During the operation of a water supply system, frequent fluctuations in the readings of a water supply differential pressure transmitter—sometimes high and sometimes low—are often not simply a matter of an “inaccurate instrument.” Once the differential pressure signal becomes unstable, variable-frequency pump interlocking control, constant-pressure water replenishment, filter differential pressure monitoring, and other processes may all malfunction accordingly: pumps repeatedly adjust speed, valves frequently start and stop, control cabinets issue alarms, and on-site personnel may find it difficult to determine the system's actual pressure condition.

In such cases, it is not advisable to immediately dismantle the transmitter or rush to replace the equipment. For operators, a more effective sequence is to first confirm whether operating conditions are truly fluctuating, then check the pressure tapping lines and valve conditions, followed by troubleshooting the power supply, wiring, signal transmission, and installation environment, and finally determine whether the water supply differential pressure transmitter itself has zero drift, diaphragm damage, or an unsuitable measuring range. This can both shorten shutdown troubleshooting time and prevent pipeline issues from being misdiagnosed as instrument failures.

First determine: Is the water pressure fluctuating, or is the displayed value fluctuating?

This is the first step in troubleshooting and also the one most easily overlooked. If pumps in the water supply system frequently start and stop, flow at water consumption points changes significantly, gas is present in the pipeline network, or valves operate too quickly, the actual differential pressure may inherently be changing. In this case, the water supply differential pressure transmitter reflects on-site operating conditions rather than a fault signal.

Comparisons can be made using a mechanical pressure gauge, control system trend curves, or backup measuring points. If the mechanical gauge and transmitter display change synchronously, and the variation generally corresponds to pump operation and valve opening, hydraulic operating conditions should be investigated first. If the mechanical gauge is relatively stable while the PLC, digital indicator, or remote transmission signal continues to fluctuate, greater attention should be given to the transmitter, pressure tapping lines, and electrical circuit.

A practical way to assess this is to observe whether the fluctuations follow a pattern. Fluctuations consistent with pump start-stop cycles are mostly related to control logic, pipeline network pressure, or buffer capacity; irregular minor jumps are commonly caused by poor contact, electromagnetic interference, air accumulation in the pipeline, or signal grounding issues.

Prioritize checking the pressure tapping lines: blockages, air accumulation, and leakage are most common

For a water supply differential pressure transmitter, whether the pressure tapping on both sides is unobstructed, stable, and symmetrical directly affects the measurement result. Especially at locations such as upstream and downstream of filters, heat exchanger inlets and outlets, and pipeline network zone differential pressure points, impurities in the water, rust, welding slag, and scale may gradually enter the pressure taps, narrowing or even blocking one side of the passage. When pressure transmission on the two sides is not synchronized, readings may lag, fluctuate, or deviate from actual conditions.

On site, focus on checking whether the pressure taps, impulse lines, three-valve manifold, or needle valves have any of the following conditions:

• Impulse lines are bent, flattened, or compressed by insulation layers or supports;
• Air bubbles are present in the piping, particularly where the impulse line has local high points;
• Fittings have slight water leakage, preventing pressure from being maintained;
• A valve on one side is not fully open, or the equalizing valve is not fully closed;
• The pressure tapping hole is blocked by debris, and the pressure tapping point was not checked at the same time after cleaning the filter.

Although the water supply medium may appear clean, deposits can still be introduced after long-term circulation. During troubleshooting, pressure should be relieved and the system isolated in accordance with on-site maintenance procedures to avoid disassembly under pressure. For installations with a three-valve manifold, confirm that the high- and low-pressure side isolation valves are in their normal operating positions and that the equalizing valve is closed. If the equalizing valve leaks internally, pressure on both sides will be partially equalized, resulting in low and unstable readings.

Valve positions and pressure tapping direction are often more critical than expected

Many reading fluctuation issues occur after equipment maintenance, pipeline modifications, or instrument reinstallation. If the high- and low-pressure sides are reversed, the readings may become negative or show an abnormal direction. If the pressure tapping point is located at a pump outlet with obvious pulsation and no damping measures are taken, the signal can easily fluctuate along with the pulsation. If the pressure tap is immediately adjacent to an elbow, reducer, valve, or pump outlet, water flow disturbance can also amplify instability in the differential pressure signal.

When checking, do not rely only on labels. Confirm based on the actual piping which side has higher pressure, whether the transmitter “+” and “-” ports correspond correctly, and whether the impulse line routing is reasonable. For upstream-to-downstream differential pressure monitoring, the two pressure tapping points should avoid areas of strong turbulence as much as possible. If pressure pulsation in the system is unavoidable, damping settings, buffering devices, or control program filtering may be evaluated according to process requirements; however, excessive filtering should not be used to conceal actual faults.

