How VESDA ECO Works
Contents
1. HOW VESDA ECO Works April 2010 Doc 18890_01 VESDA eco How VESDA ECO Works Disclaimer The contents of this document is provided on an as is basis No representation or warranty either express or implied is made as to the completeness accuracy or reliability of the contents of this document The manufacturer reserves the right to change designs or specifications without obligation and without further notice Except as otherwise provided all warranties express or implied including without limitation any implied warranties of merchantability and fitness for a particular purpose are expressly excluded Intellectual Property and Copyright This document includes registered and unregistered trademarks All trademarks displayed are the trademarks of their respective owners Your use of this document does not constitute or create a licence or any other right to use the name and or trademark and or label This document is subject to copyright owned by Xtralis AG Xtralis You agree not to copy communicate to the public adapt distribute transfer sell modify or publish any contents of this document without the express prior written consent of Xtralis General Warning This product must only be installed configured and used strictly in accordance with the General Terms and Conditions User Manual and product documents available from Xtralis All proper health and safety precautions must be taken during the installation2. commissioning and maintenance of the product The system should not be connected to a power source until all the components have been installed Proper safety precautions must be taken during tests and maintenance of the products when these are still connected to the power source Failure to do so or tampering with the electronics inside the products can result in an electric shock causing injury or death and may cause equipment damage Xtralis is not responsible and cannot be held accountable for any liability that may arise due to improper use of the equipment and or failure to take proper precautions Only persons trained through an Xtralis accredited training course can install test and maintain the system Liability You agree to install configure and use the products strictly in accordance with the User Manual and product documents available from Xtralis Xtralis is not liable to you or any other person for incidental indirect or consequential loss expense or damages of any kind including without limitation loss of business loss of profits or loss of data arising out of your use of the products Without limiting this general disclaimer the following specific warnings and disclaimers also apply Fitness for Purpose You agree that you have been provided with a reasonable opportunity to appraise the products and have made your own independent assessment of the fitness or suitability of the products for your purpose You acknowledge that you
3. years and is directly related to the amount of catalytic poisons they are exposed to Catalytic poisons include silicon based compounds e g Silicon based furniture polish electrical contact releasing sprays and lubricants like WD40 VESDA ECO uses poison resistant catalytic beads ensuring the longest possible operating life Gas name Chemical Detection Detection principle Typical sensor Formula Range life Table 1 Gas type sensing technology typical sensor life Doc 18890 01 1 How VESDA ECO Works How Catalytic Beads or Pellistors Work j _ Catalytic beads employ catalytic combustion to measure combustible gases or gt vapours in air up to the Lower Flammable Level LFL of the gas The standard sensor consists of a matched pair of elements typically referred to as a detector and compensator reference element The detector comprises a platinum wire coil embedded within a bead of catalytic material The compensator is similar except that the bead does not contain catalytic material and as a consequence is inert Bead Porous Refractory Bead with Catalyst Sealing SE Platinum i x Wire Coil 1mm Header Figure 1 Catalytic beads Pellistors Both elements are normally operated in a Wheatstone bridge circuit that will produce an output only if the resistance of the detector differs from that of the compensator The bridge is supplied with a constant dc voltage or cur
4. be connected to a simple resistor circuit that allows the voltage drop resulting from any current flow to be measured figure 2 Gas diffusing into the sensor is either oxidised or reduced at the sensing electrode and coupled with a corresponding but converse counter reaction at the other electrode a current is generated through the external circuit Since the rate of gas entry into the sensor is controlled by the capillary diffusion barrier the current generated is proportional to the concentration of gas present outside the sensor and gives a direct measure of the toxic gas present Doc 18890_01 3 How VESDA ECO Works Capillary Diffusion Barrier Sensing Es Electrode Separator CA hs Counter Electrode Current Collectors Electrolyte Sensor Pins Figure 2 Toxic Gas Sensor The central feature of the design is the gaseous diffusion barrier which limits the flow of gas to the Sensing electrode The electrode is therefore able to react all target gas as it reaches its surface and still has electrochemical activity in reserve This high activity reserve ensures each cell has a long life and excellent temperature stability 2 Electrode sensors are the simplest form of toxic gas sensors However they have limited measuring range due to polarisation of the counter electrode This polarisation effect can be eliminated by using a third reference electrode with a stable potential in the sensor design In these sensors the sensi
