RAE Systems MultiRAE Lite Gas Instruction Manual

Technical Note TN-106 05/15/VK

A GUIDELINE FOR PID INSTRUMENT RESPONSE

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.

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/m

To convert from ppm to mg/m3, use the following formula:

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* 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 H
when their ionization energies are well below the lamp photon energy.

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2O2

and NO

give weak response even

2

For air at 25°C (77°F), the molar gas volume is 24.4 L/mole and the
formula reduces to:

1

Technical Note TN-106 05/15/VK

For example, if the instrument is calibrated with a gas standard in
ppmv, such as 100 ppm isobutylene, and the user wants the display

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to read in mg/m

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:

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.

For a spreadsheet to compute the correction factor and TLV of a
mixture see the appendix at the end of the CF table.

TLVs and Alarm Limits for Mixtures

The correction factor for mixtures can be used to set alarm limits
for mixtures. To do this one first needs to calculate the exposure
limit for the mixture. The Threshold Limit Value (TLV) often defines
exposure limits. The TLV for the mixture is calculated in a manner
similar to the CF calculation:

In the above example, the 8-h TLV for benzene is 0.5 ppm and
for n-hexane 50 ppm. Therefore the TLV of the mixture is
TLVmix = 1 / (0.05/0.5 + 0.95/50) = 8.4 ppm, corresponding to

8.0 ppm hexane and 0.4 ppm benzene. For an instrument
calibrated on isobutylene, the reading corrsponding to the TLV is:

2. Pressurized gas cylinder (Demand-flow regulator):

A demand-flow regulator better matches pump speed
differences, but results in a slight vacuum during calibration
and thus slightly high readings.

3. Collapsible gas bag: The instrument will draw the

calibration gas from the bag at its normal flow rate, as
long as the bag valve is large enough. The bag should
be filled with enough gas to allow at least one minute
of flow (~ 0.6 L for a MiniRAE, ~0.3 L for MultiRAE).

4. T (or open tube) method: The T method uses a T-junction

with gas flow higher than the pump draw. The gas supply is
connected to one end of the T, the instrument inlet is connected
to a second end of the T, and excess gas flow escapes through
the third, open end of the T. To prevent ambient air mixing,
a long tube should be connected to the open end, or a high
excess rate should be used. Alternatively, the instrument
probe can be inserted into an open tube slightly wider
than the probe. Excess gas flows out around the probe.

The first two cylinder methods are the most efficient in terms
of gas usage, while the bag and T methods give slightly more
accurate results because they match the pump flow better.

B. Pressure. Pressures deviating from atmospheric pressure

affect the readings by altering gas concentration and pump
characteristics. It is best to calibrate with the instrument and
calibration gas at the same pressure as each other and the
sample gas. (Note that the cylinder pressure is not relevant
because the regulator reduces the pressure to ambient.) If
the instrument is calibrated at atmospheric pressure in one
of the flow configurations described above, then 1) pressures
slightly above ambient are acceptable but high pressures
can damage the pump and 2) samples under vacuum may
give low readings if air leaks into the sample train.

A common practice is to set the lower alarm limit to half the TLV,
and the higher limit to the TLV. Thus, one would set the alarms
to 1.3 and 2.6 ppm, respectively.

CALIBRATION CHARACTERISTICS

A. Flow Configuration. PID response is essentially

independent of gas flow rate as long as it is sufficient to
satisfy the pump demand. Four main flow configurations
are used for calibrating a PID:

1. Pressurized gas cylinder (Fixed-flow regulator):

The flow rate of the regulator should match the flow
demand of the instrument pump or be slightly higher.

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C. Temperature. Because temperature effects gas density and

concentration, the temperature of the calibration gas and
instrument should be as close as possible to the ambient
temperature where the unit will be used. We recommend
that the temperature of the calibration gas be within the
instrument’s temperature specification (typically 14° to 113° F
or -10° to 45° C). Also, during actual measurements, the
instrument should be kept at the same or higher temperature
than the sample temperature to avoid condensation in the unit.

D. Matrix. The matrix gas of the calibration compound and

VOC sample is significant. Some common matrix components,
such as methane and water vapor can affect the VOC signal.

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Technical Note TN-106 05/15/VK

PIDs are most commonly used for monitoring VOCs in air,
in which case the preferred calibration gas matrix is air.
For a MiniRAE, methane, methanol, and water vapor reduce
the response by about 20% when their concentration is
15,000 ppm and by about 40% at 30,000 ppm. Despite
earlier reports of oxygen effects, RAE PID responses with

10.6 eV lamps are independent of oxygen concentration,
and calibration gases in a pure nitrogen matrix can be
used. H

and CO2 up to 5 volume % also have no effect.