Then check the power supply and 4-20mA signal loop—do not focus only on the instrument display

When on-site pressure is stable and there are no obvious abnormalities in the impulse piping, attention should turn to the electrical system. Common water supply differential pressure transmitters use 24VDC power and 4-20mA output. Excessive power ripple, loose terminals, improper shield treatment, and damp or aging cables can all cause the control system to receive unstable signals.

Inspection can follow the chain of “power supply—wiring—input terminal—display terminal.” Measure whether the transmitter supply voltage is stable, check whether wiring terminals are oxidized or loose, and confirm that the positive and negative terminals are not reversed. Check whether signal cables are routed in parallel with variable-frequency drive power lines or motor cables over long distances. Water supply pump rooms often contain many variable-frequency drives, motors, and contactors; electromagnetic interference generated during starting and stopping often causes minor analog signal fluctuations at the control cabinet.

Where conditions permit, use a multimeter or loop calibration device to measure the current at the transmitter output terminal and compare it with the PLC acquisition value and digital indicator display value. If the output current is stable but the upper-level display fluctuates, the problem is likely in signal transmission, the isolation module, the analog input card, or system parameters. If the transmitter output itself is unstable, further inspection of the process side and the instrument itself will be more targeted.

An unsuitable installation environment can also make readings “appear faulty”

Although differential pressure transmitters are suitable for industrial sites, not every installation location is equally reliable. If an instrument is mounted directly on a vibrating pump set, pipeline, or support, long-term mechanical vibration may loosen wiring terminals or disturb internal components, causing fine reading jitter. In outdoor or humid pump room environments, poor sealing of the junction box may also cause terminals to become damp and insulation to deteriorate.

Confirm that the transmitter mounting bracket is secure, the enclosure and junction box are intact, and cable entries are waterproofed. In locations with large temperature differences or substantial condensation, impulse lines and wiring connections should be inspected regularly. If there is an obvious vibration source near the measuring point, consider relocating the instrument to a relatively stable location and using appropriate impulse connections for pressure tapping, rather than directly “hanging” the transmitter on a vibrating pipeline.

When should the water supply differential pressure transmitter itself be suspected?

After completing the above checks, if the piping is unobstructed, valve conditions are correct, and the power supply and signal loop are stable, yet the output remains continuously abnormal, the instrument itself should then be evaluated as a priority. Typical indications include: the zero point does not return after the pump is stopped and pressure is relieved; deviation from a standard gauge remains significant under the same pressure conditions; the output signal fluctuates intermittently; or actual changes within the measuring range are very small while the display frequently spans a large range.

At this point, zero-point inspection and calibration may be performed according to maintenance procedures. It should be noted that, before calibration, the high- and low-pressure sides must be under the same pressure conditions. If the system has not been depressurized, the equalizing valve is in the wrong condition, or residual pressure remains in the impulse lines, the supposed “zero-point deviation” may simply result from invalid operating conditions. For equipment that has been in operation for a long time, it should also be checked whether the measurement range remains suitable for current operating conditions. When the actual differential pressure remains close to the lower range limit for a long time, minor disturbances can appear especially pronounced; when it remains close to the upper limit for a long time, measurement margin and service life may be affected.

A troubleshooting sequence suitable for on-site implementation

When facing alarms or fluctuating readings, operators can first record the pump set condition, valve opening, range of displayed value changes, and frequency of occurrence, then proceed as follows: confirm whether actual operating conditions are fluctuating; inspect high- and low-pressure taps, impulse lines, and valve manifolds; verify piping connection direction and installation location; check the 24V power supply, terminals, and 4-20mA loop; eliminate interference sources such as variable-frequency drives; and finally perform zero-point verification, calibration, or replacement assessment.

Do not replace the transmitter directly before confirming the pressure tapping passages. Many issues that appear to be “instrument failures” are actually caused by air accumulation in impulse lines, valves not being fully open, or interference affecting control cabinet signals. Conversely, if the instrument itself is confirmed to be abnormal, a product with a measuring range, accuracy, medium compatibility, and installation method matched to the water supply operating conditions should be selected.

Xi'an Shenghongchuang Instrumentation Co., Ltd. and its subsidiary, Shaanxi Qinkong Sensor Technology Co., Ltd., have long provided industrial measurement and control products including pressure, differential pressure, level, and supporting intelligent digital display control instruments. For conventional water supply, filter differential pressure monitoring, and complex pump room retrofit applications, in addition to focusing on the parameters of an individual water supply differential pressure transmitter, selection and installation should also take into account the pressure tapping structure, signal connection method, and on-site interference conditions. Only by locating the issue in the correct link can readings become truly stable and unnecessary repeated actions in water supply control be reduced.