When selecting a current-output differential pressure sensor for industrial applications, 4-20mA output is one of the most widely used signal formats. It offers long transmission distance, strong anti-interference capability, and convenient connection to PLCs, DCSs, and intelligent digital display control instruments. However, actual selection should not only confirm that the output is 4-20mA; the measured medium, differential pressure range, static pressure rating, power supply capability, loop load, installation method, and environmental interference factors should also be evaluated together.

Differential pressure sensors are commonly used for filter blockage monitoring, air duct differential pressure detection, level measurement, flow calculation, cleanroom differential pressure control, and pressure drop monitoring across equipment. In different applications, the pressure conditions, medium characteristics, and control requirements experienced by the sensor vary considerably. Incorrect parameter selection may result in zero drift, abnormal signals, inaccurate measurement, or even damage to the sensing element.

For technical evaluation personnel, a reasonable selection process should begin with confirmation of operating conditions, followed by verification of the electrical loop and system compatibility, and finally implementation of installation, commissioning, and maintenance requirements. Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide selection and technical service support for differential pressure transmitters and supporting display and control products based on industrial automation measurement requirements.

I. First Define the Differential Pressure Measurement Object and Range

Differential Pressure Is Not Simply a Pressure Value

A differential pressure sensor measures the pressure difference between the high-pressure side and the low-pressure side. Its basic relationship is: differential pressure = high-side pressure - low-side pressure. For air pressure detection, the range may be only several tens of pascals; for level or filtration systems, it may reach several tens of kilopascals or even higher. Therefore, before selection, it is necessary to confirm where the high- and low-pressure sides are connected and what differential pressures may occur under normal, maximum, and abnormal operating conditions.

In general, the normal operating differential pressure should fall within 30% to 80% of the full-scale range. If the range is too large, small-signal resolution will decrease; if the range is too small, overrange may occur during equipment startup, valve switching, or filter element blockage. For example, where the normal differential pressure is 6kPa and the maximum expected differential pressure is 10kPa, nearby standard ranges such as 0-10kPa and 0-16kPa are generally preferred.

Positive and negative differential pressure requirements should also be considered. Some applications involve bidirectional differential pressure changes, such as supply and exhaust air systems, bidirectional filtration equipment, or test equipment, and require positive/negative differential pressure ranges such as -1kPa to 1kPa or -10kPa to 10kPa. If a unidirectional range product is selected, when the low-side pressure is higher than the high-side pressure, the output may fall below 4mA, trigger a fault condition, or cause measurement deviation.

Parameters Recommended for Recording When Confirming the Range

In addition to nominal differential pressure, maximum operating static pressure, instantaneous pressure surge, allowable overload pressure, and medium temperature should also be recorded. Especially near pipelines, pump outlets, and valves, although the actual differential pressure may not be high, the common static pressure borne by both sides may be high. Differential pressure range and static pressure resistance are two different specifications; a low differential pressure range cannot be used in place of a model suitable for high static pressure.

II. 4-20mA Output Requires Verification of Power Supply and Load

Confirm Two-Wire or Four-Wire Wiring Method

Industrial differential pressure transmitters commonly use two-wire 4-20mA wiring, meaning that the power supply and signal share two conductors. Two-wire systems simplify wiring and are suitable for long-distance transmission, but the loop supply voltage and load resistance must remain within the allowable range of the product. DC 24V is commonly used for power supply; actual requirements should be based on the power supply range specified on the sensor nameplate, manual, or technical agreement.

Some products use four-wire configurations with separate power supply and output, which are suitable for certain special control systems or applications requiring multiple output combinations. During selection, it is essential to confirm whether the PLC analog input module is an active input or a passive input, in order to avoid duplicate power supply, reversed polarity, or failure to form the signal loop. If the system already has a 24V loop power supply, the wiring scheme should be jointly confirmed by instrumentation, electrical, and automation personnel.

4mA normally corresponds to the lower range limit, while 20mA corresponds to the upper range limit. For example, for a sensor with a 0-10kPa range, an output of 12mA normally corresponds to 5kPa; for a -10kPa to 10kPa range, 12mA normally corresponds to 0kPa. When configuring the host system, the range, unit, decimal places, and linear relationship must remain consistent and must not simply be handled according to a default 0-100% setting.

Loop Load Must Not Exceed the Allowable Value

The PLC input resistance, local display instruments, ammeters, isolators, and line resistance in the loop all form the total load. A common 250Ω resistor can convert 4-20mA into 1-5V, but too many series-connected devices will increase the load. When the supply voltage is insufficient or the load exceeds the limit, the sensor may be unable to reach 20mA in the high-output range, resulting in a low full-scale reading, signal fluctuation, or alarms.

III. Accuracy, Stability, and Response Speed Should Match the Control Objective

Do Not Focus on Only One Accuracy Figure

Sensor accuracy is usually expressed as a percentage of full scale, such as 0.5%FS or 0.25%FS. For a 0-100kPa range, 0.5%FS corresponds to approximately ±0.5kPa; for a 0-1kPa low range, the same 0.5%FS corresponds to approximately ±5Pa. Low differential pressure applications are more sensitive to zero point and temperature drift. Therefore, in addition to basic accuracy, repeatability, long-term stability, and zero drift specifications should also be considered.

If the differential pressure signal is used only to indicate filter replacement, standard industrial accuracy can generally meet the requirements. If it is used for cleanroom differential pressure closed-loop control, precision gas mixing, or flow metering, accuracy class, temperature effects, and response time need to be evaluated more strictly. When the control system sampling cycle is short, slow sensor response will cause control lag; excessively fast response may directly transmit pressure pulsations into the control loop.

