A digital pressure gauge displays the current pressure directly at the measuring point. If the instrument also has one or more switching outputs, it can trigger an alarm at defined pressure limits, enable a pump or transmit a signal to a PLC.
This combines indication and simple limit monitoring in a single instrument. This can reduce wiring and equipment requirements, particularly on test benches, service systems, hydraulic power units and decentralised machines.
However, a switching output does not automatically make a digital pressure gauge a safety-related pressure limiter. Switching capacity, hysteresis, response time, measurement accuracy and fault behaviour must be suitable for the application. Safety-related shutdowns or particularly regulated media may still require dedicated pressure monitors, pressure limiters or appropriately approved safety devices.
Table of Contents
- What is a digital pressure gauge with a switching output?
- When is direct limit monitoring useful?
- Distinguishing the switching point from the reset point
- Setting the hysteresis correctly
- MIN, MAX and window monitoring
- Delays and filtering to prevent frequent switching
- Distinguishing relay, PNP and NPN outputs
- Checking the switching capacity and connected load
- Limit monitoring on a test bench
- Pump enable and pressure monitoring
- Local alarm and signalling functions
- Connection to a PLC
- Combining switching and analogue outputs
- Digital pressure gauge or pressure switch?
- Limitations for protective and safety functions
- Selecting the measuring range and accuracy
- Considering pressure peaks and dynamic processes
- Planning installation and electrical connection
- Commissioning and testing the switching points
- Documenting settings traceably
- Typical selection and configuration errors
- Practical example: Hydraulic test bench
- Information required for selection
- Which products are suitable?
- Conclusion
- Frequently asked questions
What is a digital pressure gauge with a switching output?
A digital pressure gauge with a switching output combines three functions:
- electronic pressure measurement,
- digital indication of the current measured value,
- an electrical response when defined pressure limits are reached.
The pressure sensor measures the process pressure. The electronics continuously compare the measured value with the configured limits. If a limit is exceeded or undershot, the output changes its switching state.
Depending on the instrument, the output may be designed as:
- a volt-free relay contact,
- a PNP transistor output,
- an NPN transistor output,
- an electronic switching output,
- an alarm signal to a PLC
.
Many instruments offer two switching outputs. These can be used, for example, to monitor a lower and upper pressure limit or to implement a pre-alarm and shutdown limit.
When is direct limit monitoring useful?
A digital pressure gauge with a switching output is particularly useful when the pressure must be visible locally and a simple electrical response must also be triggered.
Typical applications include:
- monitoring the minimum pressure of a hydraulic system,
- maximum-pressure alarm on a test bench,
- pump enable after the required inlet pressure has been reached,
- warning when accumulator pressure falls,
- monitoring a pneumatic clamping system,
- window monitoring between minimum and maximum pressure,
- local indication with an additional PLC signal,
- simple two-point control in service systems,
- monitoring filters or compressed-air supplies,
- stopping a test sequence if the permissible test force or test pressure is exceeded.
Direct limit monitoring reduces the equipment required when a separate pressure transmitter, digital indicator and limit relay are not needed.
It is less suitable when extensive control logic, several interdependent limits, a safety-related shutdown or particularly high switching frequencies are required.
Distinguishing the switching point from the reset point
The switching point is the pressure value at which the output changes its state. The reset point is the value at which the output returns to its original state when the pressure changes in the opposite direction.
Example of maximum-pressure monitoring:
- switching point: 8.0 bar with rising pressure,
- reset point: 7.5 bar with falling pressure.
When the pressure rises to 8.0 bar, the output switches. It is reset only when the pressure falls below 7.5 bar again.
For minimum-pressure monitoring, the logic may be reversed:
- alarm when the pressure falls below 4.0 bar,
- return to the normal state only above 4.5 bar.
The following must therefore always be clearly defined during configuration:
- rising or falling pressure,
- upper or lower limit,
- normally open or normally closed function,
- switching point,
- reset point or hysteresis.
Setting the hysteresis correctly
Hysteresis is the difference between the switching point and the reset point.
Hysteresis = switching point − reset point
In the example with a switching point of 8.0 bar and a reset point of 7.5 bar, the hysteresis is 0.5 bar.
