed at one end but can expand laterally at the other Now if the slip bearings are seized at the movable end thermal expansion of the span by an amount AL will result in a tilt of the right pier Figure 5b of 0 sin 1 AL H If the temperature change is 100C a is 10 5 C and span length L is 30 meters then AL 10 C 10 5 C 30 000 mm 3 mm For a pier that is 3 meters high the tilt will be 6 1000 microradians 206 arc seconds Now compare this 1000 microradian movement with the uncorrected temperature induced error of an Jewell Instruments tiltmeter Our high gain tiltmeters typically used in geotechnical and structural monitoring have temperature coefficients of KS 0 0004 C and KZ 1 5 microradians C A 10 C temperature change therefore produces a zero shift of 15 mi
6. consideration from their manufacturers Use the temperature range and coefficients to compute potential errors following the procedure in the previous section If these errors are smaller than your accuracy requirements no temperature compensation Is necessary 4 Use a Mechanically Stable Tiltmeter Design Choose a tiltmeter design that minimizes thermoelastic deformation of the instru ment itself Compact stiff housings are more stable and less likely to bend or vibrate than elongated beam designs with fixed ends Also the fewer the mechanical linkages between internal sensor and outer enclosure the better 5 Use a Mechanically Stable Mounting Method Use a mounting method that maximizes thermoelastic stability Three point mounting is best because it is the most rigid and prevents bending and torsion that can occur with 2 point mountings Mounting studs typically threaded rods that attach the tiltmeter to the structure should be as short as possible of the same length and of the same material In special cases thermally stable but more expensive invar studs can be used If you decide that temperature compensation of your data is still required after taking the above steps software such as the TBASE II program is available that performs the necessary corrections quickly and reliably Conclusions 1 All instruments exhibit some degree of temperature dependent behavior Thermal expansion and contraction of the sensor liquid is the largest sourc
7. croradians 1 5 of the actual pier movement The error induced by the coefficient KS is proportional to the rotation angle of the tiltmeter and the temperature change and is even smaller If the tiltmeter was leveled nulled during installation its angle after column rotation would be 1000 microradians and the KS error would be 0 0004 C 10 C 1000 microradians 4 microradians In this example the tiltmeter measures thermoelastic tilt of the pier to better than 2 accuracy with no temperature compensation Figure 6 Thermoelastic dam tilt caused by heating and cooling of the downstream face ee 01 Jan 1993 31 Dec 1993 erun ij seeesogosessseseeseeg n T X axis bh i A Resultant 11 74 270 63 2753 2765 2777 2789 2801 Time days Compensating the readings for temperature change yields even better results Although this is an hypothetical example it is typical of real field projects involving Jewell Instruments tiltmeters Most of the correlation of tilt with temperature results from thermoelastic deformation If your data still correlate with temperature change after compensating for temperature you are observing real structural or ground movement Figure 6 presents a real life example of thermoelastic deformation of a thin arch concrete dam The high gain Jewell Instruments tiltmeter is installed in a gallery inside the dam where temperatures do not cycle on a daily basis because of the insulating
8. e liquid Thermoelasticity of the tiltmeter housing and of the mechanical connections between housing and sensor is another source of tiltmeter zero shift To minimize this effect rigid housings are used Angle 8 Slope at Operating and connections between the sensor and enclosure are made as few Temperature T as possible In many designs we pot the tilt sensor directly into the Slope at eee Scai Calibration housing base eliminating mechanical connections entirely and Temperature Tcal turning the sensor and base into one unified element The temperature effects described above are partially removed compensated by the tiltmeter s electronic circuitry The apparent tilt residual error remaining after such compensation is highly repeatable and is described by two linear temperature coefficients the temperature coefficient of scale factor KS and the temperature coefficient of zero shift KZ These coefficients include contributions Figure 2 Calibration lines at two different temperatures Tcal and T l l from all sources including the tiltmeter electronics There is one additional effect of temperature on electrolytic tilt sensors The conductivity of the electrolyte changes more than five fold over the typical operating range of a tiltmeter typically 40 to 70 C By measuring sensor output ratiometrically taking output as a percentage of input Jewell Instruments tiltmeters remove this
