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How do I choose audit filters for my opacity monitor?

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     >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)
Where opacity values are expressed as decimal values. For example, an installation with an ELV of 10% and PLCF = 0.800 requires a low-level filter value between 5% (decimal 0.05) and 10% (decimal 0.10). The minimum filter value is calculated as:

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%.

In conclusion, periodic audits are essential to verify the correct operation of your opacity monitor, and you should now understand how to choose the correct audit filter values for your opacity monitor. If you have any questions, please  contact us


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