Sufficient hysteresis prevents the output from continuously switching on and off in response to small pressure fluctuations.
Insufficient hysteresis can cause:
- chattering relay contacts,
- frequent switching of valves or contactors,
- unstable PLC signals,
- increased contact wear,
- unstable machine operation.
Excessive hysteresis, by contrast, can mean that the system is not enabled again until the pressure has fallen or risen considerably further after triggering.
The hysteresis should therefore be greater than the normal pressure fluctuations, but no larger than required by the process.
MIN, MAX and window monitoring
MAX monitoring
With MAX monitoring, the output switches as soon as the pressure exceeds an upper limit.
Typical applications include:
- overpressure warning,
- aborting a pressure test,
- switching off a pump,
- sending a signal to a PLC,
- opening a relief valve as part of an operational control system.
MIN monitoring
With MIN monitoring, a signal is triggered when the pressure falls below a defined value.
Typical applications include:
- minimum-pressure monitoring,
- dry-running or supply monitoring,
- enabling a machine only when sufficient pressure is available,
- warning of pressure loss,
- monitoring a hydraulic accumulator.
Window monitoring
Window monitoring uses a lower and an upper limit. The pressure is considered permissible only within this range.
Example:
- lower limit: 4 bar,
- upper limit: 6 bar,
- permissible operating range: 4 to 6 bar.
Depending on the required logic, an output can be active either inside or outside the permissible window.
Window monitoring is suitable, for example, for:
- pneumatic clamping systems,
- machine supply pressure,
- leak and load testing,
- hydraulic holding circuits,
- monitoring regulated pressure stages.
Delays and filtering to prevent frequent switching
In addition to hysteresis, some instruments provide a switching delay or measured-value filtering.
An on-delay means that the limit must remain exceeded for a defined period before the output switches.
This allows brief pressure peaks that are not relevant to the process to be ignored.
An off-delay keeps the output active for a defined period after the measured value leaves the alarm range. This can be useful when an alarm must be reliably detected or a machine must not restart immediately.
However, filtering and delays must not be used without careful consideration. In the event of a genuine overpressure hazard, strong damping can cause the output to respond too late.
| Setting | Advantage | Possible disadvantage |
|---|---|---|
| Large hysteresis | Prevents frequent switching | Large difference between triggering and resetting |
| Strong filtering | Stable indication and switching function | Rapid pressure peaks are delayed or not detected completely |
| On-delay | Brief limit violations are ignored | Response to an actual limit violation occurs later |
| Off-delay | The alarm remains clearly detectable | Enable is delayed |
Distinguishing relay, PNP and NPN outputs
Relay output
A relay output provides an electrically isolated contact. It is frequently designed as a changeover contact with a common terminal, normally closed contact and normally open contact.
Advantages include:
- volt-free isolation,
- flexible use with AC and DC circuits within the permissible ratings,
- simple adaptation to different control systems,
- normally open, normally closed or changeover function.
Mechanical relays have a limited electrical and mechanical service life. An electronic output may be more suitable for very frequent switching.
PNP output
A PNP output switches the positive supply voltage to the PLC input or load. It is commonly used in many European machine-control systems.
NPN output
An NPN output switches the current towards 0 V. It must be compatible with the input circuit and common reference voltage of the controller.
PNP and NPN describe the electrical output circuit. Normally open and normally closed describe the logical function. A complete specification would therefore be, for example:
PNP output, normally open, active when 8 bar is exceeded.
Checking the switching capacity and connected load
The description “relay output” does not mean that every load can be switched directly.
The following must be checked:
- maximum switching voltage,
- maximum switching current,
- maximum switching power,
- AC or DC load,
- resistive, inductive or capacitive load,
- inrush current of the load,
- expected switching frequency.
Particularly critical loads include:
- solenoid valves,
- contactors,
- relays with large coils,
- motors and pumps,
- signalling devices with high inrush currents,
- long capacitive cables.
Inductive loads generate voltage peaks when switched off. Depending on the voltage and load, a flyback diode, RC network, varistor or external interposing relay may therefore be required.
Motors and larger pumps should not be switched directly through the instrument output. In such cases, the switching output controls a PLC input, an interposing relay or a suitable contactor.
