Pressure Transducer Operation; Three Sensors Available - Teledyne 4250 Installation And Operation Manual

4250 Flow Meter
Section 1 Introduction
1.4.2 Pressure Transducer
Operation
1.5 Three Sensors
Available
Standard and Extended
Range Sensors
Figure 1-2 Standard Area Velocity Probe
1-4
This frequency difference is converted by the flow meter into a
velocity reading, and the reading can indicate either forward or
reverse flow.
The pressure transducer in the area-velocity sensor contains a
resistance bridge on a silicon diaphragm. Pressure against one
side of this diaphragm causes it to flex slightly. The other side of
the bridge is referenced through a small tube to atmospheric
pressure. Flexing the chip causes the resistors on one side of the
bridge to stretch, while the resistors on the other side compress.
The result is an unbalance in the current across the bridge that
is proportional to the increase in pressure caused by a rising
level in the flow stream. This bridge is fed from a con-
stant-voltage source, so any change in its output is a result of
hydrostatic pressure against the transducer.
Teledyne Isco offers three types of AV sensor for use with the
4250. The 10-foot standard unit is intended for operation in
depths from 1 inch to 10 feet maximum. The 30-foot standard
unit is capable of operation to depths as great as 30 feet. The dif-
ference is in the sensitivity of the pressure transducer used
inside. The two are not interchangeable. You must select the
appropriate unit based on the maximum anticipated depth in
your flow stream.