All OPCs have a flow rate specification–most commonly listed as the flow rate set point plus or minus a percentage. For example, 50 ml/min. ± 10%. The actual flow rate varies by model.

OPCs are calibrated at a specific flow rate. This means that real-world particles are intended to pass through the particle counter at a calibrated flow rate. If the proper flow rate is not maintained, both sizing and counting accuracy are compromised. As a particle travels through an OPC, it scatters light. The amount of scattered light is converted to an electrical signal that is counted and measured to determine the particle concentration and size.

When the flow rate is set too high, particles travel through the OPC too fast. Since the transit time is decreased, the electronics do not have sufficient time to fully integrate the signal, compromising sizing accuracy. Counting accuracy is also compromised because the signal is now too small to exceed the particle size threshold. Further complicating the issue is that the accompanying software is normalizing the data to the specified flow rate.

Conversely, if the flow rate is too slow, transit time is increased and the particle appears to be larger than its actual size. It is therefore important that the OPC is operated at the correct flow rate. Table 1 shows the effects of changing the flow rate of an OPC. The specified flow rate is 50 ml/min.

Flow rate is controlled in the following ways, depending on the application and equipment used:

• Syringe samplers require the operator to input the correct syringe volume and sample speed. This results in extremely accurate flow control.
• Online applications require a flow controller downstream of the OPC set to the proper flow rate. These flow controllers must be calibrated with a stop watch and graduated cylinder. The accompanying software has no means to set the flow rate nor does it know if the OPC is being used at the correct flow rate.
• Compression samplers have a flow rate adjustment on the instrument. In this case, the software reports the actual flow rate at the end of a sample.