Cameron BARTON 764 - Section 3: Loop Resistance; Loop Resistance Calculations Method

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Section 3 Model 764 Differential Pressure Transmitter
Minimum resistance is based upon maximum transmitter power dissipation
(Watts) at the maximum design temperature conditions, and qualied life limited
by heat rise. For proper operation at post-accident conditions, this minimum
resistance must exist.
Figure 3.2—Power supply and loop resistance
Load and Line Resistance Calculations
Use the following method to calculate values of load and line resistance:
Total Loop Resistance (R
T
) = R
Line
+ R
Load
+ R
Ext
Power Supply Voltage = V
DC
(53 V max. for 4-20 mA or 10-50 mA Systems)
Minimum no load Transmitter Voltage = T
VDC
(15 V for both 4-20 mA and
10-50 mA Systems)
Maximum no load Transmitter Voltage = T
VDC
(47.2 V for 4-20 mA and 19 V
for 10-50 mA Systems)
Transmitter Current = I
DC
(20 mA or 50mA)
R
T =
T
VDC -
T
VDC
I
DC
Example 1 : (Maximum loop resistance for 10-50 mA system):
V
DC
= 53 Vdc T
VDC
= 15 Vdc
I
DC
= 50 mA R
T
= (53-15) / 0.05 = 760 Ohms