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CAUTION:
Environmental conditions can have an effect on sensor accuracy!

With the increasing prevalence of gas detectors used daily for ensuring workers safe entry into confined spaces, it is important that these instrument users understand the factors which may influence the reliability of the information produced by these monitoring tools. This article is intended to briefly review some of the concerns regarding the measurement for explosive atmospheres with combustible gas (LEL) indicating instruments.

There are three types of sensors which are commonly used in confined space monitors; oxygen, combustible gas (LEL), and toxic gas sensors. Each type of sensor uses a slightly different detection principle. The kinds of conditions that can affect accuracy vary from one type of sensor to the next. The type of sensor which is most prone to being affected by the atmosphere in which it is being used tends to be the combustible gas sensor. Age and usage can have a serious effect on sensitivity.

Chronic exposure to silicone containing substances (found in many lubricants), the tetra-ethyl-lead found in "leaded " gasoline, halogenated hydrocarbons (Freons®, or solvents such as trichloroethylene and methylene chloride), high concentrations of hydrogen sulfide or even very high concentrations of combustible gas may lead to degraded combustible sensor performance. In most cases all this means is that the sensitivity is adjusted upwards at the time the instrument is calibrated. In the worst case, the sensor may need to replaced. Once again, verifying the accuracy of the sensors on a regular basis is essential to assuring worker safety.

How combustible sensors work

The minimum amount of a combustible gas or vapor in air which will explosively burn if a source of ignition is present is the Lower Explosive Limit (LEL) concentration. Combustible gas readings are given in percent LEL, with a range of zero to one-hundred percent explosive. Combustible sensors contain two coils of fine wire coated with a ceramic material to form beads. The "active" bead is coated with a palladium based material that allows catalyzed combustion to occur on the surface of the bead. The "reference" bead lacks the catalyst coating, but in other respects exactly resembles the active bead.

Any combustible vapors which are present will be subject to catalytic combustion on the surface of the active bead, heating this bead to a higher temperature. The temperature of the untreated reference bead is unaffected by the presence of gas. The difference between the temperatures of the two beads is proportional to the amount of combustible gas present. Since the beads are strung on the opposite arms of a Wheatstone Bridge electrical circuit, the instrument perceives this as a change in the electrical resistance in the circuit. It is this change in resistance due to differential heating that is used by the instrument to provide a reading.

Combustible gas sensors are non-specific and respond to all combustible gases and vapors. It is not necessary for the combustible vapor to be present in LEL concentrations. Even trace amounts of combustible gas can be detected by this method. The amount of heat produced by the combustion of a particular gas on the active bead will reflect the "Heat of Combustion" for that gas. Heats of combustion may vary from one combustible gas to another. For this reason readings may vary between equivalent concentrations of different combustible gases.

A combustible gas and vapor reading instrument may be calibrated to any number of different gases or vapors. If an instrument is only going to be used for a single type of gas over and over again, it is usually best to calibrate the instrument to that particular hazard. As long as the gas that is encountered is the same gas that was used during calibration, the readings will be exactly accurate (to the tolerances of the instrument design).

Figure 1.0 illustrates what may be seen when a combustible sensor is used to monitor gases other than the one to which it was calibrated. The chart shows the "relative response curves" of the instrument to several different gases.

Figure 1.0: Relative response curves

Note that the response to the gas to which the instrument was calibrated, the "calibration standard", is still precisely accurate. For the other gases the responses are a little off. In the case of some gases the readings are a little high. This results in the instrument going into alarm a little bit early. This type of error is not usually dangerous, since it simply results in workers exiting the affected area sooner than they otherwise would have.

Gases which produce lower relative readings than the calibration standard can result in a more potentially dangerous sort of error. One way to reduce the potential for this type of error is to use a lower alarm setting. It may be seen from the graph that the amount of relative error decreases the lower the alarm point is set. If the alarm point is set at 10 percent LEL, the differences due to relative response of the combustible sensor are minimal.

Choosing the right calibration mixture

The other method for reducing the effects of this sort of error is in the choice of the calibration gas used to calibrate the combustible sensor. The best results are obtained when calibration is done using the same gas that is expected to be encountered while actually using the instrument. When it is not possible to calibrate directly to the gas to be measured, or when the combustible gas is an unknown, a mixture which provides a sensor response which is more typical of the range of combustible gases and vapors that will be encountered should be selected.

Summation

Remember that in actual practice, the relative response varies somewhat from sensor to sensor. Response ratios may also shift over the life of a particular sensor. Most importantly, if sensitivity is lost due to poisoning, it is frequently lost first with regards to methane.

In most cases the loss of occurs a little at a time. In some cases, however, the loss of sensitivity can be almost immediate. This is the reason that reputable gas detector manufacturers place so much emphasis on calibration, and why use of a certified calibration gas is a good idea.

Most gas monitoring device manufacturers recommend that the accuracy of any gas detector be verified before each day's use. In addition, accuracy should be re-checked immediately after any known exposure to contaminants.

Article Contributed by:

Timothy J. Kearney, CSP
Vice President, Argus Group

Article Provided by: The Argus Group

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