Why does the signal still drift after replacing the pressure transmitter? You might have overlooked the real cause.

Why does the signal still drift after replacing the pressure transmitter? You might have overlooked the real cause.

In the automation industry, there is a problem that is more annoying than tripping, more concealed than alarms, and more frustrating than false actions - that is signal drift. one

The essence of signal drift

Most faults of gauge pressure transmitter occur suddenly: they either break down, jump, or trigger alarms.

But drifting is not.

It's not like a malfunction but more like a "gradual change" - today it's off by a few milliamps, a week later it's off by a few percentage points, a month later the control is inaccurate, and after half a year the trend chart is already incomprehensible.

The reason why drift is difficult to detect is that it is almost never a problem with a single capacitance type pressure transmitter, but rather a gradual, minor change at one point, several points, or even the entire signal chain.

This link includes:

Instrument → Transmitter → Cable → Terminal → Isolator → PLC analog input → Power supply → Grounding → Field environment → EMC interference

If any link deteriorates even a little bit, the accumulation will lead to a drift.

This is precisely where it gets "creepy".

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Why does signal drift occur?

Although signal drift manifests in various ways, its root causes can be roughly classified into several categories.

The "zero point inconsistency" caused by ground potential difference

This is the most typical and hardest-to-think-of drift source.

There is no common zero point between the PLC end and the pressure transmitter 4-20mA end, and they are not on the same reference ground. As long as the potential difference is several tens of millivolts, the analog quantity starts to deviate. The older the factory building, the more serious this problem becomes.

It is usually manifested as the value fluctuating up and down when starting and stopping the equipment. The further the long-distance wiring, the greater the drift. Or it becomes stable immediately after adding an isolator. If you encounter a drift that "feels abnormal", it is basically this.

2. "Signal noise superposition" caused by electromagnetic interference

In environments with strong interference (such as areas with high-power frequency converters, frequent start-stop of motors, concentrated welding machines, or high-power switching power supplies), analog signals are prone to interference coupling. It is not that they "break down due to sudden changes", but rather they are continuously affected.

Signal trends often feature fine serrations, occasional "tremors", or significant irregularities in small variables. EMI is ubiquitous in factories and does not cause direct failures, but it makes signals "dirty". 40% of drifts in engineering are related to EMI.

3. "Attenuation and Poor Contact" Caused by Cables and Terminals

It is quite normal for old factories to not change their lines for over a decade. However, the cable cores may get damp and age, the insulation may deteriorate, the terminals may oxidize, the shielding layer may break, and the impedance may increase, all of which can cause the analog signal of the pressure transmitter HART to drift slowly.

This is the most easily overlooked yet most common problem.

4. Terminal loosening and insufficient wire pressure often occur.

The screws on the pressure transmitter were not tightened properly, and the terminals became loose due to vibration or oxidation at the contact points. Many engineers fail to identify such issues because they think they are "too simple".

5. "Unstable circuit" caused by power supply changes

Large power supply ripple, load changes, and multiple  air differential pressure transmitter connected to the same power supply can all cause slow drift in the analog signal, but the pressure transmitters themselves are fine?

Be sure to suspect the 24V power supply. Full load, power supply ripple, momentary voltage drop, shared power supply lines... all can cause unstable circuits.

6. The Impact of Temperature on Signal Drift

Temperature is actually the most "invisible" factor contributing to signal drift. Whether it's the pressure core, RTD, thermocouple, cables and terminals, or internal crystal structures, they are all materials that will slightly change with temperature. As the temperature rises and falls, their resistance, sensitivity, and deformation will all shift a little. These changes are not obvious in daily life, but under the repeated fluctuations of tens of degrees in a factory, these "small changes" will accumulate into a trend of "gradual deviation".

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How to Stabilize the Signal?

Drifting may seem complicated, but it all boils down to three words:

Unstable.

1. Stable reference: Establish a reliable "zero-point" environment

Isolators and isolated safety barriers can effectively cut off ground potential differences, keeping the reference point clean.

Adding isolation in the signal chain is a well-recognized and effective method for stabilizing drift in the engineering field.

Isolation can solve problems such as ground potential difference, interference coupling, inconsistent signal loops, and multiple grounding points, and can almost cure 70% of drift issues.

2. Control interference: Keep signal lines away from interference sources.

Separating the analog signal cables from the power lines, grounding the shielding layer at one end, installing EMC filters on the frequency converter, avoiding cable reels, routing power lines through power cable trays and signal lines through signal cable trays - these are all fundamental yet effective measures.

Many of the "strange drifts" on site are caused by wiring.

3. Optimize the circuit: Ensure the reliability of cables and terminals.

Using industrial shielded cables, regularly checking terminal fastening, and avoiding cable moisture are the keys to reducing long-term drift.

4. Improve power supply quality: The 24V power supply must be stable.

If a system drifts and the cause cannot be found, engineers usually say: Try a different power supply.

This is a highly professional intuition because the ripple, voltage drop, common ground, and load changes of the 24V power supply will all be "conducted" into the analog quantity.

5. Manage pressure transmitters: Regular calibration is essential.

Without calibration, natural drift cannot be eliminated. Especially for key transmitters such as those for temperature, pressure and liquid level, a good calibration system can keep the drift within a reasonable range.

The terminal screws must be tightened.

Loose terminals are one of the main culprits behind all kinds of strange malfunctions. Especially in pump rooms, fan rooms, and compressor rooms, as long as there is vibration, they will become loose.