Ion Science MiniPID - Manual (Page 8)

MINIPID 3PIN MANUAL Ion Science Ltd
Page 8 of 27
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Response Factors
What is a response factor?
Our PIDs are calibrated using isobutylene, but the PID is a broadband VOC detector with a sensitivity that
differs for each VOC. Response Factors are used to compensate for these differing sensitivities.
A response factor (RF) is a number which relates the MiniPID response to a particular VOC relative to
isobutylene. If you know what VOC you are measuring then multiplying the displayed concentration by the
RF of the VOC will result in the actual concentration of VOC.
Example: Toluene

A sensor is calibrated using isobutylene and found to have a sensitivity of 1 mV ppm
-1
.

If the sensor is exposed to 100 ppm isobutylene the output will be 100 mV.

Toluene is known to generate twice the response of isobutylene.

In order to correct the toluene response it is multiplied by the response factor for toluene of 0.5.

If the sensor is exposed to 100 ppm toluene then the displayed uncorrected concentration will be 200
ppm isobutylene. The corrected concentration would be 200 multiplied by the RF, 0.5, which gives the
correct result of 100 ppm toluene.
If response factors are programmed into an instrument, you are able to specify a volatile compound, and the
instrument will internally compensate for the response factor corresponding to that volatile, and display and
record the corrected volatile concentration.
VOC mixtures
Occasionally you will be measuring a mixture of VOCs. If the total concentration is within the linear range of
the PID, then it is reasonable to assume that the concentrations are additive without interference between
the different VOCs:
The correction factor for a gas mix containing PID detectable gases A, B, C… with response factors RF(A),
RF(B), RF(C), in fractional proportions a:b:c is given by:
RF mix = a/RF(A) + b/RF(B) + c/RF(C) …
Example:
A gas mix to be monitored contains 1 part isopropanol to 4 parts acetone:
Chemical name RF Fractional composition
Isopropanol 4.4 0.2
Acetone 0.7 0.8
Therefore the RF of the mix will be:
RF mix = (4.4 x 0.2) + (0.7 x 0.8)
= 0.88 + 0.56
= 1.44
Important: remember that if you are measuring a combination of VOCs then accurate measurement of one
of these VOCs will be difficult; without careful data analysis, you will get only a RF averaged measurement.
Be cautious when reporting actual VOC concentration if you know that there may be several VOCs present.