A discussion on the merits of Piston versus Soap Film Calibrators

The purpose of an electronic soap film calibrator is measure the flow rate of a battery powered personal air sampler. These pumps are weak when compared to electric drive pumps. This makes the industrial hygiene pumps sensitive to any backpressure. Flow readings taken at the time of calibration must not be adding to the sampling filter backpressure when using a calibrator. The soap film has no measurable backpressure.

 A piston calibrator will have a drag placing more load on the pump. Piston calibrators continuously run not giving the pump a chance to reach a stable flow prior to the piston calibrator being removed from the air stream. Without a doubt, the worst feature of piston type calibrators is they block the air stream to allow gravity to return the piston to the bottom of the cell for the next reading. All sampling pumps have some pulsing due to the single or double diaphragm pump mechanism. Take this pulsing and add the piston blocking the air in pressure spikes and the constant flow control of the pump becomes confused on the desired flow rate.

Soap film calibrators are volumetric reading devices as opposed to a mass flow device. Their accuracy is maintained with change in altitude and temperature because the soap film passes the same fixed volume. The pumps are volumetric and the readings taken by the mini-BUCK calibrator will accuracy reflect the true (site) volume.

Water vapor correction is not required on soap film calibrators. Water vapor was in the air when the mini-BUCK calibration was set against NIST traceable standards. The air-sampling pump is collecting air with water vapor, the same air the worker is breathing. The standards TLV's, PEL's are based on the water vapor being in the total volume of air collected. Historically samples were collected and standards established with the water vapor in the volume collected. Certain industries must have "dry" gases such as the Chemical Processing Industry where the true mass of gases must be known. The principles of industrial hygiene air sampling use ambient air in their standards.  The gases may be corrected to standard temperature and barometric pressure (STP) for comparison to data collected at different altitude.

At A. P. BUCK Inc. an annual verification of calibration for quality assurance is performed in the following manner. The first step is to check the calibrator readings against an NIST traceable standard. This has been proven over the 18 years of manufacturing these devices to remain stable within the +/- 0.5% accuracy. The actual measured accuracy is reported back to the owner of the calibrator "as received" on the certificate of calibration. This allows the owner to verify the accuracy of the past year of sampling. The next step is to perform any necessary maintenance and return the instrument.  There are no sensor mechanical adjustments on the mini-BUCK calibrators. The calibration is set at time of manufacture by installing a factor within the microprocessor to match the serialized flow cell. The process is proprietary and is taught only to a certified laboratory, which has been trained and certified by A. P. BUCK Inc. 

  By: Al Buck CIH (Ret)