Limit monitoring on a test bench
On a test bench, a digital pressure gauge can display the current test pressure and trigger defined process steps at the same time.
Examples include:
- starting the hold time after the test pressure has been reached,
- enabling a leak test,
- warning when the permissible test pressure is exceeded,
- aborting the test sequence in the event of pressure loss,
- switching between filling, holding and depressurisation phases.
For an accurate test, the switching accuracy must be assessed separately from the indication accuracy. The sampling rate, filtering and relay response time also influence the actual trigger point.
If the digital pressure gauge forms part of a documented test procedure, the indication and switching points should be checked regularly using a suitable pressure reference.
Pump enable and pressure monitoring
A digital pressure gauge with two outputs can, for example, monitor a pump within a defined pressure range.
Possible functions include:
- switching on the pump when the pressure falls below a lower limit,
- switching off the pump when the upper limit is reached,
- triggering an alarm if no pressure builds up while the pump is operating,
- enabling the machine only when sufficient supply pressure is available.
With simple two-point control, the limits must be sufficiently far apart. Otherwise, the pump will switch on and off continuously in response to small pressure fluctuations.
The following must also be considered:
- accumulator volume,
- pump overrun,
- pressure pulsation,
- non-return valves,
- delay times,
- maximum switching frequency of the pump.
An additional independent protective device may be required for dry-running, overpressure or personal protection.
Local alarm and signalling functions
A switching output can activate a warning light, buzzer or central alarm.
A local indication is useful when:
- operators must respond directly at the machine,
- the pressure value and alarm cause must be visible locally,
- no higher-level PLC is available,
- a decentralised service system is being monitored.
In a noisy machine environment, an audible alarm may not be sufficient. Conversely, a small warning light at a remote measuring point may go unnoticed.
The alarm and signal transmission must therefore match the actual operating procedure.
Connection to a PLC
For connection to a PLC, the output must be electrically compatible with the digital input.
The following must be checked:
- PNP, NPN or relay output,
- supply voltage,
- common reference potential,
- input current and switching threshold,
- normally open or normally closed logic,
- cable-break monitoring,
- galvanic isolation,
- pin or conductor assignment.
The PLC should not process only the normal switching state. Depending on the application, additional states may be considered:
- instrument without power supply,
- cable break,
- sensor fault,
- measuring range exceeded,
- configuration locked,
- maintenance or test mode active.
A normally closed principle can be advantageous when a broken cable must also produce a fault signal. Whether this is actually achieved depends on the complete wiring and PLC logic.
Combining switching and analogue outputs
In addition to switching outputs, some digital pressure gauges also provide a continuous output signal such as 4–20 mA or 0–10 V.
This allows two tasks to be performed separately:
- switching output for alarm, enable or shutdown,
- analogue output for measured-value indication, data logger, PLC or control system.
This combination is useful when a limit is to be generated locally by the instrument independently of the PLC, while the complete pressure profile is also recorded or visualised.
A UPS4E current-loop calibrator can be used to test a 4–20 mA measuring circuit. It can measure the output current and inject defined current values into the analogue input of the controller.
However, current simulation does not test the pressure sensor or the actual pressure switching points. For this purpose, pressure must be generated at the gauge and compared with a suitable reference.
Digital pressure gauge or pressure switch?
| Criterion | Digital pressure gauge with switching output | Electronic pressure switch | Mechanical pressure switch |
|---|---|---|---|
| Local indication | Large, clearly readable pressure indication | Usually a compact digital display | Depending on the version, no indication |
| Limits | One or more programmable limits | Usually flexibly configurable | Mechanically adjustable or factory-set |
| Output | Relay, PNP/NPN and sometimes analogue signal | Usually transistor output, sometimes analogue signal or IO-Link | Mechanical switching contact |
| Typical application | Test bench, service system and local process indication | Machine building, hydraulics and pneumatics | Simple and robust limit monitoring |
| Switching frequency | Limited with relay outputs | Electronic outputs suitable for frequent switching | Depends on the contact and load |
| Power supply | Usually required for the display and output | Required | Depending on the design, not required |
A digital pressure gauge with a switching output is particularly suitable when a clearly visible local indication is the main requirement.
