Fluke Biomedical VT Mobile - Manual (Page 55)

6-1
Chapter 6
Measuring Flow and Volume
Introduction
The Analyzer makes flow measurements by measuring the differential pressure across a
fixed orifice flow sensor. The resulting differential pressure is nearly linear with respect
to the volumetric flow. The Analyzer applies simple corrections to correct for any non-
linearities.
Also, review “Making a Measurement More Accurate” in Chapter 3 for a discussion of
the uses of the Oxygen Sensor, Temperature and Relative Humidity Sensor, and
barometric pressure readings in enhancing measurement accuracy. Chapter 3 also
discusses proper warm-up and zeroing procedures to compensate for sensor drift.
Gas Settings
The flow measurement sensors in the Analyzer are linearly dependent on the viscosity of
the applied gas. The Analyzer setup options allow you to specify the type of gas in use.
Additional choices are available using the User Defined gas settings available with the
VT for Windows software. The gas calculator in the VT for Windows software can
determine the effective viscosity for a combination of various gases.
Use of the Temperature and Relative Humidity Sensor increases flow and volume
accuracy. The Analyzer calculates the temperature dependent viscosity for the gas type
selected (except for user-defined gases). Therefore, when the sensor measures a new
flowing gas temperature, the Analyzer calculates the new viscosity. The Analyzer
viscosity calculation for the gas settings assumes the use of dry gas. Increased water
vapor content can slightly affect the flow accuracy.
Note
Never apply humidified gases to the Analyzer flow sensors. Condensation
on the flow sensors causes errors in the flow readings until you dry
condensation from the flow sensors.
Balance Gas Settings
You can use gas combinations with oxygen. For example, the Analyzer can be set to use
Helium balance Oxygen (Heliox.) In this mode, the Analyzer uses the oxygen
measurement to determine the oxygen concentration and then assumes the balance of the
gas concentration is helium. The Analyzer calculates the effective viscosity for the
combination gas at the measured temperature. The oxygen concentration used for the
balance gases updates automatically.