The pressure sensor in a refrigeration system provides plausible values when the system is at standstill, but the signal fluctuates significantly during compressor operation. Another measuring instrument fails after a few months because of corrosion at the connection, while a pressure gauge fogs up so badly at low ambient temperatures that it is barely readable.
Such problems are not unusual in refrigeration systems, heat pumps and industrial cooling systems. The pressure measuring point must simultaneously cope with refrigerant, entrained oil, low temperatures, condensation, compressor pulsations and, in some cases, considerable pressure spikes.
Selecting the instrument based solely on measuring range and thread is therefore not sufficient.
Key factors include whether the measuring point is on the high-pressure or low-pressure side, the refrigerant used, the maximum possible system pressure, temperature range, materials, seals, electrical protection rating, pulsation load and the intended installation conditions.
Suitable industrial sensors can be found under Pressure Sensors and Differential Pressure Sensors. Mechanical and digital indicators are grouped under Pressure Gauges and Digital Pressure Gauges.
Table of Contents
- Why is pressure measurement in refrigeration systems demanding?
- Distinguishing between the high-pressure and low-pressure sides
- Correctly checking refrigerant compatibility
- Considering refrigerant and oil together
- Selecting the measuring range and pressure reserve correctly
- Recognising compressor pulsations
- Distinguishing pressure spikes from normal operating pressure
- Using damping correctly
- Accounting for low temperatures and temperature changes
- Avoiding condensation at the sensor and connection
- Planning the installation position and connecting line correctly
- Selecting pressure gauges for refrigeration systems correctly
- Do not confuse pressure with saturation temperature
- Considering service connections and leak tightness
- Testing and calibrating the pressure measuring point
- Systematic troubleshooting
- Practical example on a heat pump
- Typical errors
- Which measuring instruments are suitable?
- Conclusion
- Frequently asked questions
Why is pressure measurement in refrigeration systems demanding?
A refrigeration system does not provide a calm pressure measuring point with a uniform load.
Depending on the measuring location, several conditions may occur simultaneously:
- significantly different pressure levels,
- very low or elevated medium temperatures,
- rapid pressure changes when the compressor starts,
- periodic compressor pulsations,
- short-term pressure spikes,
- refrigerant in gaseous or liquid form,
- entrained compressor oil,
- condensation or icing on cold pipes.
In addition, different refrigerants have very different chemical and thermodynamic properties.
A pressure sensor that operates without problems in an ordinary water application is therefore not automatically suitable for a refrigeration system.
Distinguishing between the high-pressure and low-pressure sides
The first important question is:
Where in the refrigeration circuit is the pressure being measured?
| Measuring Point | Typical Pressure Situation | Special Load |
|---|---|---|
| Compressor suction side | comparatively low pressure, in some cases close to atmospheric pressure or below | low temperatures, pulsations, possible condensation |
| Compressor discharge side | high pressure | strong pulsations, pressure spikes, elevated temperature |
| Condenser or gas cooler | high or system-specific pressure | temperature changes and dynamic operation |
| Liquid line | usually a comparatively stable high-pressure range | liquid refrigerant, possible pressure surges |
| Evaporator | low-pressure range | very low medium and surface temperatures |
A sensor on the suction side therefore often requires a completely different measuring range from a sensor installed directly downstream of the compressor.
Particularly for low-pressure measurements, it must also be clarified whether gauge pressure, absolute pressure or a specific reference of the measuring cell is required.
This selection must match the evaluation in the controller or PLC.
Correctly checking refrigerant compatibility
The most important rule when selecting materials is:
Media compatibility must be checked for the specific refrigerant.
This includes all wetted components:
- measuring diaphragm,
- pressure connection,
- weld seams,
- seals,
- any diaphragm seals,
- adapters and valves.
The general specification “stainless steel” is not sufficient for this purpose.