An electronic pressure switch is often the better choice when the instrument must be mounted compactly directly on a machine, switched frequently or configured via a modern automation interface.
A mechanical pressure switch can be useful when a simple volt-free contact is required without an electronic display.
Limitations for protective and safety functions
A conventional digital pressure gauge with a switching output must not automatically be regarded as a safety pressure limiter.
For a safety-related function, the following must be assessed, among other factors:
- approval and instrument category,
- fault behaviour of the sensor,
- failure of the supply voltage,
- contact failure,
- software and configuration,
- protection against manipulation,
- redundancy,
- test interval,
- the complete shutdown chain.
Steam systems, hot-water systems, combustible gases, pressure vessels, hazardous-area applications or legally regulated protective functions may require special pressure monitors or pressure limiters.
Password protection prevents unintended parameter changes, but it does not replace a functional-safety assessment.
Selecting the measuring range and accuracy
The measuring range should be selected so that the normal operating pressure is displayed with sufficient resolution while providing adequate reserve for expected pressure peaks.
A very large measuring range offers a high overload reserve but can:
- reduce resolution within the operating range,
- increase the absolute measurement deviation,
- make precise limit configuration more difficult.
A measuring range that is too small may be overloaded by normal pressure peaks.
At least the following must be distinguished during selection:
- normal operating pressure,
- upper and lower switching points,
- maximum possible process pressure,
- brief pressure peaks,
- permissible sensor overload,
- required indication and switching accuracy.
With an upper limit of 8 bar, a measuring range up to 10 bar may be too narrow if process-related pressure peaks of up to 12 bar can occur. The specific sensor must therefore be selected using its overload specifications.
Considering pressure peaks and dynamic processes
The digital pressure gauge switches only on the basis of the pressure value actually detected by its electronics.
The following are relevant in rapid processes:
- sensor measuring rate,
- display update rate,
- digital filtering,
- response time of the switching output,
- relay switching time,
- delay in the connected controller.
A very brief pressure peak can mechanically load the sensor without being present long enough to activate the display or switching output completely.
The following measures may be required in strongly pulsating hydraulic systems:
- suitable pressure damping,
- a measuring line with a small internal volume,
- an orifice or snubber,
- a higher measuring range,
- a faster sensor or peak detection,
- separate mechanical overpressure protection.
An orifice protects against rapid pressure shocks but also slows the response of the measuring instrument and switching output.
Planning installation and electrical connection
The digital pressure gauge should be installed so that the display remains clearly readable and the pressure connection is not subjected to mechanical stress.
The following must be considered:
- suitable process connection,
- permissible mounting position,
- media compatibility,
- temperature at the measuring instrument,
- vibration and pulsation,
- degree of protection,
- cable outlet and strain relief,
- accessibility for configuration.
During electrical installation, the power supply, relay contacts and signal lines must be clearly separated and documented.
The instrument must not be mechanically supported by its connection cable. Under strong vibration, installation via a flexible pressure line or remote measuring connection may be appropriate.
Commissioning and testing the switching points
Before the system is released for operation, the complete limit function should be tested using controlled pressure.
- Check the instrument data: Compare the measuring range, pressure type, output type and power supply with the ordering data.
- Check the zero point: Check the digital pressure gauge in an unpressurised condition and zero it if necessary.
- Document the switching logic: Clearly define normally open, normally closed, MIN, MAX or window operation.
- Increase the pressure slowly: Determine the actual switching point using a suitable reference.
- Reduce the pressure slowly: Determine the reset point.
- Calculate the hysteresis: Assess the difference between the switching and reset points.
- Repeat the test: Repeat the test several times.
- Check the PLC signal: Trace the signal through to the controller or control room.
- Check the connected function: Test the alarm, enable or shutdown.
- Secure the settings: Activate password protection or a configuration lock.
The pressure should be changed slowly near the switching point. If it changes too quickly, the response time, overrun and filtering can distort the determined switching value.
Documenting settings traceably
The switching points must not be stored only in the instrument. They should also be documented for maintenance, replacement and troubleshooting.
