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PID Mixtures

Calculating CF and EL for Constant Mixtures

  • The CORRECTION FACTOR (CFmix) for a mixture is calculated from sum of the fractions (Xi ) of each component divided by their respective correction factors (CFi):
    • CFmix = 1 / (X1/CF1 + X2/CF2 + X3/CF3 + ... Xi/CFi)
  • The EXPOSURE LIMIT (ELmix) is calculated similarly to the CFmix.
    • When TLV concentrations are used as the exposure limits:
    • ELmix = 1 / (X1/TLV1 + X2/TLV2 + X3/TLV3 + ... Xi/TLVi)

CFmix for Constant Mixtures

Paint: 15% Styrene and 85% Xylene

CFmix = 1/(0.15/0.4 + 0.85/.6) = 0.56

Where:

  • 0.15 is 15% styrene
  • 0.4 is the CF styrene
  • 0.85 is 85% xylene
  • 0.6 is the CF for xylene

ELmix for Constant Mixtures

Example:

Paint: 15% Styrene and 85% Xylene

ELmix = 1/(0.15/50 + 0.85/100) = 87 ppm

Where:

  • 0.15 is 15% styrene
  • 50 is the 50 ppm exposure limit for styrene
  • 0.85 is 85% xylene
  • 100 is the 100 ppm exposure limit for xylene

Alarm Limit for Constant Mixtures

  • Divide the EL in chemical units by CF to get the EL in isobutylene
Alarm limit =
ELmix
CFmix
  • In our 15% Styrene and 85% Xylene example:
    • ELmix = 87 ppm
    • CFmix = 0.56
    • Alarm limit = 87 ppm / 0.56 = 155 ppm

Constant Mixtures

Continuing with the 15% Styrene and 85% Xylene example:

  • Say the PID registers reading of 120iso (PID readings in ppm Isobutylene units)
  • Multiply by correction factor of 0.56mix
  • True concentration of mixture = 67.2mix ppm
  • This is under the calculated exposure limit of 87mix ppm for the mixture

PID Alarms: Varying Mixtures

  • The Controlling Compound
    • Every mixture of gases and vapors has a compound that is the most toxic and "controls" the setpoint for the whole mixture
    • Determine that chemical and you can determine a conservative mixture setpoint
    • If we are safe for the "worst" chemical we will be safe for all chemicals
Chemical Name 10.6eV CFExposure Limit Chemical
Ethanol
Turpentine
Acetone
10.0
0.45
1.2
1000
100
750

  • Ethanol "appears" to be the safest compound
  • Turpentine "appears" to be the most toxic
  • This table only provides half of the decision making equation
  • Set the PID for the compound with the lowest Exposure Limit (EL) in equivalent units and you are safe for all of the chemicals in the mixture
  • Divide the EL in chemical units by CF to get the EL in isobutylene
ELIsobutylene =
ELchemical
CFchemical
Chemical name CFiso(10.6eV)OSHA PEL (8 hr. TWA)ELISO (PEL)TLV®(8hr. TWA)ELISO (TLV)
Ethanol 10.01000100.01000100.0
Turpentine 0.45100222.32044.5
Acetone 1.21000833.4500416.7

  • IF you are following the Federal OSHA PEL ethanol the "controlling compound" when the Exposure Limits are expressed in equivalent "Isobutylene Units"
  • BE CAREFUL: If you are following the TLV the controlling chemical would be turpentine
  • Setting the PID to 75 ppm alarm in Isobutylene units protects from all three chemicals no matter what their ratio
  • IMPORTANT: Equivalent ELiso is a calculation that involves a manufacturer specific Correction Factor (CF)
  • Similar calculations can be done for any PID brand that has a published CF list

PID Alarms:ELiso & Unknowns

  • Of course, if there are known or suspected chemicals of higher risk a lower alarm might be called for.
  • In a potential terrorist chemical agent attack, a ELiso of 1.00 ppm might be more appropriate

What about benzene?

  • Benzene is almost never present all by its by itself
  • Benzene is usually minor fraction of total VOC present
  • Test for total hydrocarbons (TVOC), especially if the combustible liquid has an established PEL or TLV
    • Diesel - 15 ppm
    • Kerosene - 30 ppm
    • Jet Fuel (JP-8) - 30 ppm
    • Gasoline - 300 ppm
What about benzene