Troubleshooting for liquid particle counting

How to ensure that quality data is being obtained from optical particle counters (OPCs) in liquid applications

By Roger Carlone, Particle Measuring Systems

Ensuring that your liquid optical particle counter (OPC) is operating within its designed specification is critical to obtaining quality data. As data from OPCs becomes increasingly important to maintaining process control, it becomes more critical than ever for users to have confidence in the data being generated. OPC data is frequently used in qualifying process improvements, in control during chemical manufacturing, and in quality control for production of complex parts. It is not uncommon for users to question the results from their particle counters. Discussed in this article are common misunderstandings and other performance issues that would lead one to question their OPCs.


Users can take several easy steps to ensure that quality data is collected. All OPCs have defined and easily controlled specifications. These specifications are flow rate, background scatter, and concentration limit. Also important is the particle size distribution (PSD). Unless all of these parameters are correct, the data should be considered suspect. Even if one parameter is incorrect, the data quality has been compromised. This article summarizes these parameters and their effects on each other, and it also covers ultimately ensuring that quality data is achieved when analyzing ambient particle distributions in process chemicals and deionized (DI) water.

Normal particle distributions

Thirty years of particle counting experience shows that most ambient particle distributions in continuously filtered liquid systems follow a D-3 distribution (D = diameter) on total cumulative counts. Figure 1 shows a normal particle distribution. In extremely clean DI water systems, the distribution can be steeper (D-4), and in dirty systems, the distribution can be flatter (D-2). It is equally important that whatever the distribution is, it remains consistent from sample to sample.

Click here to enlarge image

When analyzing particle data, it is essential to ensure that you have a normal particle distribution. If not, it should immediately alert the user to a potential problem. Things that can lead to abnormal distributions or poor data are incorrect flow rate, excessive background scatter, and exceeding the OPC’s upper concentration limit. In addition, real particle events such as pump or valve failures, o-ring problems, damaged tubing, or other process upsets can cause abnormal particle distributions. This is why it is critical to ensure that quality OPC data is collected. Otherwise, one would not be able to identify a real particle excursion from a poorly performing OPC.

Flow rate

All OPCs have a flow rate specification. The specification is most
commonly listed as the set flow rate plus or minus a certain percentage; for example, 50 ml/min


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