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2026-09-11
Why is a pressure gauge installed alongside a pressure transmitter on the same pipeline? In some cases, a pressure switch is added as well. When first encountering these instruments, it is easy to confuse them; while they all measure pressure, they serve different functions in the field. Once their respective purposes are distinguished, however, they are not difficult to understand.
Ⅰ. Pressure Gauges
Mechanical pressure gauges are commonly found at locations such as pump outlets, storage tanks, compressed air lines, and hydraulic stations. These gauges do not require an external power supply. When the pressure of the medium acts upon an internal elastic element, a mechanical linkage drives the pointer to rotate, displaying the pressure on the dial. Their advantage is that on-site personnel can simply walk up to the equipment to see the approximate current pressure. This local indication is highly convenient during equipment startup, shutdown, routine inspections, and maintenance. However, standard mechanical pressure gauges have a limitation: the pressure data remains only at the site. If the control room needs to monitor this pressure in real time, a pressure transmitter is required.
II. Pressure Transmitters
Pressure transmitters also measure pressure, but they convert the measured pressure into a standard electrical signal. A common standard in the field is the 4–20 mA signal. For instance, if a transmitter has a measurement range of 0–1.0 MPa, the output corresponds as follows: 0 MPa maps to 4 mA, 0.5 MPa to 12 mA, and 1.0 MPa to 20 mA. Once this signal is transmitted to a DCS or PLC, the control room can monitor the pressure in real time and perform functions such as trend recording, alarming, process regulation, or integration into interlocking logic. Therefore, the installation of a pressure transmitter generally indicates that the pressure parameter is required not only for local monitoring but also for use within the control system.
Ⅲ. Pressure Switches
The operating principle of a pressure switch differs from that of the two types of instruments previously discussed. Rather than continuously reporting the current pressure level to the system, it focuses on whether the pressure has reached a specific setpoint. For instance, consider a low-pressure switch with a setpoint of 0.3 MPa; when the process pressure drops to 0.3 MPa, the switch triggers an internal contact change and transmits a signal to a PLC, relay, or interlocking circuit. It outputs a discrete (on/off) signal. Pressure switches are commonly found in applications such as monitoring low lubricating oil pressure, low instrument air pressure, high hydraulic system pressure, and compressor pressure protection. Additionally, practical pressure switches involve a distinction between the actuation setpoint and the reset point—a concept known as hysteresis or deadband—meaning the contacts do not immediately revert to their original state the moment the pressure returns to the setpoint.
IV. Why are two types of pressure instruments installed in the field?
1. Why install a pressure gauge when a pressure transmitter is already present?
This is a common scenario in the field. Pressure transmitters primarily facilitate remote monitoring, whereas pressure gauges allow for on-site verification. Having both installed actually enhances operational convenience. For instance, if the DCS displays a pressure of only 0.2 MPa while the field pressure gauge reads around 0.6 MPa, the direction of the fault becomes clear; one can prioritize checking the pressure transmitter, impulse lines, power supply, wiring, and the AI (analog input) channel. Conversely, if both the DCS and the field gauge show approximately 0.2 MPa, the issue likely lies with the process or the equipment itself. Thus, pressure gauges serve as valuable references during field troubleshooting. Of course, such comparisons only help identify the general area of the fault; they cannot replace formal calibration, as both gauges and transmitters are susceptible to errors or malfunctions.
2. Why are pressure switches sometimes installed alongside pressure transmitters?
Functionally, once a pressure transmitter's signal enters the PLC, it is entirely possible to configure high/low-pressure alarms or even incorporate the signal directly into interlock logic. Yet, some equipment setups still include dedicated pressure switches. A straightforward reason for this is the difference in signal types: pressure transmitters provide continuous analog signals, whereas pressure switches provide discrete (on/off) signals. In critical protection loops, a dedicated pressure switch detection path is often retained to prevent the protection function from relying solely on a single pressure transmitter and its associated analog input circuit. Some pressure switches can be wired to relays, equipment protection systems, or independent interlock circuits. However, one should not automatically assume that a pressure switch constitutes "independent protection." If both the pressure switch and the pressure transmitter are ultimately connected to the same PLC, they still share parts of the system. The specific field configuration depends on equipment manufacturer requirements, interlock design, and the project's specific safety standards.
V. Look Beyond Just the Measurement Range When Selecting Equipment
When it comes to the actual selection phase, there are many factors to consider. Beyond just operating pressure and measurement range, one must verify the medium type, temperature, presence of pressure pulsations, suitability of wetted materials, process connection type, and any on-site requirements for explosion-proofing or ingress protection ratings. For instance, if the normal operating pressure is only 0.4 MPa but an excessively large range is selected, the instrument will still be accurate, yet it becomes difficult to discern small pressure fluctuations during on-site readings. While it is wise to allow for some headroom in the range, simply opting for the largest possible range is not advisable. Medium conditions are also critical. High-temperature steam requires consideration of condensation and thermal insulation; corrosive media necessitate careful selection of wetted materials; and viscous or clog-prone media require the use of diaphragm seals. In locations such as pump outlets or reciprocating compressors where pressure fluctuations are significant, one must consider damping, buffering, or liquid-filled gauges to prevent the pointer from constantly oscillating. These issues are frequently encountered in the field.
VI. Summary
When a pressure switch triggers an alarm or activates an interlock in the field, one cannot automatically assume there is an anomaly in the process pressure. The cause could be a genuine pressure change, or it could stem from a clogged pressure-sensing line, poor contact, a shift in the actuation setpoint, or wiring issues. In such cases, it is best to cross-reference the local pressure gauge reading, the pressure transmitter's trend data, and the pressure switch's status. If the three signals align and correlate, diagnosis is accelerated. Consider a centrifugal pump with a normal outlet pressure of 0.8 MPa, equipped with a pressure gauge, a pressure transmitter, and a low-pressure switch. The pressure gauge provides readings for on-site personnel; the pressure transmitter sends data to the DCS for display, trend recording, and alarming; and the low-pressure switch sends an actuation signal to the protection or interlock circuit when pressure drops to the setpoint. Although they all measure the same pressure, they serve different functions. Thus, seeing these three types of instruments installed together is not contradictory. To simplify the distinction: pressure gauges handle local readings; pressure transmitters handle remote transmission and continuous measurement; and pressure switches handle discrete actuation at a specific setpoint.
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