Direct Reading Instruments for Gases and Vapors
As the name suggests, direct reading instruments provide a measurement of the gas or vapor property, usually the concentration, very quickly (within a few seconds or minutes after sampling). As such, they find use in a wide variety of applications in occupational hygiene, ranging from detecting leaks of hazardous vapors in workplaces to checking the performance of control equipment to assessing worker exposures in workplaces. Because they provide readings in near-real time, they are very useful in determining concentrations over very short time intervals and in assessing whether short-term exposure limits (STELs) are being exceeded. Thus, they can be used for personal as well as area monitoring. For the purposes of this chapter, we will classify real-time instruments into several categories: (1) instruments based on absorption or emission of electromagnetic radiation by gases and vapors, (2) instruments based on the electrochemical properties of gases and vapors, and (3) instruments based on the thermal properties of gases and vapors.