User Manual
6
To help calculate the viscosity of air at flowing temperature, a viscosity equation based on temperature is
included in this instruction manual (see Table A-32422). The equation is from “tables of Thermodynamic
and transport Properties of Air, Argon, Carbon Dioxide, Oxygen, and Steam.” All other gas viscosity
equations are obtained from "Physical and Thermodynamic Properties of Pure Chemicals" Table A-31986
lists the values of
/ µf for air from 50-159ºF at 1º intervals when the standard temperature is 70°F.
*Note: If you are flowing wet air, a humidity correction factor for viscosity must be used. The difference
between wet-air viscosity (µwet and dry-air viscosity (µdry) increases with temperature and humidity, at
80ºF and 80% relative humidity, the ratio of µwet / µdry is .997.
Figure 2 is a graph of the ratio µwet / µdry of air from 50 to 150ºF and from 10 to 90% relative humidity.
The viscosity of the flowing wet air becomes the value from the dry-air viscosity equation times the ratio
µwet / µdry
µf = µwet air =
x
Where T = ºK
The curve/table may have DP units
of
1) inches of water column (WC)
2) centimeters of WC
3) millimeters of WC
4)
pascals
5)
kilopascals
Whenever a pressure is expressed in
units of water,
the
water temperature reference
must be given.
The
calibration
curve uses 4°C for the water
temperature
reference. Some
devices
use a water
temperature reference of 20ºC and others may use other
temperature
references. If no temperature
reference is given on the DP instrument or in
its instruction manual, consult the manufacturer. If the
DP measuring device has a water
temperature reference
other than 4º, correct the
DP
reading by using
the following equation:
DP @ 4ºC = DP (device) x
Water
Temperature
Water Density (lbs/ft³)
4.0°C (39.2°F) 62.426
20.0°C (68.0°F) 62.316
15.5°C (60.0°F) 62.366
21.1°C (70.0°F) 62.301
The DP @ 4ºC value should be used with the curve/table.
Standard Volumetric Flow
Rate
The word "standard" when associated with flow
rate,
means the flow rate has been normalized to an
assigned
standard
pressure
and
temperature.
If standard
volu
metric flow rate is desired, the actual
volumetric flow rate is multiplied by the ratios.
and