Correction Factors and Ionization Energies
RAE Systems PIDs can be used for the detection of
a wide variety of gases that exhibit different
responses. In general, any compound with
ionization energy (IE) lower than that of the lamp
photons can be measured.* The best way to
calibrate a PID to different compounds is to use a
standard of the gas of interest. However, correction
factors have been determined that enable the user to
quantify a large number of chemicals using only a
single calibration gas, typically isobutylene. In our
PIDs, correction factors can be used in one of three
ways:
1) Calibrate the monitor with isobutylene in the
usual fashion to read in isobutylene equivalents.
Manually multiply the reading by the correction
factor (CF) to obtain the concentration of the gas
being measured.
2) Calibrate the unit with isobutylene in the usual
fashion to read in isobutylene equivalents. Call
up the correction factor from the instrument
memory or download it from a personal
computer and then call it up. The monitor will
then read directly in units of the gas of interest.
3) Calibrate the unit with isobutylene, but input an
equivalent, "corrected" span gas concentration
when prompted for this value. The unit will then
read directly in units of the gas of interest.
* The term “ionization energy” is more scientifically correct and
replaces the old term “ionization potential.” High-boiling (“heavy”)
compounds may not vaporize enough to give a response even when
their ionization energies are below the lamp photon energy. Some
inorganic compounds like H2O2 and NO2 give weak response even
when their ionization energies are well below the lamp photon energy.
Example 1:
With the unit calibrated to read isobutylene
equivalents, the reading is 10 ppm with a 10.6 eV
lamp. The gas being measured is butyl acetate,
which has a correction factor of 2.6. Multiplying 10
by 2.6 gives an adjusted butyl acetate value of 26
ppm. Similarly, if the gas being measured were
trichloroethylene (CF = 0.54), the adjusted value
with a 10 ppm reading would be 5.4 ppm.
Example 2:
With the unit calibrated to read isobutylene
equivalents, the reading is 100 ppm with a 10.6 eV
lamp. The gas measured is m-xylene (CF = 0.43).
After downloading this factor, the unit should read
about 43 ppm when exposed to the same gas, and
thus read directly in m-xylene values.
Example 3:
The desired gas to measure is ethylene dichloride
(EDC). The CF is 0.6 with an 11.7 eV lamp.
During calibration with 100 ppm isobutylene, insert
0.6 times 100, or 60 at the prompt for the calibration
gas concentration. The unit then reads directly in
EDC values.
Conversion to mg/m3
To convert from ppm to mg/m3, use the following
formula:
Conc. (mg/m3) = [Conc.(ppmv) x mol. wt. (g/mole)]
molar gas volume (L)
For air at 25 °C (77 °F), the molar gas volume is
24.4 L/mole and the formula reduces to:
Conc.(mg/m3) = Conc.(ppmv) x mol. wt. (g/mole) x 0.041
For example, if the instrument is calibrated with a
gas standard in ppmv, such as 100 ppm isobutylene,
and the user wants the display to read in mg/m3
of
hexane, whose m.w. is 86 and CF is 4.3, the overall
correction factor would be 4.3 x 86 x 0.041 equals
15.2.
Correction Factors for Mixtures
The correction factor for a mixture is calculated
from the sum of the mole fractions Xi of each
component divided by their respective correction
factors CFi:
CFmix = 1 / (X1/CF1 + X2/CF2 + X3/CF3 + ... Xi/CFi)
Thus, for example, a vapor phase mixture of 5%
benzene and 95% n-hexane would have a CFmix of
CFmix = 1 / (0.05/0.53 + 0.95/4.3) = 3.2. A
reading of 100 would then correspond to 320 ppm
of the total mixture, comprised of 16 ppm benzene
and 304 ppm hexane.