Core Selection Principle for Low Differential Pressure Conditions: Assess the Effective Signal First, Then Determine the Sensor Grade

Low differential pressure measurement is common in applications such as air filter resistance monitoring, cleanroom differential pressure control, indirect fan airflow measurement, slight furnace negative pressure, liquid level density calculation, and pressure balancing in closed vessels. Compared with conventional pressure measurement, the actual signal in low differential pressure conditions is typically only several hundred Pa to several kPa. Accumulated liquid in impulse lines, installation height differences, ambient temperature drift, and static pressure fluctuations may all cause significant measurement deviations.

When selecting a WIKA differential pressure transmitter, the measuring range should not be determined solely based on the “maximum possible differential pressure.” More importantly, it is necessary to confirm the normal operating differential pressure, alarm differential pressure, short-term peak differential pressure, and the static pressure jointly borne by both sides. For applications operating below 20% of full scale over the long term, even if the instrument can display a value, its relative error, the effect of zero drift, and control resolution may fail to meet requirements.

In practice, the normal operating differential pressure should preferably be within 40% to 80% of the configured range. If process fluctuations are small, operation may be close to 80%; if start-stop shocks, backflushing, or rapid valve switching are present, a larger overload margin should be reserved. Selection documentation should also specify the differential pressure range, one-sided overpressure, maximum static pressure, medium temperature, and ambient temperature to avoid mistaking the “differential pressure range” for “pressure resistance capability.”

Range Matching Recommendations

Sensor Structure Selection: Low-Range Accuracy Cannot Be Considered Separately from Static Pressure Conditions

Low differential pressure does not necessarily mean low pressure. For example, when measuring a 3kPa pressure drop across a 10MPa pipeline, the transmitter measures only a 3kPa differential pressure but continuously withstands high common static pressure. In this case, particular attention should be paid to the maximum operating static pressure, one-sided overpressure limit, and the effects of static pressure on zero point and span for the corresponding WIKA model.

For ventilation, purification, and low-pressure gas systems, low differential pressure sensor structures suitable for dry, clean gases may be evaluated first. These products generally offer high sensitivity and are suitable for small differential pressures from several tens of Pa to several kPa. For humid air, corrosive gases, steam, or media prone to crystallization, diaphragm material, sealing components, and process connection type should be treated as equally important selection criteria.

Under high static pressure, corrosive, or high-temperature process conditions, diaphragm-type differential pressure measurement structures are generally more reliable. However, their fill fluid systems, capillary length, and ambient temperature can affect response speed and zero stability. Technical personnel should determine whether flange diaphragms, remote seals, flushing rings, or condensate pots are required based on the medium condition, rather than replacing instruments solely according to connection size.

Accuracy Specifications Should Be Evaluated According to the Actual Operating Range

For low differential pressure selection, accuracy should distinguish between “full-scale accuracy” and “absolute error at the actual measurement point.” For example, a transmitter with a range of 0 to 10kPa and an accuracy of 0.5%FS has a theoretical basic error of approximately ±50Pa. When the actual differential pressure is only 500Pa, this error already equals 10% of the reading. Therefore, low differential pressure control should not only consider the accuracy percentage, but also convert it into the absolute deviation in Pa or kPa.

In addition to basic accuracy, repeatability, long-term stability, temperature drift, and zero adjustment range should also be verified. For room differential pressure control in the 20Pa to 100Pa range, a zero-point variation of several Pa may affect access control, supply and exhaust air interlocks, or alarm decisions. For filter monitoring, long-term stability is generally of greater practical value than extremely high instantaneous accuracy.

If the transmitter needs to be connected to a PLC, DCS, or building automation system, confirm whether the output is 4mA to 20mA, a voltage signal, HART, or another communication protocol, and verify the supply voltage, loop load, and display units. For low differential pressure applications, zero calibration is recommended during commissioning, followed by a recheck of the on-site zero point after stable system operation.

Reference for Error Conversion in Low Differential Pressure Measurement

Installation Conditions Determine Whether Low Differential Pressure Measurement Can Remain Stable Over the Long Term

Low differential pressure signals are very small, so additional pressure caused by field installation cannot be ignored. During gas measurement, pressure tapping points on both sides should be kept at the same elevation whenever possible, and impulse lines should have similar lengths to avoid one pipeline being significantly longer or having excessive bends. For measurement tasks from 10Pa to 50Pa, minor air leakage, loose fittings, or flattened impulse lines may all distort the data.

Liquid and steam applications require attention to hydrostatic head differences. A water height difference of approximately 100mm can produce a pressure difference of about 1kPa; therefore, the liquid level, slope, and condensation conditions of impulse lines on both sides must be as consistent as possible. When measuring steam, condensate pots should be installed in pairs and maintained at the same liquid level. When measuring liquids containing impurities, provisions should be made for drainage, flushing, and maintenance.

The transmitter should preferably be installed away from areas with strong vibration, thermal radiation, frequent washdown, and electromagnetic interference. Signal cables should be shielded and grounded in accordance with site specifications and kept at a reasonable distance from high-power lines. At locations with obvious pulsation, such as fan outlets and compressor outlets, damping, buffering, or software filtering may be configured according to process characteristics. However, the filtering time should not be set too long, so as not to affect alarm and interlock response.

On-Site Verification Checklist for Low Differential Pressure Projects

Before ordering, the medium name, whether it contains dust or condensate, normal and maximum differential pressure, static pressure on both sides, maximum temperature, minimum temperature, connection standard, installation orientation, and protection requirements should be specified. If used in hazardous areas, the explosion-proof type, certification requirements, and cable entry specifications must also be confirmed simultaneously; a standard model must not be substituted temporarily.

During commissioning, it is recommended to first close or equalize the pressure on both sides and perform a zero check. Then apply a known differential pressure or compare with a standard instrument to confirm that the 4mA to 20mA output, displayed value, and engineering units in the control system are consistent. For adjustable-range models, after modifying the range, the alarm setpoints, trend curve scale, and host computer conversion parameters must be re-verified.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can assist in clarifying the low differential pressure range, static pressure rating, medium material requirements, and installation method based on specific operating conditions. After providing the existing instrument nameplate, process flow, pressure tapping point locations, medium parameters, and control accuracy requirements, a suitable WIKA differential pressure transmitter configuration can be further matched, with targeted recommendations provided for impulse line connections and on-site calibration.