5. have not relied on any oral or written information representation or advice given by or on behalf of Xtralis or its representatives Total Liability To the fullest extent permitted by law that any limitation or exclusion cannot apply the total liability of Xtralis in relation to the products is limited to i in the case of services the cost of having the services supplied again or ii in the case of goods the lowest cost of replacing the goods acquiring equivalent goods or having the goods repaired Indemnification You agree to fully indemnify and hold Xtralis harmless for any claim cost demand or damage including legal costs on a full indemnity basis incurred or which may be incurred arising from your use of the products Miscellaneous If any provision outlined above is found to be invalid or unenforceable by a court of law such invalidity or unenforceability will not affect the remainder which will continue in full force and effect All rights not expressly granted are reserved Document Conventions The following typographic conventions are used in this document Convention Description Bold Used to denote emphasis Used for names of menus menu options toolbar buttons Italics Used to denote references to other parts of this document or other documents Used for the result of an action Doc 18890_01 i How VESDA ECO Works The following icons conventions are used in this document Caution This icon is
6. ich contains one or two gas sensors VESDA ECO uses the same well proven core sensing technologies that have been successfully used by the gas detection industry for many years see table 1 VEDSA ECO currently employs two core sensing principles Catalytic bead often referred to as Pellistors for the detection of flammable gases and electrochemical cells for the detection of toxic gases and for oxygen deficiency It should be noted that the majority of gas sensors have a finite life see table 1 The life of a gas sensor is linked to its internal chemistry and environmental conditions Manufacturing tolerances the chemical composition and operating environmental conditions all have an effect upon a gas sensors operating life This makes it difficult to precisely predict the end of life for any gas sensor To verify gas sensors continue to respond to gas and that their accuracy is within acceptable limits it is industry practice to regularly test re calibrate all gas detectors typically 6 monthly using zero gas e g nitrogen or air plus the target gas of a known concentration Typically 50 of full scale The life of a toxic gas electrochemical cell is related to the working life of its internal chemistry Oxygen fuel cells consume their internal chemistry during normal operation and their life can be measured in ppm hours based on an ambient of 20 9 v v The life of catalytic bead or pellistor based gas sensor can be many
7. ng electrode is held at a fixed potential relative to the reference electrode from which no current is drawn so both maintain a constant potential The counter electrode is still free to polarise but has no effect on the sensing electrode and does not limit the sensor in any way 3 Electrode sensors are the most widely used design of electrochemical sensors for detecting toxic gases VESDA ECO uses 3 electrode electrochemical cells How Oxygen Sensors Work Cc All electrochemical oxygen sensors are of the self powered diffusion limited metal air battery type comprising an anode electrolyte and an air cathode as shown N ail An oxygen cell can simply be considered as an enclosure either a metal can or a ur plastic moulding which holds two electrodes a flat PTFE tape coated with an active catalyst the cathode and a block of lead metal the anode This enclosure is airtight apart from a small capillary at the top of the cell which allows oxygen access to the working electrode The two electrodes are connected via current collectors to the pins which protrude externally and allow the sensor to be electronically connected to an instrument The entire cell is filled with conductive electrolyte which allows transfer of ionic species between the electrodes figure 3 Capillary Diffusion Barrier Top Plate es Sensing eee _ Working Insulator Electrode Lead Anode Sensor_ Base Current Collector
8. ocument The manufacturer reserves the right to change designs or specifications without obligation and without further notice Except as otherwise provided all warranties express or implied including by amp xtralis without limitation any implied warranties of merchantability and fitness for a particular purpose are expressly excluded This document includes registered and unregistered trademarks All trademarks displayed are the trademarks of their respective owners Your use of this document does not constitute or create a licence or any other right to use the name and or trademark and or label This document is subject to copyright owned by Xtralis AG Xtralis You agree not to copy communicate to the public adapt distribute transfer sell modify or publish any contents of this document without the express prior written consent of Xtralis Doc 18890_01
9. on of different gases the variation in output is fairly small compared to other detection techniques As safety applications are interested only in LFL measurements this is a major advantage The variation in output for the same LFL concentration of different gases is termed relative sensitivity Tests have been carried out to determine the experimental values of relative sensitivity for the catalytic beads used within the VEDSA ECO to a range of combustible gases Catalytic Poisons Certain substances are known to have a detrimental effect on catalytic sensors There are two mechanisms by which this can occur 2 Doc 18890_01 How VESDA ECO Works Poisoning Some compounds will decompose on the catalyst and form a solid barrier over the catalyst surface This action is cumulative and prolonged exposure will result in an irreversible decrease in sensitivity Typical poisons are organic lead and silicon compounds Inhibition Certain other compounds especially H2S and halogenated hydrocarbons are absorbed or form compounds that are absorbed by the catalyst This absorption is so strong that reaction sites in the catalyst can become blocked and normal reactions are inhibited The resultant loss of sensitivity is temporary and in most cases a sensor will recover after a period of operation in clean air In applications where either poisons or inhibitors are likely to be present the VEDSA ECO should be tested on a more frequent ba
