Section 1
Analysis of Air Flow Calibration
a. Physics of Measurements:
At room temperature, all gases obey - to a very close approximation - the
ideal gas equation PV = nRT, where P is the pressure in newtons per square
meter, V is the volume in cubic meters, n is the number of moles of gas,
R is the empirically determined gas constant with the approximate value
8.31 joules/mole degrees Kelvin and T is the temperature in degrees Kelvin.
The method of measuring flow rates in the DigiCal Calibrator is the
technique known as the soap film bubble test. A frictionless soap film is
suspended perpendicular to the air flow up a small bore tube. This film
forms a complete seal across the tube. The effect causes the soap film to
move along the tube at exactly the same rate as the air flow. Knowing a
measured distance and tube bore size, a volume can be calculated. Thus, a
flow rate can be determined by the movement of soap film across a fixed
volume per unit of time. This technique is classified as a Primary Standard.
1
By comparing the four variables of the Ideal Gas Law against this detection
technique, the validity of measurement can be accessed. The volume (v) is
known and fixed therefore cannot change or be considered variable. The
mass (m) of the gas being measured is not changed in this technique as the
soap film is simply suspended across the tube cross section and moves with
the gas flow rate. Mass is not affected. This leaves pressure and
temperature. These are expressed by Boyle's Law P
1
V
1
= P
2
V
2
and Charles
Law V
2
/ V
1
= T
2
/ T
1
. No pressure changes from the ambient during a test
as the soap film is practically frictionless. Temperature has no influence in
this type of flow measurement when all elements are at an ambient. This
includes the Calibrating device, the flow of gas and room temperature.
These conditions are the general circumstances in which tests are performed.
Conclusion: The detection method of measuring flow rates of gases over a
fixed volume per given unit of time is for all practical purposes independent
of all variables in the Ideal Gas Law. Thus, the DigiCal Calibrator serves as
a primary standard calibration method.
1
Lippman, Morton, "The Industrial Environment - its Evaluation and
Control," [U.S. Department of Health, Education and Welfare], NIOSH,
1973, Ch 11, 101 pp
2
11
Section 9
Parts List
A/C adapter charger 120V
230V
655112
655168
Soap solution
(8 oz. bottle of soap)
600920
Instruction manual 655103
Air inlet caps (Pkg. of
two)
655102
Calibrator battery pack 655169
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