Complete documentation includes:
- instrument designation and serial number,
- measuring range and pressure unit,
- upper and lower switching point,
- reset points or hysteresis,
- MIN, MAX or window function,
- normally open or normally closed logic,
- output type and contact assignment,
- delay and filter settings,
- function within the PLC,
- date of the test,
- actual measured switching values.
When replacing the instrument, these settings can be transferred reproducibly to the replacement device and then verified.
Typical selection and configuration errors
The hysteresis is too small
The output continuously switches on and off in response to normal pressure fluctuations.
The switching point and reset point are confused
The system is reset too late or restarts unexpectedly.
A relay output switches a motor directly
The switching current and inrush current overload the contacts.
PNP and NPN are confused
The PLC input does not respond or an unsuitable electrical connection is created.
Normally open and normally closed functions are not documented
A cable break, maintenance operation or instrument replacement produces an incorrect system state.
The indication accuracy is assumed to be the switching accuracy
The response time, filtering and output tolerance are not considered.
An excessively large measuring range is selected
The pressure can be measured safely, but the limits cannot be assessed with the required resolution.
Brief pressure peaks are not considered
The process reaches damaging peaks even though the output does not switch or responds too late.
The digital pressure gauge performs an unassessed safety function
Failure of the power supply, electronics or configuration is not adequately considered within the protection concept.
No pressure test is performed after configuration
Wiring, unit or setting errors remain undetected until an actual fault occurs.
Practical example: Hydraulic test bench
A hydraulic test bench normally operates between 50 and 180 bar. The operator must be able to read the pressure directly at the test bench. Two additional functions are required:
- enable the test sequence from 50 bar,
- abort the test and trigger an alarm at 190 bar.
A digital pressure gauge with two relay outputs is used.
Output 1 is configured as the minimum-pressure enable:
- switching point with rising pressure: 50 bar,
- reset point with falling pressure: 45 bar.
Output 2 is configured as the maximum-pressure alarm:
- switching point with rising pressure: 190 bar,
- reset point with falling pressure: 175 bar.
Neither relay is connected directly to the pump or solenoid valve. They switch digital PLC inputs. The PLC then controls the hydraulic power unit and relief valve via suitable output modules and interposing relays.
During commissioning, the actual switching and reset points are tested using a calibrated pressure reference. This reveals that strong filtering delays the upper switching point during rapid pressure increases.
The filtering is reduced and the pressure ramp of the test bench is slowed before reaching the maximum pressure. The alarm then switches reproducibly within the defined tolerance.
A separate mechanical pressure-relief valve remains installed independently of the digital pressure gauge. It protects the hydraulic system if the sensor, power supply or controller fails.
Information required for selection
At least the following information is required when selecting a digital pressure gauge with a switching output:
- medium,
- pressure type: gauge pressure, absolute pressure or differential pressure,
- minimum and maximum operating pressure,
- possible pressure peaks,
- required measuring range,
- required accuracy and resolution,
- number of switching points,
- MIN, MAX or window monitoring,
- switching and reset values,
- required hysteresis,
- relay, PNP or NPN output,
- normally open, normally closed or changeover function,
- supply voltage,
- connected load or PLC input,
- additional analogue output,
- process and electrical connection,
- temperature, vibration and degree of protection,
- calibration and documentation requirements,
- operational or safety-related function.
A meaningful enquiry could read as follows:
Digital pressure gauge for hydraulic oil, measuring range 0 to 250 bar, accuracy of at least 0.5 % of full scale, two volt-free changeover relays, minimum-pressure enable at 50 bar with reset at 45 bar, maximum-pressure alarm at 190 bar with reset at 175 bar, 24 V DC supply, G 1/4 process connection and connection to 24 V PLC inputs.
Which products are suitable?
Digital pressure gauges and test pressure gauges
The digital pressure gauges / test pressure gauges category includes battery-operated digital pressure gauges, precision pressure gauges, data-logger versions and instruments with electrical output and switching functions.
Depending on the model, available functions include:
- Min./Max. memory,
- zero-point correction,
- data logger,
- relay or transistor outputs,
- 4–20 mA or 0–10 V outputs,
- RS485 interfaces,
- adjustable limits,
- different accuracy classes and measuring ranges.
