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Frequently Asked Questions
Calibrators
What are Primary Gas flow Calibrators?
This refers to a term used in
the industrial hygiene professions. A text book title "The
Industrial Environment - its Evaluation and Control" a
NIOSH publication, is considered a bible for personal air
sampling. In the chapter on Calibration, the type of flow
measuring devices that use a "fixed volume per unit
of time" are considered Primary gas flow devices.
These are spirometers and the soap film bubble test with
a buret. If it is not one of these two technique it is
not a Primary Gas Flow Calibrator. The volume is fixed
in the flow cell and cannot change and bubble passing up
the tube measures the time. These calibrators are accurate
at any altitude because of this technique.
These calibrators have no relationship to "Primary
Standards" as defined by National Institute of Standards
and Technology (NIST).
How Do these soap film Calibrators compare to a piston Calibrator (Soap less)
The weight of the piston, approximately
22 grams, will cause some back pressure during measurements.
This can influence the accuracy of the results. The wide
dynamic range may influence the linearity of the device.
Meaning the accuracy may vary over different rates. A flow
switching system must be incorporated to return the piston
for the next stroke. Dust sucked into the calibrator can
prove difficult maintain over the long term.
A valve switching system in the piston calibrator must be changed to
allow the piston to return for the next reading. This causes flow interrupt
against the sampling pump. This disrupts constant flow pump and causes
calibration problem.
The soap film calibrators are proven and accepted technique in the scientific
community. The flow cells for the mini-BUCK Calibrator has no moving
parts to wear and is easily cleaned with plain water. The soap film is
virtually weightless and frictionless thus linear over the dynamic range
of the cell.
The piston calibrators do give a continuous read out and can provide
much higher flow readings with large bore chambers beyond the physical
capabilities of a soap film.
Piston
calibrators for use with the industrial hygiene sampling
pumps have inherent flaws.
- The
piston must return to the bottom of the flow cell to
repeat a flow reading. To do this, an internal valve
closes, placing an insertion pressure spike into the
flow stream. (a quote from a piston calibrator manufacturer)
This blocking of flow disrupts personal air sampling
pumps.
- The
glass walls of the flow cell will condense moisture
(like a window on cold winter day in a home) and the
piston cannot move.
- Piston
drag from its weight can influence the true pump reading.
Adding more backpressure at calibration time will provide
inaccuracies to the readings.
- Piston
leakage is always a concern, but is not a factor with
a soap film. Self-test must be built into piston calibrators
to check against this failure.
Is there consistency in filter cassettes from box to box or cassette to cassette ?
The variations in filter material is great. The technique of using a sample or "dummy" filter to calibrate the pumps air flow can lead to errors of 10 to 25%. Because the actual filter used was not measured for flow, this could cause it to be void in a Court of Law. The porosity of the filter material is a lower priority for the manufacturer than the purity and pore size precision. Hence, filter manufacturers don't grade the filter for flow (back pressure).