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Checking your real-time instrument

HSE Health Group maintains around 40 hand-held and benchtop aerosol instruments, which provide the capability to measure mass, number and surface area concentrations, from nm to µm.

The accurate, robust performance of real-time mass or number concentration instruments is dependent on regular maintenance and calibration. However, the performance can deteriorate within the manufacturer recommended calibration period, particularly if the instrument has been exposed to particulates in an environment of high concentration, or exposed for extended periods. In addition, low cost sensor devices are so inexpensive that service or calibration is not supported by the manufacturer. Therefore, it is even more important for those devices to have a procedure/methodology in place to check for performance drift.

As part of a wider aim to develop a system to monitor, record and maintain direct reading time-resolved instrumentation, a benchmarking method for checking the performance of such instruments using a calm air chamber, was also an objective of the work.

Instruments were simultaneously benchmarked against a calibrated reference instrument, within a calm air dust chamber, at a wide range of concentrations of a suitable test aerosol.

Real-time Calibration

Photometers and Optical Particle Counters (OPC) were benchmarked, using SPHERIGLASS ® A-5000 as the test aerosol, against a reference photometer or OPC, not necessarily of the same model or size range characteristic.

For Condensation Particle Counters (CPC) and Diffusion Chargers, measuring in the nanometer size range, the test aerosol was a NaCl solution with a mode at approximately 40 nm, generated using a Collison nebulizer. A calibrated CPC was used as a reference instrument.

For each photometer model, an average ratio of the evaluated photometer concentrations to the reference photometer concentration was calculated. A linearity curve was established to evaluate photometer concentration versus reference photometer concentration.

The concentrations measured with the CPCs all showed results within 9% of the reference CPC, and with the diffusion chargers the deviation from the reference CPC was less than 19%.

The benchmarking method is a useful test as part of a laboratory procedure checking of real-time instruments.



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