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Section 1 Model 752 and 752A Differential Pressure Transmitters
Electronic Transmitter
The electronic transmitter supplies a 4-20 mA or 10-50 mA direct current out-
put signal that is proportional to the differential pressure sensed by the DPU.
The output signal is transmitted over a two-wire transmission line to remote
receiving devices.
Power Supply
A regulated direct current (DC) power supply is required to operate the trans-
mitting loop. The voltage required will depend on the total loop resistance
(load resistor, cable wiring, and any other resistance in the loop) as shown in
Figure 3.3, page 16. Table 3.1, page 16 shows the resistances in ohms per 1000
feet of wiring for the various cable wire sizes. Once the total loop resistance
has been determined, the power supply voltage can be calculated as follows:
• For 4-20 mA output: VDC = 12 VDC + 2 VDC per 100-ohms load
• For 10-50 mA output: VDC = 12 VDC + 5 VDC per 100-ohms load
Exercise care when calculating the power supply voltage. A power supply
specied at 50 VDC ±1 volt must be considered a 49 VDC source to insure
the minimum required voltage at the transmitter. Use the actual value when
available. Otherwise, use the "worst case" value.
For power supply wiring instructions, refer to the electrical connections
shown in Figures 3.1 and 3.2, page 15.
Zero and Span Control
The transmitter has two 10-turn potentiometers—one for zero adjustments,
the other for span control. With these two controls, measurement can be made
between any two points within the rated transmitter span. However, to ensure
a high level of accuracy, combined zero and span adjustments should never
exceed ±5% of the factory calibration.
IMPORTANT Combined zero and/or span eld adjustments exceeding ±5% of the fac-
tory calibration can alter transmitter performance in direct proportion to
the changes to the factory calibration. For example, if combined adjust-
ments to zero and span change the factory calibration by a factor of 2,
transmitter performance may be decreased by a factor of 2.
Zero Control
During calibration, the zero control is used to adjust the instrument’s output
signal to 4 mA or 10 mA at the minimum pressure setting of the instrument.
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