Meaning and Impact of Zero Drift in 1151 Differential Pressure Transmitters

The 1151 differential pressure transmitter is used to measure the pressure difference between two pressure ports and can be applied in scenarios such as sealed vessel level measurement, pipeline flow measurement, filter differential pressure measurement, and air pressure monitoring. Zero drift refers to the phenomenon in which the transmitter's output current or displayed value deviates from the original zero point when the actual differential pressure between the high- and low-pressure sides is 0.

For example, for an instrument with a range of 0～10kPa and an output of 4～20mA, the normal zero point should be 4.00mA. If the output changes to 4.32mA with no differential pressure, this corresponds to a zero offset of approximately 0.20kPa. For low-range differential pressure measurements, this type of error is amplified and directly affects control decisions.

Zero drift does not necessarily mean that the sensor has failed immediately. However, if the drift continues to increase, remains unstable after repeated zero adjustment, or is accompanied by issues such as output fluctuations or slow response, systematic troubleshooting should be carried out in terms of installation, operating conditions, electrical circuits, and sensing element condition.

Zero Drift Caused by Environmental Changes

Impact of Temperature Changes on the Measurement Reference

Temperature is one of the common causes of zero drift in 1151 differential pressure transmitters. The sensor diaphragm, resistance strain elements, fill fluid, and electronic circuitry all undergo slight characteristic changes with temperature variations. Zero offset is more likely to occur particularly when on-site day-to-night temperature differences are significant, equipment is started and stopped frequently, or medium temperature fluctuates noticeably.

When the ambient temperature rises from 20℃ to 50℃, ordinary instruments may still exhibit a certain residual error even if they have temperature compensation functions. The smaller the range, the more sensitive it is to temperature effects. For example, for differential pressure measurements with a range below 1kPa, particular attention should be paid to the transmitter's temperature compensation range and temperature drift specifications.

When the installation location is close to steam pipelines, furnaces, heat exchangers, or areas exposed to direct outdoor sunlight, uneven heating on both sides of the instrument housing may also cause temporary zero changes. Heat sources should be avoided as much as possible. Insulation measures should be added when necessary, and the impulse piping should be kept under relatively consistent temperature conditions.

Vibration, Impact, and Installation Stress

Long-term mechanical vibration can loosen internal connecting components, terminals, and mounting brackets of the transmitter, thereby affecting zero stability. For instruments near compressors, pump units, fans, and high-speed machinery, zero drift is generally more pronounced if reliable fastening and vibration damping measures are not adopted.

Uneven force on mounting bolts, misaligned flange connections, or impulse pipes forcibly pulling the transmitter body can also create mechanical stress. This stress may be transmitted to the measuring diaphragm or sensor structure, causing the instrument to gradually deviate from its initial calibration value after commissioning.

It is recommended to check whether the mounting bracket is secure and whether the instrument head is loose, and to confirm that the impulse pipes have independent support. For locations with strong vibration, vibration-damping brackets may be used, or the transmitter may be moved to an area with less vibration and connected by extending the impulse pipes.

Quick Methods for Assessing Environmental Factors

Zero output can be recorded separately during equipment cold condition, stable operation, and high-temperature operation. If the output changes regularly with temperature and gradually returns after the temperature recovers, temperature effects should generally be considered first rather than immediately determining that the sensor has failed.

For instruments with 4～20mA output, a process calibrator with an accuracy of no less than class 0.05 should be used to measure loop current. Determining zero drift solely based on the control room display value can easily be affected by PLC range settings, DCS acquisition errors, and signal isolator conditions.

If the zero point gradually recovers after shutdown but the offset worsens during operation, thermal radiation, pipeline vibration, and medium pulsation should be checked as priorities. Combining on-site records with instrument calibration data can more accurately distinguish environmental issues from instrument-related issues.

Deviations Caused by the Impulse Pressure System and Process Conditions

Impulse Pipe Blockage, Leakage, or Uneven Liquid Accumulation

The 1151 differential pressure transmitter measures the actual pressure difference between the high-pressure side and the low-pressure side. Therefore, the condition of the impulse piping has a significant impact on the zero point. After crystallization, dust, oil contamination, rust, or condensate blocks one side of the piping, pressure transmission on both sides becomes inconsistent, easily creating a false differential pressure.

In gas medium measurement, liquid accumulation at the low point of the impulse pipe, or in liquid medium measurement, gas accumulation at the high point of the impulse pipe, can both cause static head imbalance. Taking water as an example, a liquid column height difference of approximately 100mm can produce a pressure difference of approximately 0.98kPa, which is already a non-negligible deviation for low differential pressure range instruments.

Minor leakage at fittings, drain valves, three-valve manifolds, and sealing gaskets can also cause pressure to decay slowly, appearing as continuous zero drift in one direction. During inspection, fitting sealing conditions should be observed under safe conditions, and pressure holding or leak testing should be performed in accordance with process specifications.

