DELTA M Corporation
3242-OM-04 3/27/02
5
0
HYDROCARBONS
WATER
0.01
0.1
1.0
5.0
1.0
10.0
100.0
500
VELOCITY (FT./SEC.)
DRAWING FILE: MTF2B0.DWG
SI
GN
AL (
m
V)
SENSOR
SIGNAL
AIR
LIQUID (F/S)
(MM/S)
GAS (F/S)
(MM/S)
0.001
0.305
0.10
3.05
3.05
30.5
30.5
304.8
304.8
3048
1524
15,240
MAXIMUM
RANGE
2.2 Flow
Switching
Most mass flow monitoring techniques calculate mass indirectly by measuring volumetric flow
such as gallons per minute or cubic cm per second, then either measure density separately or
calculate it from temperature measurements of the fluid and, finally, combine density and
volumetric flow to obtain mass flow. The DELTA M thermal-differential technique is one of two
methods that directly measure the mass flow. For ease of comparison most flow applications
are presented in terms of velocity which is independent of the flow cross sectional area (i.e.
feet per second (FPS)). The true mass flow equivalent would be FPS multiplied by density but
for simplicity FPS is used and density effects are ignored. This is normally not critical for flow
switching applications.
When the sensor is inserted into a liquid or gas the heated RTD is strongly affected by the
velocity of the medium. Flow past the heated RTD changes the heat transferred from the
surface of the sensor. This cooling effect reduces the temperature of the sensor. The
microtuf
®
compares this change to a preset flow trip point to switch the output. Figure 2B
shows the model FS4200 signal change vs. flow rate for air, light hydrocarbon liquids, and
water. The signal change vs velocity has the same general shape for all three media but the
change is larger for air and the sensitive range is different for each. For air and most
gaseous media the range is 0.1 to 500 feet per second (FPS). For most liquid media the
range is 0.01 to 5 FPS. Appendices in section 9.0 contain flow conversion information to
facilitate conversion from various units and pipe dimensions into flow velocity in feet per
second.
FIGURE 2B microtuf® MODEL FS4200 FLOW RESPONSE FOR THREE MEDIA AND
CALIBRATION SETTINGS FOR EACH
