A floating DC loop combined with an ungrounded transmitter is the “Perfect Storm” for RTD instability.

In an industrial environment, a floating system acts like a giant sponge for electromagnetic noise. Since there is no reference to Earth, the entire loop (Power Supply → Transmitter → Sensor) can “drift” to a high potential relative to the ground. This is likely why it works initially but fails “after some time”—it takes a while for the static or common-mode voltage to build up enough to saturate the transmitter’s electronics.

Why the Duplex RTD is the “Weak Link”

In a simplex RTD, you have a single circuit. In a Duplex RTD, you have two circuits in extremely close proximity (often sharing the same ceramic mandrel or separated by only a thin layer of MgO powder).

1. Capacitive Coupling: Because your loop is floating, the two elements in the duplex sensor act like the plates of a capacitor. Noise from the process (vibration, fluid flow, nearby motors) builds up a charge between Element A and Element B.
2. The “Vanish” Point: Without a ground path at the transmitter body or the DC negative, this charge has no “drain.” Eventually, the voltage difference between the RTD leads and the transmitter’s internal ground becomes too high for the A/D converter to handle. The transmitter simply “gives up” and shows No Input.

The Fix: Stabilising the Loop

You don’t necessarily need to ground the DC Negative (some systems require it to stay floating for safety), but you must give the noise a path to escape.

1. The “Drain” Ground

Connect the transmitter body to a physical earth ground. This is the most critical step. Even if the 24V loop remains floating, the transmitter’s internal EMI filters need that body ground to dump the high-frequency noise it’s picking up from the duplex sensor.

2. Shielding Strategy (The 1-Point Rule)

If you are using shielded cable, the shield should act as a “gutter” for noise.

• Connect the shield to the ground terminal inside the transmitter.
• Do NOT connect the shield at the sensor head.
• Ensure the shield doesn’t touch the DC negative wire.

3. Check for “Sensor-to-Sheath” Leakage

When the system fails, use a multimeter to measure the resistance between the RTD leads and the metal probe sheath.

• In a floating system, if the sensor element touches the sheath (due to heat expansion or moisture), it will immediately “drag” the entire 24V loop to whatever potential the machine frame is at.
• Standard: You should see >100\,M\Omega. If you see k\Omega or low M\Omega values, the sensor is grounded, and your floating loop is no longer floating.

Why the Thermocouple (TC) didn’t fail

You mentioned the TC worked fine. Thermocouples are self-powered (they generate their own mV). Most transmitters use “Galvanic Isolation” for TC inputs because they expect them to be grounded at the tip. RTDs, however, are excited by the transmitter (the transmitter sends a small current out to the sensor).

In a floating loop, that excitation current is easily disrupted by external static, whereas the TC’s mV signal is slightly more robust against “floating” DC drift.

Your Next Steps:

1. Ground the transmitter housing to the nearest steel structure.
2. Tape off the 3 unused wires of the duplex RTD individually so they cannot touch the housing or each other.
3. Check if the 24V DC Negative is truly floating at the power supply. If your local electrical code allows it, bonding the DC Negative to Ground at the power supply source often eliminates these “ghost” errors permanently.

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