Non-Standard Three-Valve Manifold Operation and Pressure Tapping Methods

During commissioning, shutdown, or zero adjustment of a differential pressure transmitter, an improper operating sequence of the three-valve manifold may cause one-side overpressure, resulting in zero offset. Especially in low-range measurements, an instantaneous large unidirectional pressure shock may affect the diaphragm stress condition and, in severe cases, cause permanent damage.

Before checking the zero point, first confirm that the pressures on the high- and low-pressure sides have been balanced, then observe whether the output is stable. If on-site operating conditions cannot truly achieve equal pressure on both sides, a standard pressure source, calibration device, or compliant isolation method should be used for verification. Zero adjustment must not be performed blindly.

An unreasonable pressure tapping point location can likewise cause measurement deviations. For example, in flow measurement, if severe vortices, deposits, or localized blockage exist near the pressure tapping hole, the differential pressure transmitted to the transmitter will be distorted. In such cases, the process pressure tapping issue should be addressed before instrument calibration is performed.

Electrical Circuit and Internal Instrument Causes

Power Supply, Grounding, and Electromagnetic Interference Issues

The 4～20mA two-wire 1151 differential pressure transmitter has certain requirements for power supply quality. Insufficient supply voltage, excessive ripple, or increased loop contact resistance may cause unstable output or apparent zero drift. For common 24VDC power supply systems, it should be confirmed that the actual voltage under load meets the requirements specified in the instrument manual.

When signal cables are routed in parallel too close to variable-frequency drive power lines or high-voltage cables, electromagnetic interference can easily be introduced. Interference usually appears as display fluctuations or periodic variations, with amplitudes possibly reaching 0.02mA to 0.10mA, which can affect low-range measurement accuracy after conversion.

Improper shield grounding methods, multi-point grounding, or damp and oxidized terminals may also cause abnormal signals. The sealing of the junction box, terminal tightness, and shield wire handling method should be checked, and corrective actions should be taken in accordance with the grounding specifications of the on-site control system.

Sensor Aging and Changes in Electronic Component Performance

When exposed for long periods to high temperature, high pressure, corrosive media, or frequent pressure shocks, the sensing elements of the sensor may experience performance degradation. Problems such as corrosion of the isolation diaphragm, fill fluid leakage, and aging seals can gradually worsen zero stability and are often accompanied by increased range errors.

Components in electronic amplification circuits can also experience parameter drift over their service life. If the transmitter has exceeded its calibration interval, or has operated for more than 3 to 5 years without comparative calibration, verification, calibration, or performance evaluation should be arranged based on actual operating conditions.

It should be noted that repeatedly masking faults by adjusting the zero screw or using digital zero adjustment cannot resolve diaphragm damage, abnormal fill fluid conditions, or circuit aging. If the zero point cannot be maintained after calibration, further inspection should be performed by professionals, and the sensor assembly or complete instrument should be replaced if necessary.

Handling and Prevention of Zero Drift in 1151 Differential Pressure Transmitters

Troubleshoot Step by Step and Avoid Blind Zero Adjustment

When zero drift occurs, first confirm whether the process has zero differential pressure conditions. Then check the three-valve manifold status, liquid or gas accumulation in the impulse pipes, valve leakage, and pressure tapping pipe blockage. Zero adjustment of the transmitter should only be performed after external factors have been eliminated.

Next, check the supply voltage, wiring terminals, shield grounding, and signal loop. When necessary, isolate the instrument from the field loop and use standard calibration equipment for bench testing. If the field condition is abnormal but the bench test is stable, the issue is mostly concentrated in the installation environment, impulse pressure system, or control loop.

If bench calibration finds that the zero point, full scale, and linearity all exceed allowable errors, or that zero return is unstable after pressurization, continued use should be stopped and repair should be arranged. For critical interlocks, metering, or safety monitoring points, adjustments must not rely solely on experience; calibration records and handling procedures should be retained.

Establish a Periodic Maintenance Mechanism

It is recommended to establish maintenance intervals based on medium cleanliness, temperature and pressure ratings, frequency of use, and control importance. Under normal operating conditions, calibration can be performed once every 6 months to 12 months. For high-temperature, corrosive, blockage-prone, or critical control conditions, the interval may be appropriately shortened to 3 months to 6 months.

Maintenance should include impulse pipe purging or condensate draining, three-valve manifold sealing inspection, junction box moisture protection inspection, zero and range calibration, and installation fastening inspection. By recording the zero offset for each maintenance cycle, it is possible to determine whether instrument performance is gradually deteriorating.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide product selection, technical consultation, calibration recommendations, and non-standard customization support for pressure, differential pressure, level, and flow measurement applications. When the zero point of a 1151 differential pressure transmitter repeatedly drifts, it is recommended to conduct a targeted diagnosis based on the on-site medium, range, installation method, and loop parameters to restore the stability and accuracy of the measurement system as soon as possible.