No. 17 - Concentration Measurement with Photometers

A question that has arisen many times over the past thirty some years is whether or not the ATI Photometer can be used to measure concentration levels.

The answer to the question is an emphatic yes, but its given with a very big caveat.

An aerosol photometer reacts to particulate that is drawn through the viewing area, (optical convergence point) and scatters light forward. The greater the amount of particulate, the greater the light scattered forward. This light is optically focused on a photomultiplier tube which converts the light to an electrical current. This current is sent to the amplifier for amplification, signal processing, and display. Therefore an aerosol photometer is an excellent, instantaneous concentration indicator.

The mathematical formula for the light scattered forward by the particle is not a very difficult, complex formula. Basically, it says that the larger the particle, the more light scattered forward and it is exponential. For example, an aerosol photometer will give you the same signal for ten one-micron particles as it would for one ten-micron particle. Since the aerosol photometer looks at particulate matter en masse, it would be a good instrument for indicating concentration. To obtain accurate results, the photometer must be calibrated to the aerosol whose concentration is to be measured.

A good example of this is, in the early 60's the Naval Research Laboratory (NRL) started calibrating ATI photometers against the aerosol generated in a Q-127 Aerosol Penetrometer which generates a monodispersed aerosol. David W. Crosby, of ATI, designed and patented an adjustable Light Leak that could be set at any point and used to adjust the photometer sensitivity to any particulate concentration level for a specific aerosol. In modern ATI photometers this is currently called the Internal Reference.

Here's how the Internal Reference works. When an aerosol photometer is returned to ATI for recalibration it is thoroughly cleaned, optically realigned, electronically calibrated, performance checked, and then it is calibrated to a specific aerosol. The current default aerosol ATI uses is DOP. ATI generates a polydispersed aerosol of a known NIST traceable concentration. After the photometer has been thoroughly warmed up, a sample of this known concentration aerosol is drawn into it and the photometer sensitivity is adjusted for a reading of 100%. If a DOP polydispersed aerosol sample is then drawn in and a reading of 50% is obtained it indicates that there is a concentration of 50 micrograms per liter. It is easy to assume that the same response would be obtained if another aerosol were measured that was generated by the same nozzle and a similar liquid. Unfortunately this is not true. Some information on this subject is available in a paper David W Crosby presented at the 21st International Department of Energy/Nuclear Air Cleaning Conference in 1990. (This paper is also is available in the 1993 Proceedings of the Institute of Environmental Sciences on page 559.) Basically, even if the aerosol size is similar, the refractive index is different for different liquids and therefore photometric responses will differ from the gravimetric real time concentration measurement.

This explains the "large caveat" mentioned earlier. To have an accurate indication of the concentration, a photometer must be calibrated to the specific aerosol that will be sampled.

Warm up the photometer for a minimum of 30 minutes so that it is thermally stable. This will minimize any temperature or electronic drift.
Adjust the photometer with the Internal Reference to a 100 microgram per liter concentration of DOP aerosol.
Using the record out jack of the photometer, feed the signal to a recorder for hard copy results.
Sample the aerosol and, while sampling the aerosol with the photometer, also draw a sample through a personal sampler cassette, or similar device. If there are large concentrations of particulate you may only need to run this test 10 to 15 minutes. If the concentrations of particulate are very low you may want to run this over an 8-hour period to get quantifiable results.
Look at the data to calculate a factor that you must use with your photometer for future measurements of this type.

A good example of this application occurred in a Washington, DC, suburb where an ATI customer had to guarantee that during construction no dirt would get into the computer system and cause a head crash. ATI decided to continuously monitor the area in the vicinity of the computer main frame heads of their class 10,000 clean room. Since the area was a clean area, very low readings were obtained on the photometer after it was set up and calibrated for a 100 microgram per liter concentration level. ATI sampled the air in the same area through a 47- millimeter gravimetric sample pick up, as outlined in the above referenced paper. At the same time ATI started sampling with the photometer and feeding the signal to a strip chart recorder for hard copy data. ATI ran this test for 8 hours and weighed the gravimetric pickup sample. The results obtained measured 0.0094 micrograms per liter, rounded off to .01 micrograms per liter while the photometer reading averaged 20 on the 0.1% range or a reading of 0.02%. This 0.02% is equivalent to a 0.02 microgram per liter concentration indication. This worked out quite well since there was a factor of 50 percent.. In other words, the photometer would read twice the actual concentration in the area. In this case the factor was quite high since the monitored area was environmentally controlled. In the general work place and with other aerosols the factor usually is not as large since more particulate and larger size particulate are found in these areas.

Even if the photometer can't be calibrated to the actual aerosol being monitored, the concentration, in most cases, can be instantly read within approximately 20%. A gravimetric test can be performed to establish a correlation factor to make the result of the photometer more accurate.