How to Read the Wiring Diagram of a Relay Output Pressure Transmitter

A relay output pressure transmitter integrates pressure measurement, numerical display, switch alarm, and interlock control into one device. It is commonly used for water pump start/stop control, air compressor protection, hydraulic system overpressure alarms, tank pressure monitoring, and similar applications. Correctly reading the wiring diagram is an important prerequisite for ensuring stable equipment operation.

Compared with standard pressure transmitters that output only a 4-20mA analog signal, relay output models typically include an alarm setting menu and relay terminals. When pressure reaches the set threshold, the internal relay energizes or de-energizes, transmitting a switching signal to a contactor, PLC input point, audible-visual alarm, or solenoid valve control circuit.

Before on-site wiring, verify the nameplate parameters, terminal markings, supply voltage, and relay contact rating. Common products use 24VDC power supplies, while some models use 220VAC; terminal numbering may vary by model, so wiring must not be based solely on wire color or experience.

Industrial pressure transmitters and intelligent digital display control instruments supplied by Xi'an Shenghongchuang Instrumentation Co., Ltd. can provide suitable solutions according to the on-site pressure range, installation connection, output method, alarm logic, and complex operating conditions. For high-temperature, high-pressure, or high-interference environments, shielding, protection, and isolation requirements should also be considered during configuration.

Terminal Functions and Basic Wiring Logic

How to Identify Power Supply and Signal Terminals

The terminals of a relay output pressure transmitter can generally be divided into power supply terminals, analog output terminals, relay output terminals, and grounding terminals. On 24VDC models, supply polarity is often marked as “V+, V-”, “+24V, 0V”, or “L+, M”. Reverse connection of a DC power supply may prevent the device from starting or cause damage.

If the product has a 4-20mA output, common terminal markings include “I+, I-”, “OUT, GND”, or “AO, COM”. Two-wire transmitters typically use the same loop for power supply and signal transmission; three-wire and four-wire products may separate the power supply and output, so wiring must follow the corresponding manual.

Relay terminals are usually marked COM, NO, and NC, representing the common terminal, normally open terminal, and normally closed terminal respectively. When the alarm actuation condition has not been reached, COM and NC are connected; after an alarm or control condition is triggered, COM and NO are connected. This status may also be affected by forward or reverse control mode and should be verified through powered testing.

The protective grounding terminal is generally marked PE or with a grounding symbol. Grounding of the metal housing, shielding layer, and control cabinet should be handled in accordance with specifications. Especially in areas with strong electromagnetic interference from variable-frequency drives, motors, welding machines, and similar equipment, proper grounding can significantly reduce display fluctuations and false alarm risks.

Typical Wiring Arrangement for a Relay Output Pressure Transmitter

For example, when an intelligent pressure transmitter powered by 24VDC is connected to a PLC system, connect the positive power supply terminal to V+, the negative terminal to V-, and connect the 4-20mA output to the PLC analog input channel; relay COM and NO can be connected in series with an alarm lamp or intermediate relay coil circuit.

For automatic water pump control, a COM and NO connection is generally used for low-pressure start. When pressure falls below the set value, the contact actuates and starts the water pump; another relay output can be used for high-pressure pump shutdown. A reasonable pressure differential should be maintained between the two thresholds, for example, a start pressure of 0.30MPa and a stop pressure of 0.50MPa.

When the relay needs to drive a high-power motor, heater, or electromagnetic actuator, the main load must not be driven directly by the pressure transmitter contacts. Isolation should be provided through an intermediate relay, AC contactor, or PLC control circuit to prevent contact burning, sticking, and loss of equipment control.

How to Set the Trigger Threshold of an Alarm Pressure Transmitter

First Determine the Measuring Range, Control Objective, and Safety Limits

The alarm threshold should not simply be set to the system rated pressure. It should be determined by considering the allowable equipment pressure, normal operating pressure, pressure fluctuation range, and safety margin. For example, if the normal system pressure is 0.60MPa and the design upper limit is 0.80MPa, the overpressure alarm can initially be set between 0.72MPa and 0.75MPa.

The pressure transmitter range should cover both the normal value and possible peak values. It is generally recommended that normal pressure fall within 30% to 80% of the measuring range. For example, where the commonly used pressure is 1.0MPa, a 0-1.6MPa or 0-2.0MPa range should be considered first rather than an excessively large 0-10MPa range.

For adjustable pressure transmitters, first confirm the zero point, full scale, and unit before entering the menu to set alarm points. If the device supports switching among MPa, kPa, bar, and psi, ensure that the control drawings, PLC program, and on-site display use consistent units to prevent incorrect interlocks caused by unit misinterpretation.

In dual high- and low-pressure alarm applications, the low-pressure point is mainly used for leakage warning, pump set startup, or insufficient supply pressure protection, while the high-pressure point is mainly used for overpressure alarms, pump set shutdown, or safety interlocks. After setting, record the parameters and retain clear terminal and alarm point markings inside the control cabinet.

