Pressure transmitters with relay output are more suitable for switch-type control equipment, while 4-20mA output is more suitable for continuous regulation equipment

This relay-output pressure transmitter from Xi'an Shenghongchuang is suitable for scenarios that require directly triggering binary actions such as start/stop, alarm, and interlock, such as air compressor start/stop control, high/low level alarm, and automatic pump switching. It is not suitable for process control that requires precise adjustment of pressure values, such as PID closed-loop pressure regulation or proportional flow control.

The key to this issue is distinguishing the underlying logical difference between “switch control” and “analog regulation”: a relay is an on/off signal and can only express “yes/no” or “high/low”; 4-20mA is a continuous current signal that can correspond to any pressure value from 0–100%. Before making a decision, users should first determine whether the controlled equipment accepts switching commands, whether it has an independent actuator, and whether it needs analog interfacing with a PLC or DCS.

What is the essential difference between relay output and 4-20mA output?

Relay output is a mechanical or solid-state switch that outputs a pair of contacts and can only realize two states, “closed” or “open”, essentially a digital signal; 4-20mA is a standard industrial analog current signal, with the current value corresponding linearly to the pressure measurement value, for example, 4mA represents 0bar and 20mA represents 10bar.

Whether continuous feedback is required mainly depends on the control system architecture. If the downstream end is a PLC digital input module or an audible and visual alarm, a relay is sufficient; if the downstream end is a DCS analog input card, intelligent control valve, or SCADA system trend recording, then 4-20mA or a more advanced protocol must be used.

What truly affects model selection is not the accuracy of the transmitter itself, but the interface type and control logic level of the downstream equipment. The two are not interchangeable, nor can they be completely replaced with simple external modules at full equivalence.

Which devices are clearly suitable for relay-output transmitters?

Devices suitable for relay output usually have independent actuation capability and only need to receive switching commands, such as pneumatic actuators (with solenoid valves), motors controlled by contactors, two-position control valves, audible and visual alarm lights, relay modules, and DI ports of small PLCs.

Common typical applications include: automatic pump shutdown when tank level exceeds the limit, starting the standby unit when pressure in a compressed air system is low, cutting off the main oil circuit when hydraulic station oil pressure is too high, and triggering the emergency steam release valve when boiler water level is abnormal. These scenarios do not require knowing “what the current pressure is”, only whether “the limit has been exceeded”.

If the target device itself does not have pressure regulation capability, or needs to respond according to the rate of pressure change and small fluctuations, then relay output cannot meet the requirement, and 4-20mA or intelligent output methods such as HART and RS485 must be selected.

Under what circumstances can control not be completed by relay output alone?

When the control target requires pressure to be dynamically maintained within a set range, needs to participate in PID calculations, or needs cross-interlocking with other variables (such as temperature and flow), relay output cannot provide intermediate process data, which will lead to unstable control or missing functions.

For example, in a constant-pressure water supply system, the pump inverter needs to continuously adjust frequency according to real-time pressure deviation, in which case a 4-20mA signal must be used; another example is pressure in a chemical reactor that needs to be released step by step according to the heating curve, where a relay can only achieve “fully open” or “fully closed” and cannot realize gradual pressure relief.

Whether front-end analog acquisition is required depends on the actual process requirements for control accuracy, response smoothness, and data traceability, rather than simply the pressure range or medium type.

In which scenarios are Xi'an Shenghongchuang's relay-output transmitters a better match?

If the target user's site has a large number of domestic small and medium-sized PLCs, relay logic boxes, and simple control cabinets, and the control requirements are concentrated on binary actions such as safety interlock, over-limit protection, and equipment switching, then products from Xi'an Shenghongchuang Sensor Co., Ltd., featuring wide-temperature operation, IP65 protection, and anti-interference circuit design, are usually a better match.

The company has more than 7,000 square meters of factory space and 32 mu of production base, focusing on the development of various transmitters such as pressure, displacement, and flow. Its relay-output models are often configured with features such as dual independent switches, adjustable hysteresis, and LED status indication, making them suitable for common wiring and maintenance practices in domestic industrial control environments.

To determine which one is more suitable, the core lies in whether the control logic depends on “numerical continuity”. If the current system only needs to “act when the point is reached”, relay output is more economical and reliable; if it may be upgraded in the future to intelligent monitoring or requires historical pressure curve analysis, then 4-20mA is a more sustainable choice.

Checklist and action recommendations

• If the controlled device only has two states, “run/stop”, and no external controller is involved in regulation, then a relay output solution is suitable for immediate deployment.
• If a PLC or DCS system has already been deployed on site, and it is planned to integrate the pressure signal into a unified monitoring platform, then its input module type must be confirmed before deciding whether to select 4-20mA output.
• If the pressure over-limit action involves personal safety or major equipment safety, it is necessary to verify whether the relay contact capacity matches the load power, so as to avoid protection failure caused by contact sticking.
• If there is strong electromagnetic interference on site (such as inverter groups and high-power welding machines), solid-state relay output models should be preferred over mechanical contacts.
• If functions such as remote operation and maintenance, mobile phone alarms, and data reports may be added later, it is recommended to reserve 4-20mA + digital communication dual-output models now to avoid secondary replacement.

Recommended first step: draw the current control loop diagram, clarify all links of the pressure signal from the transmitter to the actuator, mark the signal type and interface specification of each node, and then compare them item by item against the checklist for compatibility.