For applications involving pump vibration, compressor pulsation, or frequent valve operation, damping settings, buffer devices, or software filtering can be added according to actual operating conditions. Filtering parameters should be adjusted through on-site commissioning and should not excessively delay actual changes merely to stabilize displayed values. Generally, stable operating values, startup shock values, and signal curves under abnormal conditions should all be observed.

Temperature Changes Affect Long-Term Measurement Results

Installation ambient temperature, medium temperature, and day-night temperature differences can all affect measurement stability. High-temperature media should not act directly on the pressure port of ordinary sensors; impulse tubing, condensate bends, or isolation devices should be used to reduce heat transfer. In low-temperature or humid environments, condensation should also be prevented from entering the terminal chamber and pressure passages to avoid reduced insulation, blockage, or zero offset.

IV. Medium, Material, and Static Pressure Rating Determine Product Reliability

Pressure Port Materials Must Be Compatible with the Medium

Air, clean gases, ordinary water, and hydraulic oil have relatively standard compatibility requirements. For acid and alkali media, salt-mist-containing gases, corrosive vapors, viscous liquids, or particle-containing fluids, wetted materials must be carefully confirmed. Common stainless steel materials have good general applicability, but they are not suitable for all chemical media. Specific suitability must be determined in conjunction with medium concentration, temperature, and pressure.

For media that are prone to crystallization, solidification, contain impurities, or have high viscosity, narrow pressure tapping holes should not remain exposed to the medium for long periods. Where necessary, diaphragm structures, flange mounting, flushing ports, or supporting impulse piping may be used. The protection rating of the sensor body also needs to match the site environment. In outdoor, humid, or dusty environments, attention should be paid to terminal box sealing and waterproof treatment at cable entries.

Both sides of a differential pressure sensor may be subjected to high common static pressure. For example, in vessel level measurement, the actual level differential pressure may be small, while the internal vessel pressure may be high. In this case, it is necessary to confirm the maximum static pressure, single-side overload capacity, and simultaneous pressure capacity on both sides; the model must not be determined solely based on the differential pressure range. For high-pressure, high-temperature, or flammable and explosive areas, relevant explosion-proof, protection, and safety requirements should be further confirmed.

Installation Direction and Pressure Tapping Position Affect Measurement

For gas measurement, condensate accumulation in impulse tubing should be avoided as much as possible; for liquid measurement, air bubbles should be prevented from remaining; for steam measurement, condensation conditions on both sides should be kept as consistent as possible. Reversed high- and low-pressure connections will reverse the output direction, so verification should be performed using a known differential pressure after piping is completed. Before installation, it should also be confirmed that there is no welding slag, iron filings, or residual sealing material in the piping to prevent blockage of sensor ports.

V. Wiring, Grounding, and System Commissioning Must Not Be Overlooked

Although 4-20mA Resists Interference, Standardized Installation Is Still Required

Compared with 0-10V signals, current signals are more suitable for long-distance transmission in industrial sites, but this does not mean electromagnetic interference can be ignored. Sensor signal cables should preferably use shielded twisted pairs and should be routed separately from variable-frequency drive power cables, motor cables, and relay control cables whenever possible. If crossing is unavoidable, cables should cross at close to 90 degrees to reduce the length of parallel routing.

The shielding layer should generally be grounded at one end in accordance with system grounding specifications to avoid ground loop currents. The grounding relationship among field equipment, control cabinets, and the power supply negative terminal should be planned in a unified manner. For areas with frequent thunderstorms, long outdoor pipelines, or strong electromagnetic environments, suitable surge protectors, signal isolators, or safety barriers should be considered in the signal loop, while ensuring that their voltage drop does not affect loop power supply.

During commissioning, process effects can first be isolated while checking the 24V supply, terminal polarity, and loop continuity. Then, use a standard current source to verify whether the PLC or display instrument correctly reads 4mA, 12mA, and 20mA. Next, connect the sensor to actual operating conditions and verify the displayed values at zero differential pressure, normal differential pressure, and near full scale, confirming that engineering units and alarm threshold settings are correct.

Establish a Periodic Inspection Mechanism

For critical process points, it is recommended to regularly inspect impulse tubing flowability, fitting sealing, zero drift, and output current in conjunction with the equipment maintenance plan. For filter differential pressure monitoring, warning and interlock values can be set based on historical data, for example, issuing a warning at 70% to 80% of the rated differential pressure to avoid waiting until complete blockage occurs. If the sensor exhibits persistent zero abnormalities, output jumps, or obvious lag, pipeline, power supply, and wiring issues should be investigated first before determining whether calibration or replacement is required.

VI. Differential Pressure Sensor Selection Information Checklist

When submitting a selection request, it is recommended to provide the medium name, temperature, maximum static pressure, normal differential pressure, maximum differential pressure, connection specification, installation location, supply voltage, output signal, site protection requirements, and control system model at one time. The more complete this information is, the more conducive it is to quickly determining a suitable differential pressure sensor structure and parameters and reducing later modification costs.

For conventional air pressure, filter differential pressure, level differential pressure, and industrial pipeline monitoring, industrial-grade products with a matching range, DC 24V power supply, 4-20mA output, and stable anti-interference performance can be prioritized. For high static pressure, corrosive, high-temperature, blockage-prone, or explosion-proof areas, special operating condition confirmation should be added based on conventional parameters, and customized solutions should be adopted where necessary.

Xi'an Shenghongchuang Instrumentation Co., Ltd. can assist in confirming the differential pressure range, output method, connection material, pressure resistance rating, and supporting digital display control instrument solution according to the actual measurement object and control system conditions. Please provide on-site operating condition parameters and wiring requirements before purchase to obtain more accurate selection recommendations for 4-20mA current-output differential pressure sensors.