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E. Concentration. Although RAE Systems PIDs have electronically

linearized output, it is best to calibrate in a concentration range
close to the actual measurement range. For example, 100 ppm
standard gas for anticipated vapors of 0 to 250 ppm, and 500 ppm
standard for expected concentrations of 250 to 1000 ppm. The
correction factors in this table were typically measured at 50 to
100 ppm and apply from the ppb range up to about 1000 ppm.
Above 1000 ppm the CF may vary and it is best to calibrate
with the gas of interest near the concentration of interest.

F. Filters. Filters affect flow and pressure conditions and therefore

all filters to be used during sampling should also be in place
during calibration. Using a water trap (hydrophobic filter)
greatly reduces the chances of drawing water aerosols or
dirt particles into the instrument. Regular filter replacements
are recommended because dirty filters can adsorb VOCs
and cause slower response time and shifts in calibration.

G. Instrument Design. High-boiling (“heavy”) or very reactive

compounds can be lost by reaction or adsorption onto materials
in the gas sample train, such as filters, pumps and other
sensors. Multi-gas meters, including EntryRAE, MultiRAE
and AreaRAE have the pump and other sensors upstream of
the PID and are prone to these losses. Compounds possibly
affected by such losses are shown in green in the table, and
may give slow response, or in extreme cases, no response at
all. In many cases the multi-gas meters can still give a rough
indication of the relative concentration, without giving an
accurate, quantitative reading. The ppbRAE and MiniRAE series
instruments have inert sample trains and therefore do not
exhibit significant loss; nevertheless, response may be slow for
the very heavy compounds and additional sampling time up to
a minute or more should be allowed to get a stable reading.

TABLE ABBREVIATIONS

CF = Correction Factor (multiply by reading to get corrected

value for the compound when calibrated to isobutylene)

NR = No Response

IE = Ionization Energy (values in parentheses are not well

established)

C = Confirmed Value indicated by “+” in this column; all others
are preliminary or estimated values and are subject to change

ne = Not Established ACGIH 8-hr. TWA

C## = Ceiling value, given where 8-hr.TWA is not available

DISCLAIMER

TN-106 is a general guideline for Correction Factors (CF) for use
with PID instruments manufactured by RAE Systems. The CF may
vary depending on instrument and operation conditions. For the best
accuracy, RAE Systems recommends calibrating the instrument to
target gas. Actual readings may vary with age and cleanliness of
lamp, relative humidity, and other factors as well. For accurate work,
the instrument should be calibrated regularly under the operating
conditions used. The factors in this table on the following pages
were measured in dry air (40 to 50% RH) at room temperature,
typically at 50 to 100 ppm. CF values may vary above about 1000 ppm.

Updates
The values in this table on the following pages are subject to change
as more or better data become available. Watch for updates of this
table on the Internet at http://www.raesystems.com.

IE data are taken from the CRC Handbook of Chemistry and Physics,
73rd Edition, D.R. Lide (Ed.), CRC Press (1993) and NIST Standard Ref.
Database 19A, NIST Positive Ion Energetics, Vers. 2.0, Lias, et.al.,
U.S. Dept. Commerce (1993). Exposure limits (8-h TWA and Ceiling
Values) are from the 2005 ACGIH Guide to Occupational Exposure
Values, ACGIH, Cincinnati, OH 2005. Equations for exposure limits
for mixtures of chemicals were taken from the 1997 TLVs and BEIs
handbook published by the ACGIH (1997).

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3

Technical Note TN-106 05/15/VK

Compound Name Synonym/Abbreviation CAS No. Formula 9.8 C 10.6 C 11.7 C IE (eV)TWA

