Effects of high humidity on PIDs
Conventional PIDs may use humidity suppression/compensation techniques in an attempt to resolve the issues caused by the effects of humidity, 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.
There are a few important issues to look out for when choosing an instrument. As mentioned above, traditional PIDs can be affected by the adverse environmental conditions such as those found at fire scenes i.e. dirt, water and humidity. The situation in the immediate aftermath of a fire is therefore not ideal and in particular the presence of high humidity can disrupt PID measurements leading to false low or conversely high readings.
Looking at Diagram 1, the presence of the porous membrane should be noted. It is made from a hydrophobic material which means that it rejects the ingress of water and mitigates the chance of low readings.
To further deal with high humidity, the addition of a fence electrode (also shown in Diagram 1.) overcomes the possibility of high readings since it behaves as a conductive break and stops the excess current flow caused by the presence of high humidity and contamination which would otherwise lead to a false positive.
The cause of low readings is because water vapour absorbs the photons normally released by ionisation within the sensor as can be seen in the simplified cross sections of a PID (See diagram 2).
The release of photons is what occurs during photoionisation, but the issue is that some of these photons are absorbed by water vapour, which worsens as humidity increases. Contamination can also build up between the electrodes effectively short circuiting them, leading to a high, ‘false positive’ reading at high humidity with no VOC present i.e. > 90% RH (see Diagram 3.)