Hysteresis and Delay Determine Alarm Stability

When pressure is close to the setpoint, pipeline pulsation, water hammer, mechanical vibration, or pump start/stop operation may cause the value to fluctuate repeatedly. Without hysteresis, the relay may energize repeatedly within a short period, causing alarm lamps to flash, contactors to operate frequently, and even shortening equipment service life.

Hysteresis can be understood as the difference between the reset threshold and the trigger threshold. For example, if the high-pressure alarm is set to 0.75MPa and the hysteresis is set to 0.05MPa, the alarm is triggered when pressure rises to 0.75MPa and resets only after pressure drops to 0.70MPa.

The delay function is suitable for systems with transient pressure shocks. For hydraulic equipment that experiences a brief pressure rise during startup, an alarm delay of 1 second to 5 seconds can be set; for overpressure protection applications with higher safety risks, delay should be used cautiously to avoid postponing protective action.

After the settings are completed, use a standard pressure source for pressure-rise and pressure-drop testing, and verify at minimum that the trigger value, reset value, relay status, and external actuator operation are consistent. Commissioning records should include the set value, actual actuation value, hysteresis value, and test date.

Comparison of Electronic Pressure Transmitters and Mechanical Pressure Switches

The advantage of electronic pressure transmitters is that measurement, display, remote transmission, and switching control can be completed simultaneously. For projects requiring connection to a PLC, DCS, host computer, or remote operation and maintenance platform, the 4-20mA signal and relay alarm can simultaneously meet process monitoring and local protection requirements.

Mechanical pressure switches have a relatively simple structure and still offer value in equipment with single functional requirements and no need for continuous pressure data. However, after long-term use, spring fatigue, mechanical wear, and environmental vibration may cause actuation point drift, requiring periodic calibration.

When selecting and purchasing, do not compare unit prices only. Accuracy, measuring range, media compatibility, connection size, protection rating, number of outputs, contact rating, and maintenance cost should also be evaluated. For corrosive media, high-viscosity media, or sanitary applications, select diaphragm materials and process connections accordingly.

When local-display pressure transmitters are used in humid, dusty, or outdoor areas, the protection rating should be determined according to the actual environment. For general applications inside industrial cabinets, IP54 or above may be considered; outdoor locations exposed to rain, washing, or water accumulation should generally use higher protection ratings such as IP65 or IP67, and cable gland sealing should be checked.

Key Points for On-Site Wiring, Commissioning, and Troubleshooting

Avoid Common Wiring Errors During Installation

Relevant power supplies must be disconnected before wiring, and a multimeter should be used to confirm the voltage level and terminal status. Equipment powered by 24VDC must not be connected to 220VAC; although relay outputs are dry contacts, the voltage and current of the external control circuit must remain within the product's rated range.

Analog signal cables should preferably be twisted-pair shielded cables and kept at an appropriate distance from power cables and variable-frequency drive output cables. A separation installation principle of more than 300mm can be referenced in engineering applications; where strong interference sources cannot be avoided, use metal cable trays for isolation or add signal isolators.

Common causes of signal attenuation or fluctuation in remote pressure transmitters include excessively long wiring, insufficient wire diameter, oxidized terminals, multipoint grounding of the shield layer, inadequate power supply, and incorrect analog module parameter settings. The actual current of the 4-20mA loop should be measured, and the load resistance should be verified to ensure it does not exceed the allowable range.

For wireless pressure transmitters, signal coverage, battery capacity, reporting interval, and on-site temperature must also be assessed. The higher the reporting frequency, the lower the ambient temperature, or the weaker the wireless signal, the shorter the battery life is generally. Critical safety control should not rely solely on wireless links; reliable local protection logic should be retained.

What to Do When the Reading of a Local-Display Pressure Transmitter Is Inaccurate

First confirm whether the pressure source is genuinely stable, then compare it with a calibrated standard pressure gauge or pressure calibrator. If the deviation is basically consistent across the full measuring range, check the zero setting; if the deviation increases as pressure rises, check the full-scale calibration, sensor element condition, and whether the measuring range selection is appropriate.

If the displayed value fluctuates, investigate media pulsation, pipeline vibration, loose connections, and electromagnetic interference. At locations with obvious pulsation, such as hydraulic pump and air compressor outlets, a damper, buffer tube, or throttle orifice can be added; the instrument installation location should also avoid severe vibration sources whenever possible.

If relay alarm operation is abnormal, check the high and low limits, hysteresis, delay, forward or reverse mode, and relay status in the device menu. Then use a multimeter to test the continuity relationship among COM, NO, and NC to determine whether the issue is caused by parameters, external wiring, or relay contact failure.

After wiring and parameter settings are completed, it is recommended to perform acceptance item by item according to the following five steps: “power supply check, display check, analog signal verification, relay operation, and interlocked equipment response.” If you require a relay output pressure transmitter wiring diagram, selection parameters, or on-site technical support, contact Xi'an Shenghongchuang Instrumentation Co., Ltd. Provide the medium, measuring range, power supply, connection, and control logic, and professional personnel will match a suitable industrial sensing and measurement-control solution.