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Calibration Basics for Gas Detection EquipmentB Y MICHAEL D. SHAWIt is quite unlikely that you will ever use an absolute method for gas detection. Rather, you will employ any one of dozens of “relative” methods — that is, methods that produce some output that must be calibrated against a known standard. Then, its display can be directly read out in units of concentration, usually parts-per-million (ppm).Even though proper calibration is 90 percent of successful gas detection, it is a subject that has been neglected–often purposely – by the majority of instrument manufacturers. There’s a good reason for this, of course: Proper calibration can often be difficult and expensive. Early occupational health, toxic gas detection focused on carbon monoxide (CO) and hydrogen sulfide (H 2S). The calibration standards were supplied as gas blends in cylinders, and in the case of CO, at least, things worked out pretty well. This is because CO is not very reactive, and, within reason, maintains a stable concentration in the cylinder, as the pressure drops with use.On the other hand, H2S is quite reactive, and the original simplistic techniques used to create the cylinder gas blends could not provide a stable product. The problems observed with H2S blends were soon seen in blends for many other toxics. To make matters worse, improper analogies were drawn between experiences in combustible gas detection and toxic gas detection, establishing a false sense of security about poorly prepared gas blends. In fact, other than the obvious point that both combustible and toxic gas detection get involved with detecting gases, the two fields of endeavor could not be more different. The combustible gases of interest are nearly all stable (unless they are ignited by some external source), while nearly all toxic gases are unstable, and in many cases are extremely reactive. Most importantly, though, combustible gas detection is done in percent level concentrations, while toxic gas detection is done in parts-per million, and even parts-per-billion concentrations, 10,000 and 10 million times lower, respectively. Fortunately, calibration gas blending technology has improved, encompassing specialized techniques for passivating the cylinders, as well as logging experience to determine how long a blend must age to become stable, and how long stability can be guaranteed. Much of the technological development has been done with aluminum cylinders, since this material seems to be less prone to wall effects and unwanted chemical reactions than steel. Here are some helpful hints: • Order the blend so that the concentration is about 50 percent of the instrument’s measuring range; • Ensure that the blend’s analysis is ± 2 percent accurate (or better); • Insist on NIST-traceability; and • Obtain a written guarantee as to how long the blend will be stable. Since most of the cost of the blend is in the analysis labor, order the largest cylinder you can use. Stay away from disposable cylinders, which just become a solid waste problem. After all, we ARE in the environmental business. Permeation Devices Certain toxic gases, especially at low concentrations, are not well suited to being stored in cylinders, and cylinder blends are cumbersome to re-standardize, in that a separate (usually wet chemical) analytical method is required. An alternative is permeation devices. Permeation devices are small, inert capsules containing a pure chemical compound in a two-phase equilibrium between its gas phase and its liquid or solid phase. At a constant temperature, the device emits the compound through its permeable portion at a constant rate. This rate can always be determined via differential weighing at constant temperature. The final concentration is adjusted by varying the flow rate of an inert carrier gas. Permeation devices are typically employed within specially made systems that offer temperature and carrier gas flow rate control. Zero Gas As you can imagine, if your measurement range is in the low ppm (or less), accurately zeroing the instrument is of vital importance. Consider that it is not a trivial matter to remove contaminants such as carbon monoxide from air below tenths of a ppm. Zero air can be obtained from the same vendors who manufacture gas blends. We would make the following recommendations: • Tell your supplier your target gas and measuring range, and have him suggest the proper zero gas for your application; and • Ask for a written analysis of the zero gas. Ideally, there will be specific information and not just a series of “less thans.” In general, the lower your measuring range, and the greater accuracy you desire, then the more frequently you should calibrate. Calibration monthly is a good median recommendation, and bi-monthly is even better. When we say “calibration,” we mean a good patient effort that allows for sensor and instrument stabilization, to get a solid, reproducible reading. So-called bump tests, that challenge the instrument with some unknown, but high concentration of gas prove little, and can often be misleading. For the most part, these are NOT recommended. In certain cases, less frequent calibration will still afford satisfactory results. This should be discussed with your instrumentation supplier. FSM Michael D. Shaw is executive vice president and director of marketing for Interscan Corp., a manufacturer of toxic gas detection instrumentation and related software. |
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