Setra System Model 256 - Manual (Page 6)

3.2 Current Output Units
The 4-20 mA current output units are designed to have current flow in one
direction only - PLEASE OBSERVE POLARITY.
We suggest that the electrical conduit shield be connected to the system’s loop
circuit ground to improve electrical noise rejection.
The Model 256 is a two-wire loop-powered 4 to 20mA current output unit and
delivers rated current into any external load of 0-800 ohms. (See Diagram 2 for
location of +EXC and -EXC current output screw terminals.) The current flows
into the + terminal and returns back to the power supply through the
- terminal. (The center GND terminal may be used for shielding.) The power
supply must be a DC voltage source with a voltage range between 9 and 30
measured between the + and - terminals. The unit is calibrated at the factory
with a 24 VDC loop supply voltage and a 250 ohm load.
Minimum Supply Voltage (VDC) = 9 + 0.02 x (resistance of receiver plus line).
Maximum Supply Voltage (VDC) = 30 + 0.004 x (resistance of receiver plus line).
4.0 CALIBRATION
The 256 transducer is factory calibrated and should require no field
adjustment. Whenever possible, any zero and/or span offsets should be
corrected by software adjustment in the user’s control system. However, both
zero and span adjustments are accessible by removing the screw top cover and
the adjustment access cover and turning the potentiometer screw inside. (See
Diagram 1 for the location of the Zero and Span potentiometers.)
4.1 Voltage Output Zero Adjustment
While monitoring the voltage between the positive output (+OUT) and
negative output (– OUT), and with the pressure port open to atmosphere, or
with zero pressure applied, the zero may be adjusted by turning the zero
potentiometer screw. The factory setting is 0.1 VDC (
±25mV).
Note: –OUT and –EXC are commoned on the circuit.
4.2 Voltage Output Span Adjustment (Complete the zero adjust-
ment before setting span.)
Span or full scale output adjustments should only be performed by using an
accurate pressure standard (electronic manometer, digital pressure gage, etc.),
with at least comparable accuracy to the 256 transducer. With full
range pressure applied to the pressure port, the span may be adjusted by
turning the span potentiometer screw. The factory setting is 5.1 VDC (
±50mV).
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