Loop That Reads Low Only at High Output
A two-wire 4-20 mA pressure transmitter on a new fuel-gas line has a datasheet range of 0-100 psig and a minimum terminal voltage of 12 VDC. The cable run is 250 m of 1.5 mm2 copper, one way, with one field junction box mid-run, landed on a DCS analog input card with a 250 ohm sense resistor. Loop power is a dedicated 24 VDC supply shared with four other transmitters. The DCS has been reading roughly 7 psig low at high pressure and correctly at low pressure. Today the line is at 80 psig: the transmitter's local display reads 80.0 psig, the DCS reads 73.0 psig, and a series mA meter at the junction box reads 15.7 mA. A calibrated DMM shows 24.0 VDC at the panel supply and 11.9 VDC at the transmitter terminals.
1. Calculate the total loop resistance and the expected resistance. What single fault signature do the numbers reveal?
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Confirm the current is consistent. DCS 73 psig on a 0-100 psig span corresponds to 4 + (73/100 x 16) = 15.68 mA, matching the 15.7 mA measured in series. So the loop current itself is genuinely low - the transmitter is not delivering the 16.8 mA that 80 psig requires.
Compute total loop resistance from the measured voltages:
R_total = (V_supply - V_transmitter) / I = (24.0 - 11.9) / 0.0157 = 12.1 / 0.0157 = 771 ohm
Compute what it should be:
- Sense resistor: 250 ohm
- Copper conductor: R = 0.0172 x (2 x 250 m) / 1.5 mm2 = 5.7 ohm
- Terminations, allowance: ~2 ohm
- Expected total: ~258 ohm
Excess: 771 - 258 = ~513 ohm.
Signature: the transmitter terminal voltage is 11.9 V, right at the 12 V minimum. The transmitter cannot regulate; it clamps near its compliance limit and the loop current falls below the commanded value. At low pressure the current is low, the IR drop is small, and terminal voltage is adequate, so the reading is correct. At high pressure the drop grows, the voltage collapses, and the DCS reads low. The tell is the split between the local display (80.0 psig - the sensor is fine) and the loop current (wrong). This is a voltage-compliance problem, not a sensor or calibration problem.
2. Where do you look for the extra resistance, and what measurement sequence proves the location?
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Sequence (under a valid work permit; avoid breaking the loop live where possible):
- Measure the DC voltage across each segment while the loop is at a steady current: supply to panel terminal block, panel terminal block to field junction box, field junction box to DCS card, and across the 250 ohm sense resistor.
- Convert each reading to resistance (R = V/I). The segment that accounts for hundreds of ohms is the fault.
- Alternatively, with the loop de-energized and the transmitter and resistor disconnected, measure conductor-to-conductor resistance end to end and compare against the calculated 5.7 ohm.
Most likely findings given ~513 ohm of excess:
- A second 250 ohm resistor left installed in the field junction box or marshalling panel from a retired chart recorder, plus a corroded or loose termination contributing the remainder.
- A corroded splice, a terminal landed on an oxidized screw, or moisture in the junction box.
- A damaged conductor (partially broken strands) after the cable reroute.
What to rule out: a failing transmitter. Confirm by noting that the transmitter's local PV is correct and that the excess drop appears in the wiring segments, not across the transmitter. A bad transmitter would normally show a wrong PV locally as well.
Record the as-found voltage drops and the measured segment resistances in the loop folder before repair.
3. What is the correct permanent repair, and evaluate the two shortcuts: raising the supply to 30 VDC, or replacing the 250 ohm resistor with 100 ohm.
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Permanent repair: remove the parasitic resistance. Delete the extra resistor, re-terminate the corroded joint with proper lugs, dry and reseal the junction box, and re-measure the loop resistance back to ~258 ohm.
Verify at the top of the range: force 20 mA (or use the process at high pressure) and confirm the transmitter terminal voltage is comfortably above 12 V - target 16 V or more for margin.
Compliance check: V_supply(minimum) = V_transmitter(minimum) + I(max) x R_loop = 12 + 0.021 x 258 = 17.4 V. A 24 VDC supply with the correct loop resistance has ample margin.
Shortcut 1 - raise the supply to 30 VDC: legal only if it stays within the transmitter maximum (typically 42 VDC) and the supply and card ratings. It is acceptable as a temporary measure to restore indication, but it does not fix a bad joint. That joint will keep degrading, can heat, and can fail open. It also increases power dissipation in the sense resistor.
Shortcut 2 - drop the sense resistor to 100 ohm: this reduces the card's signal voltage and degrades noise immunity over a 250 m run; the AI card may also be specified for 250 ohm. It also masks the real fault. Not acceptable.
Close out: re-run the loop check at 4/12/20 mA from the marshalling cabinet, confirm the DCS reads 0/50/100%, and update the loop drawing to reflect the removal of the retired resistor.