ABB XSeriesG4 User Manual page 114

Again, the 4-20 ma transmitter must be fully operational at voltages as low as VCC – 5 Vdc. VCC is the source voltage available
at the site and 5 Vdc represents the voltage dropped across the meter's 250 Ohm resistor at maximum loop current (20 mA).
Totalflow can provide a 12 to 24 Vdc converter (part # 2100358-001). This converter can be powered from the meter's VBATT.
The converter's 24 Vdc output can then be used to power the 4-2 mA transmitter loop. In other words; VCC (Figure 1-1) would
now be 24 Vdc. Subtracting the 5 Volts dropped across the 250 Ohm resistor would leave 19 Vdc to power the transmitter.
NOTE: Another possible solution would be to reduce the 250 Ohm resistor to a smaller value, possibly 125 Ohms.
An under-powered transmitter could more easily drive 20 mA into the lesser resistance. Field calibrating the AI
would negate any differences between the 250 Ohm and the 125 Ohm resistors. However, as the size of the
resistor is reduced, the overall resolution is diminished.
A.2 Example 1
If the 4-20 mA transmitter can drive full range (20 mA through the 250 Ohm resistor) a 4-20 mA current range has been
converted into a 1-5 Vdc voltage range. The analog to digital (AD) converter used in Totalflow equipment expects a 1-5 Vdc
signal. During field calibration, we might have the 4 mA of loop current represent 10 bbls. of fluid per hour and 20 mA represent
100 bbls. The conversion to voltage (using the 250 Ohm resistor) would be as follows:
1 Vdc = 10 bbls/hour
5 Vdc = 100 bbls/hour
Assuming that the AD converter is ranged between 0 and 5 Vdc, and that the converter is resolving 15 bits, expect to resolve
about 150 μVdc (or .0006 mA). These numbers translate to about .003 bbl (3 thousandths of a barrel, or about one half fluid
ounce).
Flowmeter
AI +
To A/D Converter
A
AI -
Flowmeter
AI +
To A/D Converter
1
3
Onboard 250Ω
resistor selected
using three-pin
250Ω
jumper
AI -
To PCCU Control Flowmeter
AI +
To A/D Converter
250Ω
AI -
External to
flowmeter
2-wire (line powered)
4-20ma transmitter
User supplies 250Ω
resistor and wires it
250Ω
across the AI input
pins
External to
flowmeter
2-wire (line powered)
4-20ma transmitter
B
External to
VCC
flowmeter
3-wire 4-20ma transmitter
Onboard 250Ω resistor is
selected in PCCU setup
C
102
Appendix A 4–20 mA transmitters
VCC
VCC