Portable gas monitoring instruments are typically operated in a passive (diffusion) mode or in an aspirated (pumped) mode. How do I know which one I should use and whether or not one mode is better than the other?
This is a common question that really has a simple answer, but it does come with some detail.
Most sensors intended for use in portable gas monitoring instruments are designed to operate in the passive mode. These designs are such that gas in air diffuses through normal air currents into openings on the face of the sensor and accumulates on and reacts with the sensor’s working electrode. The sensors will function and perform normally in a properly designed diffusion based instrument with no help from a sampling pump at all.
However, there are many that believe that a pump is necessary to draw air into the instrument and sensors and that an instrument with a pump can detect gas in a wider area than a simple diffusion monitor. These are simply myths of gas detection and the true fact is that the flow rates of pumps used with portable instruments are so relatively low that the only gas that is sensed or air that is drawn in is from the immediate end of the sampling hose or inlet of the sampling pump. Truly whether the instrument is operated in a diffusion mode or in a pumped mode, it is only a point detector and can only detect gas that is at the immediate face of the sensors or inlet to sampling pump.
So exactly when should I use an instrument with a sampling pump? As I said earlier, the answer is quite simple. The only time that it is necessary to use a sampling pump with a portable gas monitoring instrument is when it is necessary to sample the conditions of the air in an area located remotely from the location of the instrument. Confined space regulations require that atmospheres of confined spaces are tested prior to entry and to do that requires a pump to draw the air from within the space out to the instrument.
The rule of thumb then for whether or not you need to use a pump with your instrument is simply that if you are standing at Point A and need to know the gas concentration Point B, you need a pump.