Refrigeration systems may use, for example:
- fluorinated refrigerants,
- hydrocarbons,
- R717 or ammonia,
- R744 or CO₂,
- various refrigerant blends.
These media place different requirements on pressure rating, materials and sealing.
When ordering, it is therefore not sufficient to specify only “pressure sensor 0 to 40 bar”. At least the following should also be stated:
- refrigerant,
- measuring range,
- maximum possible pressure,
- medium temperature,
- ambient temperature,
- process connection.
Considering refrigerant and oil together
In a real refrigeration system, a pressure sensor is often not exposed exclusively to pure refrigerant.
Compressor oil can be transported through the circuit and reach the measuring point.
When checking chemical resistance, the combination of:
refrigerant plus the lubricant used
should therefore be considered.
The following are particularly relevant:
- seal materials,
- elastomers,
- plastics in the process connection,
- any coatings.
A seal material that appears suitable for a refrigerant in principle may behave differently in combination with temperature and oil.
The specific resistance should therefore be checked using the manufacturer specifications for the sensor, refrigerant and oil.
Selecting the measuring range and pressure reserve correctly
The normal operating pressure is not automatically the correct full-scale value for the sensor.
The following must be taken into account:
- normal operating pressure,
- maximum operating condition,
- start-up processes,
- standstill pressure,
- pressure spikes,
- permissible sensor overload.
A sensor should not operate continuously directly at the upper end of its measuring range during normal operation.
At the same time, an excessively large measuring range is unfavourable if it reduces the resolution required for control or diagnostics.
CO₂ refrigeration systems require particular attention. These can reach significantly higher pressure levels than many conventional refrigeration circuits.
A 40 bar sensor designed for a conventional refrigeration system must therefore not be used in a CO₂ system without further verification.
The following are always decisive:
measuring range plus permissible overpressure plus the actual maximum possible system pressure.
Recognising compressor pulsations
Compressors do not generate a completely uniform pressure.
Depending on compressor type, speed, piping system and load, periodic pressure fluctuations can occur.
Typical indications include:
- a vibrating pressure gauge pointer,
- a rapidly fluctuating sensor signal,
- unstable control behaviour,
- different values at different sampling rates,
- strong fluctuations directly at the compressor but considerably calmer values further away.
These pulsations are not necessarily measurement errors.
The sensor may actually be detecting a genuinely dynamic pressure profile.
The decisive question is therefore:
Should the dynamic pressure profile be measured, or is a stable average process pressure required?
The necessary damping depends on this.
Distinguishing pressure spikes from normal operating pressure
In addition to periodic pulsations, short pressure spikes can occur.
Possible causes include:
- compressor start and stop,
- fast-switching valves,
- sudden changes in flow,
- liquid hammer,
- switching operations,
- defrosting or control processes.
A slow mechanical pressure gauge may not indicate such spikes at all.
A fast electronic pressure sensor, on the other hand, may detect them.
This can create the impression that the electronic sensor is “more unstable” than the mechanical pressure gauge.
In reality, the two instruments simply measure with different dynamic response characteristics.
If sensors are repeatedly overloaded, it should therefore be checked whether the actual peak pressure exceeds the intended measuring range.
Using damping correctly
For strong pulsations, mechanical or electronic damping can be useful.
Possible solutions include:
- restrictor or snubber,
- a small suitable measuring line,
- integrated pulsation damping in the sensor,
- digital filtering in the sensor, controller or PLC,
- a suitable installation position with a lower pulsation level.
The following principle applies:
As much damping as necessary, but no more than required.
Excessive damping can cause:
- rapid process changes to be detected too late,
- control loops to respond sluggishly,
- critical pressure spikes to remain hidden.
The damping must therefore match the purpose of the measuring point.
For a simple operating display, the measurement can often be smoothed more strongly than for a protection or control function.
Accounting for low temperatures and temperature changes
In a refrigeration system, at least two temperatures must be considered separately:
- medium temperature,
- ambient temperature of the measuring instrument.
