For significant zero drift in ceramic capacitive differential pressure sensors, first identify the drift phenomenon

Under conditions where the pressures on both sides of a ceramic capacitive differential pressure sensor are equal and the pressure tapping valves are correctly positioned, the output value should theoretically remain stable near zero. If the displayed value continuously deviates from zero, or changes noticeably after shutdown, heating, or cooling, a zero-drift issue can be identified.

Before on-site troubleshooting, first record the measuring range, current output, ambient temperature, medium temperature, supply voltage, and installation time. For products with 4-20mA output, an inspection should be arranged when the zero deviation exceeds 0.5% of the measuring range. If fluctuations recur repeatedly within a short period, re-zeroing alone is not sufficient.

It is necessary to distinguish between “actual differential pressure changes” and “abnormal instrument zero point.” For example, vessel liquid level, filter differential pressure, and pressure differences before and after a fan may vary with process changes. Maintenance personnel should first confirm that the process condition is stable before assessing the zero point, to avoid misdiagnosing a sensor fault.

Basic method for determining drift

First isolate the process pressure on the high-pressure side and low-pressure side, and confirm that both sides are at the same pressure reference. For differential pressure transmitters equipped with a three-valve manifold or five-valve manifold, strictly follow the sequence of depressurizing, balancing, and isolating to prevent one-sided overpressure from damaging the ceramic sensing element.

After pressure balancing, allow the system to stand for 5 to 10 minutes and observe whether the output tends to stabilize. If the output gradually returns to zero from a large deviation, focus on checking for accumulated liquid, blockage, or temperature effects in the impulse lines. If the output remains stable at a fixed deviation, further inspect zero calibration and the circuit reference.

It is recommended to use a calibrated pressure source and a high-precision multimeter for verification. Comparing the on-site display, the control system acquisition value, and the measured value at the sensor terminals can quickly distinguish whether the drift originates from the sensor itself or from signal transmission, power supply, or host-system scaling settings.

Step 1: Check the installation and impulse line system

In a differential pressure measurement loop, the impulse line system is often more likely than the sensor itself to cause a false zero point. Differences in the length, pipe diameter, routing, or insulation conditions of the impulse lines on both sides can cause liquid-column differential pressure, condensate height differences, or trapped gas, ultimately resulting in zero offset.

When measuring gases, impulse lines should avoid low points where liquid can accumulate; when measuring liquids, high points where gas can accumulate should be avoided. Under steam conditions, the condensate pots on both sides must be at the same height, filled with condensate, and maintained at similar temperatures; otherwise, zero-point changes during startup and shutdown will be particularly noticeable.

Also check for internal leakage in the valve manifold, incompletely closed drain valves, leakage at fittings, and crystallization blockage at pressure tapping points. Minor leakage may not cause an obvious process fault, but it can slowly release pressure on one side, causing the differential pressure output to shift persistently in one direction.

Key points for impulse line and installation inspection

For low-range differential pressure measurements, the effect of liquid-column height differences is more pronounced. Taking water as an example, a liquid level height difference of approximately 100mm can produce a differential pressure of about 0.98kPa, sufficient to cause significant zero offset in low-range differential pressure sensors. Therefore, the arrangement of condensate pots and impulse lines must remain symmetrical.

After completing the inspection, rebalance the pressure on both sides and confirm that the output is stable before proceeding to the next step. If the zero point returns to normal after clearing condensate and draining, the issue mainly originates from the process connection, and immediate removal or replacement of the sensor is not recommended.

Step 2: Investigate electrical and environmental interference

The signal processing circuit of a ceramic capacitive differential pressure sensor has certain requirements for power supply stability, grounding quality, and the electromagnetic environment. Excessive fluctuations in a 24VDC power supply, high ripple from a switching power supply, or sharing a power supply with high-power loads may all cause an unstable output zero point.