Acetaldehyde 75-07-0 C2H4O NR + 6 + 3.3 + 10.23 C25

Acetic acid Ethanoic Acid 64-19-7 C2H4O

Acetic anhydride Ethanoic Acid Anhydride 108-24-7 C4H6O

Acetone 2-Propanone 67-64-1 C3H6O 1.2 + 0.9 + 1.4 + 9.71 500

Acetone cyanohydrin 2-Hydroxyisobutyronitrile 75-86-5 C4H7NO 4 + 11.1 C5

Acetonitrile Methyl cyanide, Cyanomethane 75-05-8 C2H3N 100 12.19 40

Acetylene Ethyne 74-86-2 C2H

2

Acrolein Propenal 107-02- 8 C3H4O 42 + 3.9 + 1.4 + 10.10 0 .1

Acrylic acid Propenoic Acid 79-10-7 C3H4O

Acrylonitrile Propenenitrile 107-13 -1 C3H3N NR + 1.2 + 10.91 2

Allyl alcohol 107-18 -6 C3H6O 4.5 + 2.4 + 1.6 + 9.67 2

Allyl chloride 3-Chloropropene 107-05-1 C3H5Cl 4.3 0.7 9.9 1

Ammonia 76 64-41-7 NH

Amyl acetate mix of n-Pent yl acetate &

628-63-7 C7H14O

3

2-Methylbutyl acetate

Amyl alcohol 1-Pentanol 75-85-4 C5H12O 5 10.00 ne

Aniline Aminobenzene 62-53-3 C6H7N 0.50 + 0.48 + 0.47 + 7.72 2

Anisole Methoxybenzene 100-66-3 C7H8O 0.89 + 0.58 + 0.56 + 8.21 ne

Arsine Arsenic trihydride 77 84-42-1 AsH

3

Benzaldehyde 100-52-7 C7H6O 1 9.49 ne

Benzene 71-4 3-2 C6H

6

Benzonitrile Cyanobenzene 100-47-0 C7H5N 1.6 9.62 ne

Benzyl alcohol

α-Hydrox ytoluene,

100 -51-6 C7H8O 1.4 + 1.1 + 0.9 + 8.26 ne
Hydroxymethylbenzene,
Benzenemethanol

Benzyl chloride

α-Chlorotoluene,

100- 4 4-7 C

Cl 0.7 + 0.6 + 0.5 + 9 .14 1

7H7

Chloromethylbenzene

Benzyl formate Formic acid benzyl ester 104-57-4 C

Boron trifluoride 7637-07-2 BF

Bromine 7726-95-6 Br

8H8O2

3

2

Bromobenzene 108-86 -1 C6H5Br 0.6 0.5 8.98 ne

2-Bromoethyl methyl ether 6482-24-2 C3H7OBr 0.84 + ~10 ne

Bromoform Tribromomethane 75-25-2 CHBr

3

Bromopropane,1- n-Propyl bromide 106-94-5 C3H7Br 150 + 1.5 + 0.6 + 10.18 ne

Butadiene 1,3-Butadiene, Vinyl ethylene 106-99-0 C4H

6

Butadiene diepoxide, 1,3- 1,2,3,4-Diepoxybutane 298-18-0 C4H6O

Butane 106-97-8 C4H

10

Butanol, 1- Butyl alcohol, n-Butanol 71-36-3 C4H10O 70 + 4.7 + 1.4 + 9.99 20

Butanol, t- tert-Butanol, t-Butyl alcohol 75-65-0 C4H10O 6.9 + 2.9 + 9.90 100

Butene, 1- 1-Butylene 106-98-9 C4H

Butoxyethanol, 2- But yl Cellosolve, Ethylene

111-76-2 C6H14O

8

glycol monobutyl ether

Butoxyethyl Acetate, 2- 2-Butoxyethyl acetate; 2-Butoxy-

112- 07-2 C8H16O
ethanol acetate; But yl Cellosolve
acetate; Butyl glycol acetate;
EGBE A; Ektasolve EB acetate

Butyl acetate, n- 123-86-4 C6H12O

Butyl acrylate, n- Butyl 2-propenoate,

141-32-2 C7H12O
Acrylic acid but yl ester

Butylamine, n- 109-7 3-9 C4H11N 1.1 + 1.1 + 0.7 + 8.71 C5

Butyl cellosolve see 2-Butoxyethanol 111-76-2

Butyl hydroperoxide, t- 75-91-2 C4H10O

NR + 22 + 2.6 + 10.66 10

2

NR + 6.1 + 2.0 + 10.14 5

3

2.1 + 11.40 ne

2

12 + 2.0 + 10.60 2

NR + 10.9 + 5.7 + 10 .16 25

11 + 2.3 + 0.95 + <9.9 100

2

1.9 + 9.89 0.05

0.55 + 0.47 + 0.6 + 9.25 0.5

0.9 + 0.73 + 0.66 + ne

NR NR NR 15 .5 C1

NR + 1. 30 + 0.74 + 10.51 0.1

NR + 2.7 + 0.5 + 10.48 0.5

0.8 0.6 + 1.1 9.07 2

25 + 3.5 + 1.2 ~10 ne

2

67 + 1.2 10.53 800

0.9 9.58 ne

1.8 + 1.2 + 0.6 + <10 25

2

3

2

2

2.0 + 1.6 + <10 1

2

1.27 + 20

2.6 + 10 150

1.6 + 0.6 + 10

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