Out of Tolerance - Internal Reference (Digital Photometers)

An “out of Tolerance” condition can occur with a digital photometer's internal reference. This condition can result in either a low reference response or a high reference response. Each of these scenarios is discussed below.

A low reference response condition is where the photometer reports a concentration value less
than physically present. This has the effect of underreporting leakage levels and potentially passing filters that have failing characteristics.

Conditions of example:

• Photometer has a reference value set to 100
• Reference response is low. (i.e. the photometer reads 75‐ug/l when 100‐ug/l is sampled)
• The 100% set point for response is not obtained by sampling a real‐time aerosol upstream of the filter under test.

A leakage rate of 0.010% would be reported as 0.0075% due to the 25% reduction in response.

A high reference response condition is where the photometer reports a concentration value greater than physically present. This has the effect of over‐reporting leakage levels and potentially failing filters that have passing characteristics.

Conditions of example:

• Photometer has a reference value set to 100
• Reference response is high. (i.e. the photometer reads 125‐ug/l when 100‐ug/l is sampled)
• The 100% set point for photometer response is not obtained by sampling a real‐time aerosol upstream of the filter under test.

A leakage rate of 0.010% would be reported as 0.0125% due to the 25% increase in response.

Relevant Notes:
When utilized for filter leakage certification, every effort should be taken to use an actual sample of the “Upstream” aerosol concentration. If an actual sample of the “Upstream” aerosol challenge concentration is used to set the 100% response, an Out of Tolerance condition has no effect on the leakage values obtained. Sampling the Upstream aerosol concentration for the 100% setting is typically referred to ratio-metric mode. The unit reports a ratio of the detected leakage directly as a percentage of the available span at published levels of instrument accuracy. For this reason, the Internal Reference feature should be used for obtaining a working 100% set point only when an upstream sample cannot be achieved.

Displayed % Leakage values may be limited to 135% of the chosen 100% set point regardless of whether an aerosol sample or Internal Reference was used. A digital photometer having a value of 65 chosen as the reference factor has a reading ceiling of up to 135% of the 65 chosen as the reference factor, or 87.8‐ug/l of the reference base. The 135% display limit is due to range limitations of the amplifier.