10. rent that heats the elements to 500 550 C Combustible gases are oxidised only on the detector element where the heat generated increases its resistance producing a signal proportional to the concentration of combustible gas The compensator helps to compensate for changes in ambient temperature pressure and humidity which affect both elements equally Most catalytic based detectors have their pair of elements housed in separate metal cans In a complete gas detector the cans will normally be mounted inside an enclosure consisting of a metal sinter and housing This enclosure allows gas to reach the sensor whilst ensuring that the hot sensor elements cannot ignite an explosive gas mixture Detection and alarm of flammable gases relies on the accurate measurement of flammable gases below the LFL concentration Safety applications therefore are not generally concerned with measuring the volume concentration of gas Measurements are more usually expressed as a percentage of the LFL concentration of the gas LFL Most combustible gas detection techniques are designed to detect a wide range of gases Ideally the output of a sensor will be independent of the gas being measured In reality however the variation in physical properties affects the output Catalytic oxidation sensors are no exception so the response a catalytic bead gives to the same volume concentration of different gases will vary However when exposed to the same LFL concentrati
11. s e aai Sensor Pins Figure 3 Schematic of oxygen sensor 4 Doc 18890_01 How VESDA ECO Works The rate at which oxygen can enter the cell is controlled by the size of the capillary hole at the top of the sensor When oxygen reaches the working electrode it is immediately reduced to hydroxyl ions These hydroxyl ions migrate through the electrolyte to the lead anode where they are involved in the oxidation of the metal to its corresponding oxide As the two processes above take place a current is generated which can be measured externally by passing it through a known resistance and measuring the potential drop across it Since the current produced is proportional to the rate at which these reactions occur its measurement allows accurate determination of the oxygen concentration As the electrochemical reaction results in the oxidation of the lead anode these sensors have a limited life Once all the available lead has been oxidised they no longer work Typically oxygen sensors have 1 2 year life times www xtralis com The Americas 1 781 740 2223 Asia 852 2916 8894 Australia and New Zealand 61 3 9936 7000 Continental Europe 32 56 24 19 51 UK and the Middle East 44 1442 242 330 j TM The contents of this document are provided on an as is basis No representation or warranty either express or implied is made as to VE D A O B the completeness accuracy or reliability of the contents of this d
12. sis to ensure its ability to respond to gas Limitations of Use Catalytic beads are designed to detect flammable gases within a background of ambient air and require the presence of Oxygen to operate Catalytic beads will not detect flammable gases ina background of inert gas e g Nitrogen or Helium Cautionary Note Regarding ASD Pipework Installations To ensure ease of maintenance VESDA ECO must not be glued into ASD sampling pipework It should be noted that some glues used to permanently join ASD sample pipework and fittings contain solvents which may result in a positive response on catalytic bead based detectors Gas commissioning procedures should only be carried out after these glues have fully cured LFL Note The LFL of a gas is the minimum concentration of that gas in air at which an ignition source will cause an explosion How Electrochemical Cells Work Electrochemical toxic gas sensors are micro fuel cells designed to be maintenance free ud and stable for long periods They have a direct response to volume concentration of gas rather than partial pressure A The simplest form of electro chemical toxic sensor comprises two electrodes sensing Sr and counter separated by a thin layer of electrolyte This is enclosed in a plastic housing that has a small capillary to allow gas entry to the sensing electrode and includes pins which are electrically attached to both electrodes and allow easy external interface These pins may
13. used to indicate that there is a danger to equipment The danger could be loss of data physical damage or permanent corruption of configuration details Warning This icon is used to indicate that there is a danger of electric shock This may lead to death or permanent injury Warning This icon is used to indicate that there is a danger of inhaling dangerous substances This may lead to death or permanent injury Contact Us The Americas 1 781 740 2223 Asia 852 2916 8894 Australia and New Zealand 61 3 9936 7000 Continental Europe 32 56 24 19 51 UK and the Middle East 44 1442 242 330 www xtralis com How VESDA ECO Works Contents How Catalytic Beads or Pellistors Work GalalylUc POISONS oie nn nee ee ewe stile SESE SENEE EEE Poisoning Inhibition Limitations of Use Cautionary Note Regarding ASD Pipework Installations How Electrochemical Cells Work How Oxygen Sensors Work Doc 18890_01 How VESDA ECO Works Scope This document explains how the core sensing technologies used within the VESDA ECO gas detection product range work along with any limitations of use Overview VESDA ECO adds gas detection capability to existing or new installations of VESDA Aspirated Smoke Detection ASD systems that utilize 25mm or BSP air sampling pipe networks The VESDA ECO can detect a range of flammable gases toxic gases and oxygen deficiency Each VESDA ECO detector houses a replaceable sensor cartridge wh
Download Pdf Manuals
Related Search