A sensor may, for example, be installed on a very cold suction line while its electrical connection is surrounded by warm, humid room air.
When the system is switched off, the temperatures may then change rapidly again.
The permissible temperature range must therefore be observed for:
- measuring cell,
- electronics,
- seals,
- connector,
- cable.
For mechanical pressure gauges, the case filling must additionally be suitable for the low ambient temperature.
An unsuitable filling liquid can become very viscous at low temperatures and significantly slow down the pointer movement.
Avoiding condensation at the sensor and connection
Particularly on the low-pressure side, the surface of a pipe can be significantly colder than the surrounding air.
If its temperature falls below the dew point of the ambient air, condensation forms.
This can affect not only the pipe but also:
- sensor connection,
- plug connector,
- cable gland,
- pressure gauge housing,
- terminal boxes.
The appropriate ingress protection rating and electrical connection design should therefore be taken into account during installation.
It is also advisable to:
- route cables so that water cannot run towards the connector,
- install and seal plug connectors correctly,
- avoid damaged cable glands,
- avoid installing measuring instruments unnecessarily in areas where condensate can collect.
For very cold pipes, complete insulation of the measuring point must also be planned.
Planning the installation position and connecting line correctly
The pressure sensor should preferably be installed so that the measuring point remains accessible for service and unnecessary leakage points are avoided.
The following should be avoided, for example:
- long adapter chains,
- poorly supported measuring instruments,
- tight pipe loops with high mechanical stress,
- installation directly on strongly vibrating compressor housings if a calmer measuring point is available.
In the presence of strong vibration, a short external measuring line or mechanically decoupled installation may be useful.
However, the measuring line also changes the dynamic behaviour of the measuring point.
A long, thin line can significantly damp pressure pulsations. This may be undesirable for rapid pressure spike measurement.
The position of a shut-off or service valve must also be taken into account. If the sensor is trapped behind a closed valve, it may indicate a completely different pressure from the actual process.
Selecting pressure gauges for refrigeration systems correctly
Mechanical pressure gauges are still frequently used in refrigeration systems for local indication and service work.
Important selection criteria include:
- refrigerant compatibility,
- pressure range,
- temperature range,
- pulsation load,
- case filling,
- connection position,
- material of the wetted parts.
For strong compressor pulsations, liquid filling of the case is often useful.
It reduces pointer flutter and mechanical loading of the measuring mechanism.
At low ambient temperatures, however, the correct filling liquid must be selected.
Special low-temperature versions are required for very cold applications.
A standard glycerine-filled pressure gauge must therefore not automatically be used at arbitrarily low temperatures.
Do not confuse pressure with saturation temperature
Many conventional refrigerant pressure gauges feature additional temperature scales for specific refrigerants alongside the pressure scale.
These are based on the relationship between saturation pressure and saturation temperature.
However, this second scale does not mean that the pressure gauge measures the actual pipe or gas temperature.
It merely derives the saturation temperature associated with the measured pressure for the respective refrigerant.
Actual temperature measurements are therefore normally also required for superheat or subcooling calculations.
In addition, the pressure-temperature relationship used must match the refrigerant exactly.
After changing the refrigerant, an existing temperature scale may therefore become unusable even though the pressure indication itself remains correct.
Considering service connections and leak tightness
Every additional connection increases the number of potential sealing points.
For a permanently installed pressure measuring point, an unnecessarily complex arrangement such as:
service connection → adapter → reducer → extension → sensor
should therefore be avoided.
A short and clearly specified connection is preferable.
The following must be checked during installation:
- thread type,
- sealing principle,
- seal material,
- permissible tightening torque,
- pressure rating of all adapters,
- refrigerant compatibility.
Protective caps and service valves are also part of the sealing chain.
Even a small leak can affect both system operation and pressure measurement in the long term.
Testing and calibrating the pressure measuring point
If a pressure measuring point provides implausible values, the sensor should not be replaced immediately.
