Properly calibrating catalytic bead sensors: A practical guide

Transcription

1 Properly calibrating catalytic bead sensors: A practical guide Catalytic explosives (CatEx) sensors offer precise and reliable detection of flammable gases provided they are optimally set for each detection task. When performing surrogate-gas calibration, it is important to observe not only the latest regulations, but also a few detection-related characteristics. 1

2 Sensors based on the calorimetric principle are often used whenever an atmosphere needs to be tested or monitored for explosive substances like methane or nonane. The advantage of these so-called catalytic bead sensors is that they measure any flammable gas. But they only display a precise measurement for the gas they are set to detect. Their sensitivity varies depending on the substance. For example, they are generally less sensitive to vapours than to gases. If a gas detection device with a catalytic bead sensor is calibrated to detect a substance to which the sensor is sensitive, the displayed result (in terms of percentage of LEL) for any substance to which the sensor is not sensitive 1 will be lower than that substance s actual concentration. Conversely, if a sensor is calibrated to a gas to which it is not sensitive, then for all other gases, the device will display higher concentrations than actually present. For example, catalytic bead sensors are considered very insensitive to the vapour nonane. If a sensor is calibrated to nonane and detects, for example, methane, the device will show an increased test result for methane because of its stronger sensitivity to this substance. A device calibrated to nonane provides an early alarm for all flammable gases present. What does this mean in practical terms? Before a device is ready for use, it must be configured for the intended task. It is important to consider the following questions: Which gases do you expect to encounter? How and with which gas should the (daily) display test be carried out? How and with which gas should the calibration be carried out? Which alarm thresholds should be set? How should this be documented? In many cases, it is safe to assume that multiple flammable gases and vapours will be present. Therefore, it is advisable to select a calibration that keeps you on the safe side. This can be done using the target gas intended to be measured, but it can also be done using a surrogate gas. Target- or surrogate-gas calibration? There are various reasons for choosing a test gas other than the target gas (e.g. nonane) when performing calibrations and display tests: The test gas is more manageable: Nonane, for example, is liquid at room temperature and is not available in test gas cylinders. Performing a vapour calibration would be labour intensive, not to mention imprecise without proper expertise and equipment. Possible occurrence: A test gas may also be used when it, in addition to the target gas, appears in the operational environment as a possible measuring gas. More comprehensive verification: The substitute gas allows additional functions of the gas detection device to be verified. Because of its properties, nonane, for example, is the test gas of choice for applications that require the utmost safety precautions and extremely sensitive calibration. 2

3 Nonane is liquid at room temperature. For target-gas calibration, the vapour arising from liquid nonane would normally be used. However, in practical terms, this is difficult to achieve with the necessary accuracy using direct vapour calibration. Therefore, calibration to nonane is usually performed using a different test gas, i.e. a so-called surrogate gas. Correctly converting the sensitivity can be done either manually, using the manufacturer s technical specifications, or automatically by the gas detection device. MEASURING NONANE: IS YOUR DEVICE SUITABLE? A metrological approval is the only way to be sure. Proof of metrological suitability for flammable gases and vapours is provided by EN standard (VDE ), Explosive atmosphere - Part 29-1: Gas detection devices Requirements for operational behaviour of devices for measuring flammable gases. Each manufacturer decides for which gas the metrological performance will be tested. Only a few manufacturers are metrologically approved for nonane including Dräger, with its detection devices in the Dräger X-am series. Proper calibration when there are sensor poisoning risks sensor contamination risks One disadvantage of catalytic bead technology is the risk of sensor poisoning. Due to environmental influences, the sensor can lose sensitivity, inhibiting its responsiveness. Polymerising substances, such as styrene or acrylonitrile, as well as chlorinated or fluoridated hydrocarbon substances, silicones, hybrids or sulphuric compounds (e.g. hydrogen sulphide 2 and mercaptanes) can poison the sensor s catalytic element. It is often assumed that the daily bump test will indicate whether any of these contaminations have occurred, but this is incorrect. For all operations of catalytic bead sensors in which methane may occur, EN standard recommends that gas detection devices also be calibrated and tested for functionality using methane even if the device is actually set to a different target gas. A poisoned detection device will fail a functionality test in which methane is used as a surrogate gas to test the CatEx sensor. If the device is calibrated with methane, it would display higher concentrations than actually present for all other substances, assuming it can still be calibrated. The higher sensitivity provides greater safety. In practical terms, there are two methods for offsetting possible poisoning effects. 1. Calibrating the catalytic bead sensor to methane and lowering the alarm threshold for the explosives sensor. Usually, explosives sensors are set to alarm thresholds of 20% of LEL (pre-alarm) and 40% of LEL (main alarm). If a device calibrated to methane is set to 10% and 20% of LEL instead, the alarms will be triggered earlier without making any change to the device s actual sensitivity. The disadvantage of this method is that low concentrations of vapours could possibly not be displayed because the sensor s sensitivity is not increased and it is suppressed by the sensor s capture range. 2. Surrogate-gas calibration using manual or automatic calculation of the sensitivity difference. With this solution, sensitivity to methane must also be regularly tested. Poisoning causes catalytic bead sensors to become selectively insensitive to methane at first; later the insensitivity spreads to other gases or vapours like propane or nonane. This is known as selective poisoning. 3 In extreme cases, a selectively poisoned catalytic bead sensor may no longer react to methane at all. 3

