Meriam Laminar Flow Elements - Manual (Page 6)

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
μstd / 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 = ቆ
14.58 + ( ܶ )32
(110.4 + ܶ)
ቇ x
ቀ
ஜ୵ୣ୲
ஜୢ୰୷
ቁ
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
ୢୣ୬ୱ୧୲୷ ୭୤ ୌଶ୓@୲ୣ୫୮ୣ୰ୟ୲୳୰ୣ ୲ୣ୫୮ୣ୰ୟ୲୳୰ୣ ୰ୣ୤.
ୢୣ୬ୱ୧୲୷ ୭୤ ୌଶ୓@ ସ°େ
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 volumetric flow rate is desired, the actual
volumetric flow rate is multiplied by the ratios.
ୗ୲ୟ୬ୢୟ୰ୢ ୲ୣ୫୮ୣ୰ୟ୲୳୰ୣ (୘ୱ୲ୢ)
୤୪୭୵୧୬୥ ୲ୣ୫୮ୣ୰ୟ୲୳୰ୣ (୘୤)
and
୤୪୭୵୧୬୥ ୮୰ୣୱୱ୳୰ୣ (୔୤)
ୱ୲ୟ୬ୢୟ୰ୢ ୮୰ୣୱୱ୳୰ୣ (୔ୱ୲ୢ)