The measured value can first be compared with a suitable reference instrument.
A suitable test procedure is:
- Document the system condition.
- Select a reference instrument with a suitable pressure range.
- Safely isolate the measuring point from the process or test it via a designated service connection.
- Compare several pressure points.
- Assess zero point and span.
- Compare the measured value with increasing and decreasing pressure.
- Evaluate dynamic pulsations separately.
A static calibration can show whether the sensor and evaluation electronics are fundamentally operating correctly.
However, it does not automatically reproduce the dynamic conditions directly at a running compressor.
A sensor can therefore work perfectly on a pressure calibrator and still exhibit abnormal behaviour when installed because of pulsations or pressure spikes.
Systematic troubleshooting
- Identify the refrigerant: Document the medium and, where applicable, the oil used.
- Identify the measuring point: Clearly determine whether it is on the high-pressure or low-pressure side.
- Check the measuring range: Compare operating pressure and possible pressure spikes with the sensor design.
- Check the temperature: Record medium and ambient temperature.
- Check the materials: Verify the compatibility of wetted parts and seals.
- Observe the signal at standstill: Check zero point or static pressure.
- Start the compressor: Observe any fluctuations that occur.
- Assess pulsations: Determine whether they are real or electrically induced.
- Check damping: Document any existing snubber, filter or PLC filtering.
- Check for condensation: Inspect connectors, cables and housings.
- Check the connection for leakage: Inspect adapters and service connections.
- Perform a reference measurement: Statically compare the sensor with a suitable reference instrument.
Practical example on a heat pump
An industrial heat pump has an electronic pressure sensor installed on the discharge side of the compressor.
When the system is at standstill, the sensor indicates a plausible value.
After the compressor starts, however, the PLC signal fluctuates strongly.
An electrical problem is initially suspected.
The inspection shows:
- stable supply voltage,
- correct wiring,
- sensor behaves normally during static reference testing,
- strong fluctuations occur only while the compressor is running.
A high-speed recording of the raw signal shows periodic pressure fluctuations.
The measuring point is located directly downstream of the compressor and is in fact exposed to strong pulsations.
For control purposes, however, the instantaneous pressure of every individual pulsation is not required. A stable process pressure is needed instead.
The measuring point is therefore equipped with pulsation damping suitable for the application, refrigerant and pressure rating.
The raw signal is then checked again.
| Condition | Observation |
|---|---|
| Compressor off | stable pressure value |
| Compressor on without damping | strong periodic fluctuation |
| Compressor on with suitable damping | more stable measured value while maintaining sufficient dynamic response |
The original sensor was therefore not defective. It had simply detected the actual dynamic pressure profile.
The solution was to correctly match the measuring point, damping and required measurement dynamics.
Typical errors
| Error | Possible Consequence | Suitable Corrective Action |
|---|---|---|
| Sensor selected only according to pressure range | Refrigerant or seal is incompatible | Check medium and materials separately |
| Oil content not considered | Incorrect seal material selected | Assess refrigerant and lubricant together |
| Sensor on high-pressure side sized too close to normal operating pressure | Overload during pressure spikes | Take maximum pressure and overload limit into account |
| CO₂ system designed like a conventional refrigeration system | Measuring range or pressure rating too low | Check the system-specific pressure level |
| Fluctuating sensor signal immediately interpreted as a defect | Actual compressor pulsations are overlooked | Analyse the dynamic raw signal |
| Damping selected too strong | Important rapid pressure changes are delayed | Match damping to the measurement task |
| Standard pressure gauge used at very low temperature | Sluggish indication or poor readability | Select a low-temperature version |
| Condensation at the electrical connection ignored | Corrosion or electrical faults | Use suitable ingress protection and cable routing |
| Long adapter chain at the service connection | More potential leakage points and mechanical loading | Simplify the connection arrangement |
| Refrigerant temperature scale interpreted as actual temperature | Incorrect process assessment | Measure actual temperature separately |
| Only static calibration performed | Dynamic problem remains undetected | Also check behaviour during compressor operation |
Which measuring instruments are suitable?
