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How Do I Perform an Audit on 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.

Once the correct filters have been selected, there are two methods for performing an audit, depending on whether the unit must comply with US EPA Procedure 3 Quality Assurance Requirements for Continuous Opacity Monitoring Systems at Stationary Sources.

If Procedure 3 compliance is required, an audit on an AMETEK Land 4500 MkIII must be completed using an External Zero Device (EZD). This is a device which is not part of the instrument, and which is usually stored in a controlled environment such as an office or workshop. Otherwise, the audit can use the instrument’s built-in reflector. In both cases, the procedure is similar

1. Place the instrument in Maintenance mode by entering the unlock code. This signals to the data acquisition system that the live data does not relate to the current process conditions.

2. If the process pressure is greater than the ambient atmosphere (i.e. a positive-pressure process), close the fail-safe shutters or otherwise isolate the instrument from the process. This will prevent the process gases from contaminating the instrument and harming the person performing the audit.

3. Open the transceiver by undoing the clips to reveal the main lens and zero calibration assembly.

4. The next step depends on whether compliance with Procedure 3 is needed
a.  Procedure 3 needed – fit the EZD to the front of the zero-cal assembly, ensuring the arrow points up and the serial number matches the instrument in question.
b.  If procedure 3 is not needed - Go to menu Calibration: Cal Audit: Fast Response and wait for the zero reflector to run into position

5. Check that the instrument reads 0.0% ± 0.5% opacity.

6. Insert the audit filters as specified in the relevant regulation. For US EPA Procedure 3, three filters each must be inserted three times non-consecutively for a total of nine readings. It is useful to write down the values displayed values and compare them with the expected values, but these are not the official values for inclusion in the audit report.

7. Each filter must remain in place long enough that its value is recorded on the compliance data acquisition system (DAS). Many DAS record measured values once per minute. In this case, waiting three minutes is enough to guarantee at least one valid reading.

8. When the data recording is complete, remove the EZD or press Stop to exit audit mode.

9. Close the transceiver case and fasten the clips.

10. Open the fail-safe shutter or remove the stack isolation, if necessary.

11. Exit Maintenance mode by selecting Lock on the instrument home screen.

12. Download the audit data from the site DAS. 

Step (12) is essential because it confirms that the DAS is recording the measured data correctly. Without this step, it is possible that the DAS could be configured for a different measurement range from the instrument. For example, the DAS could be configured for 0 to 50% opacity when the instrument is measuring 0 to 100%. The error would not be obvious in normal operation if the process opacity is close to zero but could cause an apparent non-compliance when the measured value increases.

Periodic audits are essential to verify the correct operation of your opacity monitor, and you should now understand how to perform a calibration audit on your opacity monitor.

If you have any questions, please contact your local AMETEK Land sales and service centre. 

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