PID Mixtures

Calculating CF and EL for Constant Mixtures

• The CORRECTION FACTOR (CF_mix) 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 (EL_mix) is calculated similarly to the CF_mix.
  • When TLV concentrations are used as the exposure limits:
  • ELmix = 1 / (X1/TLV1 + X2/TLV2 + X3/TLV3 + ... Xi/TLVi)

CF_mix for Constant Mixtures

Paint: 15% Styrene and 85% Xylene

CF_mix = 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

EL_mix 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

• In our 15% Styrene and 85% Xylene example:
  • EL_mix = 87 ppm
  • CF_mix = 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 120_iso (PID readings in ppm Isobutylene units)
• Multiply by correction factor of 0.56_mix
• True concentration of mixture = 67.2_mix ppm
• This is under the calculated exposure limit of 87_mix 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

• 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

• 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 EL_iso 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:EL_iso & 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 EL_iso of 1.00 ppm might be more appropriate

What about benzene?