MSA ALTAIR 5X PID Operating Manual page 12

Span Gas
Span gas is a reference gas used during calibration to determine the slope (response per unit concen-
tration) of the calibrated response curve.
For the 0-2000 ppm PID sensor the allowable calibration gas is 100 ppm isobutylene.
See Chapter 4.10 for calibration instructions.
Response Factors
When a compound is ionized by photoionization, the ionized molecules are collected and converted to
a current. This response is a characteristic property of the specific compound which is influenced by
its molecular structure. The slope of the response curve (defined in picoamperes per ppm) is different
for different chemicals. To properly report the concentration for a given sample gas, the ALTAIR 5X
PID uses response factors. See Chapter 10, for instructions on using the pre-programmed list of
response factors.
It is very important to have an understanding of PID basics when changing PID settings. Failure to
properly identify the VOC gas being measured and/or failures to select the correct Response Factor
alarm values (exposure, STEL, TWA) that match the desired Response Factor and/or the correct
lamp will result in erroneous readings or erroneous alarm limits that could cause death or serious
personal injury.
The response factor is defined as the ratio of the detector response for isobutylene to the detector
response for the sample gas. Response factors for a wide range of substances have been determined
experimentally. These response factors are programmed into the device. Note that the calibrated
response curve, and all programmed response factors are relative to isobutylene. Isobutylene has a
response factor of one.
The response factor is a multiplier that compensates for the difference between the response of the
sample gas and the response of isobutylene at 100ppm. Whenever the device detects the presence
of a VOC, it uses the response factor for the user-assigned target gas to convert the signal to the
correct, concentration. This is done by multiplying the equivalent isobutylene response by the response
factor for the set sample gas. The isobutylene response curve is calculated at every calibration.
If the response factor is known, a device calibrated with isobutylene can be used to calculate the actual
concentration of a target gas.
For example:
An operator is using a device that has been calibrated on isobutylene. The sample gas is set to isobuty-
lene. While using this device to sample for hexane, the display reads 100 ppm. Since the response
factor for hexane is 4.5, the actual concentration of hexane is:
Actual hexane concentration = 4.5 x 100 ppm = 450 ppm.
Calculating a Response Factor
To determine a response factor for a target chemical, perform the following simple procedure:
(1) Calibrate the ALTAIR 5X PID using isobutylene as the span gas.
(2) On the device, set the sample gas name to isobutylene.
(3) Apply a known concentration of the target chemical to the device and note the concentration
reported in the display.
The response factor for the target chemical relative to isobutylene:
RF target gas =
For example:
A device is calibrated on isobutylene, and has isobutylene defined as the sample gas. When sampling
106 ppm of benzene in air, the device reports a concentration of 200 ppm. In this example, the
response factor for benzene relative to isobutylene would be:
RF benzene =
WARNING
Actual known concentration
Concentration reported by device
106 ppm known concentration benzene
200 ppm reported
ALTAIR 5X PID
PID Theory and Definitions
=0.53
US
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