Check whether the signal cable runs in parallel over a long distance with variable-frequency drive cables, motor power cables, or high-frequency equipment cables. The shielding layer should generally be reliably grounded at one end in accordance with site grounding specifications. If both ends of the shielding layer are connected arbitrarily, ground-loop interference may instead be introduced.

Also check for oxidation of wiring terminals, water ingress into cables, and loose waterproof connectors. Reduced insulation in humid environments can easily cause fluctuations in milliampere signals. For outdoor installations, also confirm that the enclosure sealing, venting structure, and lightning protection measures are intact.

Effects of temperature and vibration

Although ceramic capacitive sensing elements offer good corrosion resistance and stability, rapid changes in ambient temperature can still affect the thermal balance of the capacitive structure, potting materials, and electronic components. After a sensor is moved from a low-temperature environment to a high-temperature site, allow no less than 30 minutes for temperature stabilization before zero calibration.

If the transmitter is installed close to a steam pipeline, furnace, hot-air duct, or location exposed to direct sunlight, insulation, shading, or remote mounting measures should be added. Uneven heating of the high-pressure and low-pressure sides can also be transmitted through the impulse lines as additional differential pressure, so attention should not be limited to the instrument enclosure temperature.

Continuous vibration can cause loose wiring, bracket resonance, or poor contact at internal solder joints. For measurement points near pumps, compressors, or fans, check the rigidity of the mounting bracket. If necessary, use a vibration-damping bracket and shorten unnecessary suspended impulse line sections.

Step 3: Verify the measuring range, calibration, and sensing element condition

Confirm whether the actual on-site differential pressure remains close to the upper limit of the sensor range for extended periods, or whether it is frequently subjected to reverse differential pressure and transient impact. An undersized selected range can affect zero stability and repeatability, while severe one-sided overpressure may cause irreversible performance changes in the ceramic diaphragm.

During calibration, first perform the zero adjustment, then conduct point-by-point pressure increase and decrease tests at 25%, 50%, 75%, and 100% of the measuring range, recording hysteresis and repeatability. Adjusting only at the zero point without checking full-range linearity may conceal reduced sensor sensitivity or abnormalities in the signal amplification circuit.

If the sensor has local buttons, a magnetic pen, or communication-based adjustment functions, perform zero shifting only when pressure is fully balanced and permitted by the process. After zero adjustment, the full-scale output must be verified again to ensure that the correspondence between 4mA and 20mA has not been incorrectly changed.

How to evaluate calibration results

If the zero deviation continues to increase under balanced conditions and cannot be stabilized after repeated calibration, or if insufficient full-scale output, obvious nonlinearity, or excessive hysteresis occurs, stop forced zero adjustment to prevent incorrect data from entering the control system.

For products suspected of having been affected by one-sided overpressure, corrosive medium ingress, or prolonged high temperatures, it is recommended that qualified personnel conduct pressure cycling, insulation performance, and full-range accuracy tests. During on-site removal and installation, protect the process connections to prevent impurities from entering the pressure chamber.

Establish a traceable maintenance mechanism

Zero-drift troubleshooting should not end with a single adjustment. It is recommended to establish equipment records documenting the date of each verification, zero deviation, environmental conditions, process status, and handling results. Continuous data can determine whether the drift is an occasional installation issue or a gradually developing component performance issue.

For critical differential pressure measurement points, preventive inspections can be scheduled in conjunction with production cycles, focusing on valve manifolds, impulse lines, power supply circuits, and calibration status. For high-temperature, high-pressure, highly corrosive, and high-vibration conditions, range margin, temperature resistance requirements, and isolation solutions should be clearly defined during the selection stage.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide product selection, technical support, calibration recommendations, and non-standard customization services for pressure, differential pressure, liquid level, and industrial automation measurement and control projects. When persistent zero drift occurs on site, the measuring range, medium, wiring method, output data, and installation photos can be compiled for technical personnel to assist in determining the troubleshooting sequence and solution.