aerosol component, use of the photometer would be appropriate for monitoring lead exposure.
At high humidity, water droplets can exist in the air for extended periods of time and be detected by a photometer. These droplets can change size rapidly in response to small changes in humidity. Therefore, care should be taken when measuring aerosols near water sprays and other high humidity locations. It has been found in some cases, such as in cotton mills, that the aerosol produced by dried water sprays can be a significant component of the workplace aerosol.
Aerosol photometers require that the aerosol be carried to the detection volume in some fashion. Because of the inertial and electrical properties of aerosol particles, there may be errors in transporting the aerosol to the detection volume. Most photometers have a sampling pump that draws the aerosol into an inlet, through a length of duct, and to the detector. Some instruments include a preclassifier (cyclone or impactor) to make the overall response more similar to the respirable dust definition. Some photometers rely on air convection or motion of the photometer to bring the aerosol to the detector (passive sampling). In either case, there will generally be some particles that are not detected due to losses in the instrument before they reach the detector. Particles larger than 10 µm are especially likely to be lost. However, these losses will generally be small for smaller particles unless there is high local air velocity or unless the aerosol particles are highly charged.
A list of instruments is provided at the end of this chapter. Other instruments are listed in various references [7,8]. Since instrument development is an active field, it is suggested that current literature sources and the manufacturers be contacted for the latest information.
4. DATA ACQUISITION AND TREATMENT
a. Calibration
Most photometers are factory-calibrated by comparing the instrument response in a well-defined aerosol to measurements by the gravimetric method (e.g. Method 0600). The instrument response at one or more concentrations is compared with the gravimetric method result. In most cases, the photometer response is modified to read directly in mg/m3 . However, it should be remembered that this calibration is only valid for the specific calibration aerosol and may differ by as much as a factor of ten when used with an aerosol from a different source, different composition, or size distribution. The factory calibration of this type is primarily useful to ensure that the instrument is operating properly and responding the same as other similar instruments. It does not ensure that the photometer will respond accurately to another aerosol.
An aerosol photometer measures a single parameter that is dependent on many variables, e.g., particle size distribution, particle agglomeration, particle refractive index, that can and do change in field situations. Therefore, it is necessary that the aerosol photometer be calibrated in conditions closely approximating the aerosol to be monitored. Calibration is carried out by comparing the time-weighted average photometer readings directly with field measurements using Method 0600. Most photometers do not protect the optical surfaces with a clean air sheath and should be checked frequently for zero drift during calibration and routine monitoring.
b. Operation Data can be collected in either manual or automated mode. Most direct-reading photometers have a digital or analog readout indicating concentration. This allows manual observation of measured concentration. In this mode the instrument is useful for observing relative concentrations between different locations, for detecting leaks in processes, for evaluating work procedures, etc. Some photometers also have an output port that can be
connected to a recorder or other data acquisition system. This allows the instrument to
