Ongoing quality assurance is important for any piece of measuring equipment. From a voltmeter to a spectrophotometer, calibration checks are needed to demonstrate that it is continuing to function within its design specification. An opacity monitor is no different, and the usual method for verification is to perform a calibration error check using traceable audit filters. These are neutral density filters which are designed to have a stable opacity value, and which are periodically calibrated to a traceable national standard. A traceable calibration is important because it specifies both the filter value and its associated uncertainty.
In most cases, the required audit filter values are determined by the typical measurement range of the opacity monitor and will often be specified by a national or international standard. An opacity monitor used for compliance measurements in the USA must comply with the ASTM D6216 standard. This specifies low-, mid- and high-level values depending on the emission limit value for the installation.
|Emission Limit Value||>20% Opacity|
|Low||5% to 10%||10% to 20%|
|Mid||10% - 20%||20% to 30%|
|High||20% to 405%||
30% to 40%
Note that the specified values are the instrument reading after any applicable pathlength correction factor (PLCF) has been applied. Filter suppliers and calibration labs can calculate the required corrections, but it is a good idea for the instrument user to verify the correct value. The following equations allow you to calculate the relevant value. If Opacitycorr is the value in the table above and the required filter value is Opacityreq
ODcorr = -log10 (1 – Opacitycorr)
Opacityreq = 1 – 10(-PLCF x ODcorr)
ODcorr = -log10 (1 – 0.05) = 0.022
Opacityreq = 1 – 10(-0.022 x 0.800) = 1 – 0.960 = 0.040 = 4.0%
The maximum filter value is calculated as:
ODcorr = -log10 (1 – 0.10) = 0.046
Opacityreq = 1 – 10(-0.046 x 0.800) = 1 – 0.919 = 0.081 = 8.1%
Therefore the required value on the calibration certificate must be between 4.0% and 8.1%.