| TECH-TIP: Online Instrument Verification
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TECH-TIP: Online Instrument Verification

Continuously monitoring instrumentation is an important part of water treatment plant and distribution system operation and control.  Online instrumentation provides real-time information about water quality and treatment conditions, which may be used for monitoring, data trending, and process control purposes.  Water utility staff routinely rely on the data from online instrumentation, so it is important that the data produced by these instruments is as accurate as possible.  One way that operators can help to ensure data accuracy is through routine verification of online instrumentation. 

 

A common way to do this is by comparison of the online instrument results with a grab sample that is then typically analyzed using a laboratory or handheld instrument.  Grab sample comparisons generally can be easily run at any time, allowing utilities to set a regular schedule for performing this verification procedure.  For example, grab samples to verify the accuracy of online turbidity analyzers may be run on a weekly basis, between quarterly (or monthly) primary standard calibrations.  Grab sampling is also easy enough to conduct that it may be used as a component of instrument troubleshooting processes.

 

The exact procedures for conducing grab sample comparisons will vary somewhat, based on the parameter that is being tested and its specific application.  It is recommended that utilities develop standard operating procedures (SOPs) for the grab sample verification procedures that are most commonly used at their facilities.  However, the following general steps apply to the majority of grab sample comparison procedures:

 

  • Collect a sample that is representative of the sample being measured by the online instrument. 
    • This typically means that the sample should be collected in close proximity to the instrument itself, possibly even from the sample inlet line, if a sample can be collected without disrupting flow to the instrument.  If samples are collected a significant distance from the analyzer’s sampling point (such as from a laboratory sample tap), changes in sample concentration may occur that can impact the accuracy of the comparison sample results.  
    •  Be sure to note the instrument’s reading at the time the grab sample is collected.  This may also be a good time to verify that the local reading on the instrument screen matches the SCADA reading.  If the readings do not match, it may be necessary to adjust the scaling of the instrument’s 4-20mA output – either at the instrument controller or in the SCADA system.
  • Measure the parameter of interest using a handheld or laboratory instrument. 
    • Be sure to follow the correct sampling and analysis procedures for the parameter being testing.  For example, for chlorine samples, the grab sample should be collected in glass containers (preferably in the instrument’s sample cell itself if a colorimetric analysis method is used) and analyzed as soon as possible to avoid chlorine loss which can cause a negative error in grab sample results. 
    • Ensure that you are comparing “apples to apples” – so if an online analyzer is measuring total chlorine, a total chlorine test procedure should also be used to test the grab sample.  Be aware of any potential interferences that may impact online analyzer or grab sample testing results. 
    • When analyzing a filter effluent sample for low-level turbidity, pay close attention to measurement technique to ensure the accuracy of the grab sample analysis.  Remember that anything that can potentially scatter light, such as fingerprints, air bubbles, or scratches on a sample cell, can interfere positively with a turbidity reading. 
  • Record the results obtained from the laboratory analysis and compare them with the online analyzer readings. 
    • Acceptable comparisons will vary between different utilities and from parameter to parameter.  As a general rule, results should typically agree to within about 10-15%.  For samples with relatively low numerical values, such as turbidity that are less than 0.10 NTU, consider an acceptable agreement of a given comparison to be within a specific numerical range, such as +/- 0.03-0.05 NTU. 
    • Some states may have specific regulatory requirements regarding the frequency of grab sample comparisons and the guidelines for acceptable comparison measurements.  It is important to follow any state or local regulatory requirements for grab sample instrument verification procedures.
  • If the sample results are within the given tolerances for the comparison – congratulations, you are done!

 

However, grab sample results do not always end up within the acceptable ranges for a specific comparison measurement.  It is important that utility SOPs provide guidance regarding steps to take when a grab sample result does not compare favorably with the online instrument reading.  Some initial steps to take in this situation include the following:

 

  • Resample
    • Before making any changes, proceed with the relatively simple step of collecting a repeat sample and running the test one or two more times.  The discrepancy may have been the result of error in the sample collection and/or analysis technique.  However, multiple grab sample results that do not compare favorably with online instrument readings indicate the need for additional troubleshooting.
  • Verify
    • If the repeat readings continue to indicate a discrepancy, it is recommended to verify the accuracy of the instrument used to analyze the grab sample.  Standards are available to verify the accuracy of many instruments and parameters. 
    • Secondary standards, such as gel based standards, may be used to verify the accuracy of the accuracy of the instrument reading for many common parameters, such as turbidity, chlorine, and fluoride.  Follow the manufacturer’s instructions for proper use of these standards.
    • Primary standards may also be used to verify accuracy.  Primary standards are typically analyzed in the same manner as a sample and help to verify the accuracy of the analytical system – which includes the analyst technique, instrument, and reagents.
    • This verification step will either confirm the accuracy of the grab sample analysis or identify errors that need to be addressed prior to proceeding.  If the grab sample verification is not successful, continue to troubleshoot the laboratory measurement until the issue has been resolved (or repeat the test with a backup instrument if one is available).  Don’t forget to check the expiration date for the reagents in use.
  • Troubleshoot/Make Additional Corrections
    • If verification of the grab sample measurement method was successful, it may be time to turn operator attention back to the online instrument to begin troubleshooting.  Some considerations during the troubleshooting process:
      • When was the instrument last calibrated?
      • Has the instrument been properly maintained?  Check instrument maintenance logs to confirm that the instrument has been maintained as required.  This includes steps such as cleaning, changing tubing, replacing reagents, and replacing light bulbs/light sources.  Maintenance should be performed according to the manufacturer’s instructions.
      • Check the instrument’s flow rate to confirm that it is within the optimal range specified for the instrument.  Flow rate can have a dramatic impact on the accuracy of analytical results.
      • Consider verification of the online analyzer reading with a primary standard, if this is an option for the instrument/parameter in use.
    • If recalibration is required, carefully follow the manufacturer’s instructions for instrument calibration.  Consider repeating the grab sample verification process once recalibration has been completed.
Consider developing a regular schedule and SOP for the verification of online instrument readings to help ensure continued accuracy of these critical analyses.  Refer to Self-Assessment for Water Treatment Plant Optimization, the Partnership’s new water treatment plant self-assessment guidance for self-assessment questions relating to online instrumentation and SCADA.  Updated guidance for this topic is included in Chapter 6 of the book – Application of Operational Concepts – which also includes newly updated and expanded self-assessment material for process control testing, operator application of concepts, and communication.  All of these topics are included to assist water treatment plants with collecting the accurate data that plays an important role in monitoring, troubleshooting, and process control.

 

Article contributed by Barb Martin, Partnership Senior Manager
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