9. e of temperature dependency in Jewell Instruments tiltmeters 2 The effect of temperature change on tiltmeter output is predictable and repeatable It is quantified by two constants the temperature coefficient of scale factor KS and the temperature coefficient of zero shift KZ These constants enable the user to predict the magnitude of potential temperature induced errors and to correct compensate for such errors during data analysis 3 The large thermoelastic movements of civil engineering structures are easily detected by tiltmeters and are sometimes mistaken for measurement errors 4 Before beginning an instrumentation project the user should first establish the required measurement accuracy then estimate the measurement error over the expected temperature range using the instrument s temperature coefficients If the error is smaller than the accuracy requirements no temperature compensation is necessary 5 Simple precautions such as installing tiltmeters in the shade or underground can reduce or eliminate temperature effects 6 Temperature compensation of Jewell Instruments tiltmeter readings is performed using equations 4 and 5 Compensation is carried out automatically by a commercial software package and may also be built into spreadsheets and user defined programs Angle Conversion Factors 1 degree 60 arc minutes 3600 arc seconds 17453 microradians 0 01745 radians 1 arc second 4 85 microradians 1 microradian 1 micr
10. effect entirely However in designs that incorporate the sensor as part of a Wheatstone bridge electrolyte conductivity change can be a major source of measurement error Temperature Coefficients Defined Scale factor is the proportionality constant between tilt angle and tiltmeter output It is determined in the factory by calibrating the tiltmeter rotating it through a range of known angles and recording the output voltage at each angle The slope of the best fit straight line through the calibration data is the scale factor Scal that is reported in the tiltmeter user s manual In reality the Slope is slightly different at each temperature Figure 2 The change of slope per unit temperature change is the temperature coefficient of scale factor 1 KS S Scal Scal T Tcal where Scal is the scale factor at the calibration temperature Tcal and S is the scale factor at a different temperature T Temperature change can also shift the zero crossing of the calibration line in the absence of any real tilt of the structure to which the tiltmeter is attached In Figure 2 the zero offset voltage is VT which leads to an apparent tilt angle of OT Scal VT at temperature T The zero shift is therefore Scal VT Ocal The zero shift per unit temperature change is defined as the temperature coefficient of zero shift KZ 2 KZ Scal VT Ocal T Tcal The coefficients KS and KZ are determined in the laboratory by performing calibrations at t
11. oinch per inch 1 micron per meter 1 mm per km If Canada had a hinge at Winnipeg and a man standing in Vancouver lifted the west coast to chest height he would tilt the western half of the country by 1 microradian Jewell Jewell Instruments LLC 850 Perimeter Road Manchester NH 03103 Instruments sales jewellinstruments com www jewellinstruments com e Tel 800 227 5955
12. wo or more temperatures and include contributions from all sources Their values are specific to each of the several classes of tiltmeters made by Jewell Instruments and are available on request For tiltmeters with the designation high gain the ones most typically used in geotechnical engineering KS 0 0004 CC and KZ 1 5 microradians C 0 3 arc second C Temperature coefficient values should decline in the future as sensor and electronic designs advance Procedure for Temperature Compensation For tiltmeter measurements made at the calibration temperature Tcal the tilt angle 0 is simply 3 0 Scal V where Vis the measured voltage For measurements at a different temperature T the scale factor is first adjusted using the temperature coefficient KS before computing 60 X axis Y axis Figure 3 Ten days of ground tilts during a Resultant pump test without temperature compensation Figure 4 Same data as in Figure 3 with temperature compensation X axis Y axis Resultant E 26 02 1997 B A yp 08031997 4 S Scal 1 KS T Tcal The zero offset is then removed using the temperature coefficient KZ and the true tilt angle computed as follows 5 0 SV KZ T Tcal where 0 true angular position tilt and T the temperature at which your measurement was made Tcal the calibration temperature reported in the tiltmeter user s manual S scale factor
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