4 Automatic surrogate-gas calibration for greater flexibility and safety The CatEx 125 PR catalytic bead sensor from Dräger includes an extensive dataset which allows the sensor to be calibrated with methane to almost all other flammable gases or vapours. Methane is set as the testing and calibration gas while another gas (hexane or nonane, for example) is set as the gas to be measured. Other gases in the dataset can also be used as the testing and calibration gas. Whether calibrating manually or using the Dräger X-dock to automatically test and calibrate, the device independently performs the necessary conversion calculation and sets the required sensitivity. Because of the extensive data stored in the device, any temperature influences are also automatically taken into account. A surrogate-gas calibration is generally less accurate than a targetgas calibration the tolerance ranges from +/-30%. However, because gases often occur in mixed form in real life, a substitute sensitivity to at least one gas is necessary in any case. The goal is therefore to achieve the necessary sensitivity while also excluding the possibility of selective poisoning, to ensure the user will be safely warned. This is reliably achieved using the procedure shown in Table 1: AUTOMATIC SURROGATE-GAS CALIBRATION WITH METHANE TARGET-GAS CALIBRATION (e.g. to Nonane) (e.g. to Pentane and not to Methane) Detecting selective Yes No methane poisoning Maintaining high sensitivities Yes Depending on target gas High accuracy for a (target) gas Tolerance of surrogate-gas calibration: +/- 30 % Yes Possible to use an automatic Yes Yes testing station Vapour as detection gas Yes, depending on manufacturer* Only possible with intensive use of equipment Documentation of detection, Depending on manufacturer s design* Depending on manufacturer s design calibration and function test gases Use with gas mixtures Yes Yes Comparison of methane surrogate-gas calibration with target-gas calibration * Yes, for Dräger X-am 2500 and X-am

5 The multi-gas detection device from Dräger can also be configured to display only Ex 4 as a gas indication, even though it is set in the background to detect a specific gas. Ultimately, the goal of this calibration is to detect any possibly flammable gases or vapours. If selective poisoning occurs, the calibration/ bump test will either fail or the device will be calibrated to be more sensitive than necessary therefore erring on the safe side. That is why surrogate-gas calibration can only be performed in the direction of greater sensitivity. The system prevents setting methane as the target gas if another gas has already been selected for calibration or testing. This approach ensures that methane is safely displayed according to the EN standard while also providing the necessary sensitivity for detecting other gases or vapours. Plus, the process is fully documented because the testing, calibration and detection gases are depicted individually and listed in the test reports along with the concentrations actually set. By using gas detection devices that give the option of performing surrogate-gas calibration for selected gases or vapours while also choosing methane as the test gas for the function test/calibration, users comply with the requirements of EN standard The automatic surrogate-gas calibration for the Dräger X-am 2500 and Dräger X-am 5000 devices are a simple, safe solution for CatEx 125 PR model catalytic bead sensors. Authors: Rüdiger Weich Product Manager Malte Berndt Product Manager Bettina Runge Product Manager MEASURING WITH A GAS DETECTION DEVICE CALIBRATED TO NONANE (EX) MEASURING WITH A GAS DETECTION DEVICE CALIBRATED TO METHANE (EXPLOSIVE) Display (% of LEL) Display (% of LEL) Measured gas: Nonane Measured gas: Methane Methane 2 Propane 3 Nonane Simplified example depiction for illustrative purposes, based on Bergdoll & Rudolph, BRANDSchutz 12/07, p Concentration (% of LEL) Methane 2 Propane 3 Nonane Simplified example depiction for illustrative purposes, based on Bergdoll & Rudolph, BRANDSchutz 12/07, p Concentration (% of LEL) 3 Catalytic bead sensors for measuring flammable gases and vapours have different sensitivities when calibrated to different measurement gases. Example depiction for illustrative purposes: The image to the left depicts a gas detection device calibrated to nonane, along with a catalytic bead sensor. The concentrations 100% LEL of methane and propane are displayed before the actual lower explosive limit is reached. The image to the right depicts a detection device calibrated to methane. The device is less sensitive to the gases propane and nonane. 5

6 1 Insensitive means that the sensor produces only a minimal measurement signal (e.g. in mv/% of LEL). 2 Poisoning with hydrogen sulphide is of lesser importance to modern catalytic bead sensors even when bump testing with mixed gases containing hydrogen sulphide during function testing or calibration. Example: Sensor Dräger CatEx 125 PR: Max. 2% loss of sensitivity during exposure to 1,000 ppm*h. Because bump testing is generally short (a matter of seconds) and involves low concentrations (low ppm), a daily test with mixed gas has practically no impact on sensitivity. 3 For selective methane poisoning, there are theoretical explanations related to activation energy. 4 Ex stands for explosive gases and vapours SOURCES/REFERENCES: EN standard : Explosive atmospheres - Part 29-2: Gas detection devices - Selection, installation, use and maintenance of devices for measuring flammable gases and oxygen EN standard : Explosive atmospheres Part 29-1: Gas detection devices - Requirements for operational behaviour of devices for measuring flammable gases Bergdoll, Rudolph: What is the correct LEL value? in BRANDSchutz/ Deutsche Feuerwehr-Zeitung 12/2007, p. 865ff; Published by: W. Kohlhammer GmbH ST Discover our full range of portable gas detection technologies and find the right solutions for the challenges you face every day: IMPRINT GERMANY Revalstraße Lübeck PDF