Technical Note TN-106 rev 13d wh.01-05 Correction Factors, Ionization Energies*, And Calibration Characteristics Correction Factors and Ionization Energies RAE Systems PIDs (photoionization detectors) can be used to detect 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 (CFs) have been determined that enable the user to quantify a large number of chemicals using only a single calibration gas, typically isobutylene. In RAE Systems 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 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. * Some inorganic compounds like H2O2 and NO2 give weak response at photon energies well above those of their ionization energies. The term "ionization energy" replaces the old term "ionization potential." measured is m-xylene (CF = 0.43). After downloading this Correction 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 is 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) 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 is 5.4 ppm. 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, consisting of 16 ppm benzene and 304 ppm hexane. For a spreadsheet to compute the Correction Factor and Threshold Limit Value (TLV) of a mixture, see the appendix at the end of the CF table. Example 2 With the unit calibrated to read isobutylene equivalents, the reading is 100 ppm with a 10.6 eV lamp. The gas 1 RAE Systems Inc. 3775 N. First St., San Jose, CA 95134-1708 USA Phone: +1.888.723.8823 Email: raesales@raesystems.com Web Site: www.raesystems.com