IDM-K80 digital contact pressure gauge
The IDM-K80 digital contact pressure gauge combines a digital pressure display with two changeover relays.
Its main features include:
- two freely adjustable upper or lower limits,
- relay outputs as changeover contacts,
- adjustable switching differential,
- password protection against unintended changes,
- zero-point correction,
- selectable pressure units,
- accuracy of up to 0.25 % of full scale, depending on the version.
The IDM-K80 is particularly suitable for test benches, hydraulic power units and decentralised systems requiring local pressure indication with two volt-free limit contacts.
IPDM80-1 digital pressure gauge
The IPDM80-1 has a large LED display and two integrated relays.
Depending on the version, it also provides:
- Min./Max. storage,
- measuring ranges starting at low millibar pressures,
- capacitive ceramic measuring cell,
- optional 4–20 mA, 0–20 mA or 0–10 V output,
- stainless-steel process connection,
- IP65 degree of protection.
The instrument is suitable for applications in which the pressure must remain clearly readable from a greater distance and limits as well as a continuous output signal are required.
Pressure switches
The pressure switches category includes mechanical and electronic instruments for pressure monitoring in machines, pumps, compressors, hydraulic systems and pneumatic systems.
A pressure switch may be preferable to a digital pressure gauge when:
- compact machine integration is more important than a large display,
- very frequent switching is required,
- PNP, NPN or IO-Link functions are required,
- an accurate local indication is not required,
- a specific approval or instrument function is required.
Conclusion: Indication and limit functions can be combined effectively
A digital pressure gauge with a switching output is particularly suitable for applications in which the pressure must be clearly visible locally while also generating a simple limit signal.
Two outputs can be used, for example, to monitor minimum and maximum pressure, a pre-alarm and shutdown level or a permissible pressure window.
For stable operation, the switching point, reset point and hysteresis must match the actual pressure fluctuations in the system. Filters and delays can reduce unwanted switching, but must not prevent a required rapid response.
Relay, PNP and NPN outputs must be electrically compatible with the PLC and connected load. Pumps, motors, contactors and larger solenoid valves should generally be controlled through suitable interposing or power-switching devices.
A digital pressure gauge with a switching output does not automatically replace an approved pressure monitor, pressure limiter or mechanical overpressure protection device. Safety and protective functions must be assessed on the basis of the complete installation.
Before commissioning, the actual switching and reset points should be tested using controlled pressure and then documented clearly.
Frequently asked questions about digital pressure gauges with switching outputs
What is a digital pressure gauge with a switching output?
It measures and displays the pressure and changes the state of a relay or transistor output when a configured limit is reached.
What is the difference between the switching point and reset point?
The output is activated or deactivated at the switching point. The reset point determines when it returns to its original state as the pressure changes in the opposite direction.
Why is hysteresis required?
It prevents the output from switching continuously on and off in response to small pressure fluctuations around the limit.
Can a digital pressure gauge switch a pump directly?
Only if the voltage, current, inrush current and load type are explicitly within the permissible contact ratings. In practice, the output is usually connected to a PLC, interposing relay or contactor.
Can I monitor minimum and maximum pressure simultaneously?
Yes, provided that the instrument has two independently configurable outputs. This also allows window monitoring to be implemented.
Which is better: a relay or PNP output?
A relay provides a volt-free contact and is electrically flexible. A PNP output is particularly suitable for frequent switching with compatible 24 V PLC inputs.
Does a digital pressure gauge with a switching output replace a pressure switch?
It can perform the function of an operational pressure switch. A dedicated pressure switch may be more suitable for compact machine applications, very high switching frequencies or special approvals.
Is the instrument suitable for a safety shutdown?
Not automatically. For a safety-related shutdown, the approval, fault behaviour, power supply, controller and complete shutdown chain must be assessed.
How are the switching points tested?
The pressure is increased and reduced slowly using a suitable reference. The switching point, reset point, hysteresis and repeatability are documented.
Which information does ICS Schneider require for selection?
The required information includes the medium, pressure range, possible pressure peaks, required switching points, hysteresis, output type, switching logic, power supply, electrical load, process connection, accuracy, ambient conditions and intended function within the installation.