4.5 Sampling methods

The LCL and UCL are calculated differently depending on the type of sampling method used. Sampling methods can be classified into one of three categories:

4.5.1 Full-Period, Continuous Single Sampling

Full-period, continuous single sampling is defined as sampling over the entire sample period with only one sample. The sampling may be for a full-shift sample or for a short period ceiling determination.

4.5.2 Full-Period, Consecutive Sampling

Full-period, consecutive sampling is defined as sampling using multiple consecutive samples of equal or unequal time duration that, if combined, equal the total duration of the sample period. An example would be taking four 2-hour charcoal tube samples.

There are several advantages to this type of sampling. If a single sample is lost during the sampling period due to pump failure, gross contamination, etc., at least some data will have been collected to evaluate the exposure. The use of multiple samples will result in slightly lower SAE. The collection of several samples leads to conclusions concerning the manner in which differing segments of the workday affect overall exposure.

4.5.3 Grab Sampling

Grab sampling is defined as collecting a number of short-term samples at various times during the sample period that, when combined, provide an estimate of exposure over the total period. Common examples include the use of detector tubes or direct-reading instru­mentation (with intermittent readings).

4.6 Calculations

If the initial and final calibration flow rates are different, a volume calculated using the highest flow rate should be reported to the laboratory. If compliance is not established using the lowest flow rate, further sampling should be considered.

Sampling is generally conducted at approximately the same temperature and baro­metric pressure as calibration, in which case no correction for temperature and pressure is required, and the sample volume reported to the laboratory is the volume actually meas­ured. Where sampling is conducted at a substantially different temperature or pressure than calibration, an adjustment to the measured air volume may be required depending on sampling pump used, in order to obtain the actual air volume sampled. The actual volume of air sampled at the sampling site is reported and used in all calculations.

For particulates the laboratory reports milligrams per cubic meter of contaminant using the actual volume of air collected at the sampling site. This value can be compared directly to OSHA Toxic and Hazardous Substances Standards (e.g., 29 CFR 1910.1000).

The laboratory normally does not measure concentrations of gases and vapors directly in parts per million. Rather, most analytical techniques determine the total weight of con­taminant in a collection medium. Using the air volume provided by the industrial hygien-ist, the lab calculates the concentration in milligrams per cubic meter and converts this to parts per million at 25°C and 760 mmHg. This result is to be compared with the PEL with­out adjustment for temperature and pressure at the sampling site.

ppm(NTP) = mg/m³(24.45)/(Mwt) where

• 24.45 = molar volume at 25°C (298 K) and 760 mmHg

• Mwt = molecular weight

• NTP = normal temperature and pressure at 25°C and 760 mmHg

If it is necessary to know the actual concentration in parts per million at the sam­pling site, it can be derived from the laboratory results reported by using the following equation:

ppm(PT) = ppm(NTP) (760)/(P) (T)/(298) where

• P = sampling site pressure (mmHg)

• T = sampling site temperature (K)

• 298 = temperature in К

Since ppm(NTP) = mg/m³ (24.45)/(Mwt) ppm(PT) = mg/m³ X 24.45/Mwt X 760/P X T/298

Note: When a laboratory result is reported as milligrams per cubic meter contaminant, concentrations expressed as parts per million (PT) cannot be compared directly to the stan­dards table without converting to NTP.

Note: Barometric pressure can be obtained by calling the local weather station or air­port and requesting the unadjusted barometric pressure. If these sources are not available, then a rule of thumb is for every 1000 ft increase in elevation, the barometric pressure decreases by 1 in.Hg.