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Model 763A Gage Pressure Transmitter
Section 2
Section 2—Theory of Operation
Basic Components
Gage Pressure Sensing Element
The C-type Bourdon tube is an arc-shaped metal tube that is elliptical in
cross-section and sealed at one end. The opposite end, which is open, is at-
tached to the pressure source. Pressure applied inside the tube affects its shape
and actuates the sensing element.
Electronic Transmitter
The 763A uses a wire (larger gage than that used in the Model 763 transmit-
ter) that is welded to the Bourdon tube and cantilever beam. Increased inter-
nal pressure applied to the Bourdon tube tends to straighten the tube, which in
turn bends the cantilever beam proportionally. Motion of the free end of the
beam applies tension to one gage, increasing its resistance, and compression
to the other, decreasing its resistance. The two gages are connected to form
two active arms of a bridge circuit. The bridge output signal is conditioned
and converted to a 4-20 mA or 10-50 mA output signal by the transmitter
electronics.
Basic Operation
The electronic transmitter is basically a loop current regulating device, where
loop current is controlled by an input of mechanical force or motion. Figure
2.1,
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the electrical currents. As shown, the transmitter, power supply, and load (line
plus receiving device) are connected in series.
The current from the power supply enters the transmitter, passes through the
reverse polarity protecting diode, then divides into two separate paths. The
The remainder of the current passes through the electronic regulator, where it
divides into two paths, then passes through the bridge circuit and the voltage
through the load and back to the power supply.
Reverse Polarity Protection
Reverse input polarity protection is provided by the forward-conducting
diode. In the event the polarity of the input is reversed, the diode blocks the
input and prevents the reversed input power from damaging the electronic
circuit components.
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