In addition to fixed and portable solutions, personal monitoring of volatile organic compounds is a vital component in the protection of worker’s health from the potentially toxic effects of short term high or longer term, lower level repeated exposure. The photoionisation detector (PID) has proven to be an ideal tool but there are considerations when choosing the right instrument.

A major limitation to the use of PIDs in the past was their susceptibility to ambient humidity, which is often found in hazardous environments. This is a particular issue in refineries which are typically located near water, or when measuring VOCs in steam, wastewater, or contaminated soil (which is usually moist).
Conventional PIDs may use humidity suppression/compensation techniques but each of them has disadvantages:

• Humidity sensor – these typically have a slower response than the PID sensor itself which causes a drifting compensation
• Desiccant tube – these both slow the PID response and also reduce it by adsorption plus they need replacing from time to time which adds cost
• Humidify the calibration gas – this only works at one level of humidity and is no longer accurate when the humidity changes

Importantly none of these solutions solves a false positive at high humidity.

ION Science has developed a new generation of PIDs with almost no humidity effects, without the need for RH sensor compensation, desiccant tubes, or calibration gas humidification. This is accomplished by a combination of:

• an improved sample inlet system to reduce dust and moisture entering the sensor;
• thinner sensor cell to suppress the loss of photons by water adsorption;
• the patented “Fence-Electrode” design that grounds any spurious current and thus eliminates humidity-induced drift

The humidity- resistance has been extended to fixed PIDs as well. Fixed systems are useful for applications where VOCs are expected on a routine basis, such as in chemical process control, fence line monitoring, wastewater monitoring, chemical storage areas, or locations of frequent chemical use. In some of these instances monitoring is done outdoors, where rain or night-time condensation can result in liquid water being drawn into the PID.
– Petro Industry News