Pressure sensors and pressure transmitters
Under Pressure Sensors and Differential Pressure Sensors, various sensors are available for gauge, absolute and differential pressure.
For refrigeration systems, the following should be checked in particular:
- measuring range,
- overload resistance,
- wetted materials,
- seal materials,
- medium temperature,
- ambient temperature,
- ingress protection rating,
- output signal,
- pulsation and vibration load.
Depending on the application, 4 to 20 mA sensors, voltage outputs or digital interfaces may be suitable.
In the presence of strong pressure pulsations, sensors with suitable integrated damping or external pulsation damping can be used.
Pressure gauges and digital pressure gauges
Under Pressure Gauges and Digital Pressure Gauges, you will find instruments for local indication, service and reference measurements.
Mechanical pressure gauges are particularly suitable for permanently visible local indication.
Depending on the version, digital pressure gauges offer additional functions such as:
- minimum and maximum values,
- peak detection,
- higher resolution,
- data logging,
- digital interfaces.
For dynamic refrigeration systems, it is particularly important to determine whether the instrument is intended to measure an average value or rapid pressure spikes.
Accessories for pulsation damping
Depending on the application and refrigerant, the following can be used for highly dynamic measuring points:
- snubbers,
- restrictors,
- suitable measuring lines,
- shut-off and service valves.
All additional components must meet the same requirements for pressure rating, temperature and media compatibility as the actual sensor.
ICS Schneider Messtechnik provides support in selecting pressure sensors, pressure gauges, process connections, materials, seals and pulsation damping for refrigeration systems, heat pumps and industrial cooling systems.
Conclusion
Reliable pressure measurement in refrigeration systems does not begin with selecting the thread, but with considering the entire measuring point.
First, the refrigerant, lubricant, high-pressure or low-pressure side, maximum system pressure and temperature range must be known.
Materials and seals must be suitable for the specific refrigerant. A general material specification alone is not sufficient confirmation of media compatibility.
On the compressor discharge side, pulsations and short pressure spikes must be given particular attention. A strongly fluctuating sensor signal is therefore not automatically an indication of a defective sensor.
If a smoothed process pressure is required for control or indication, suitable pulsation damping can be used. However, it must not be so strong that safety- or control-relevant pressure changes are concealed.
Low temperatures and condensation must also be taken into account on cold pipes. The electrical connection, cables, seals and, where applicable, the pressure gauge filling must be suitable for these conditions.
Service connections should also be kept as short, leak-tight and mechanically clean as possible. Every additional connection increases the number of potential leakage points.
Finally, when measured values are implausible, a distinction should be made between a static sensor fault and dynamic process loading. A reference calibration checks zero point and span, while high-speed recording during compressor operation makes pulsations and pressure spikes visible.
Frequently asked questions about pressure measurement in refrigeration systems
Can any stainless-steel pressure sensor be used with refrigerants?
No. Media compatibility must be checked for the specific refrigerant, lubricant and all wetted materials and seals.
Why does my pressure sensor fluctuate while the compressor is running?
The compressor can generate actual pressure pulsations. It should therefore first be checked whether the fluctuation is genuinely present in the process or is caused by electrical interference.
How can compressor pulsations be damped?
Depending on the application, snubbers, restrictors, suitable measuring lines or electronic filters can be used. The damping must match the required measurement dynamics.
Why should I not size a pressure sensor too close to the normal operating pressure?
In addition to normal operating pressure, the sensor must also withstand the maximum possible operating conditions and short-term pressure spikes.
Do I need special sensors for CO₂ refrigeration systems?
Because of the system-specific high pressure levels, the measuring range and pressure resistance must be selected particularly carefully. Explicit compatibility with R744 must also be verified.
What must I consider when using ammonia?
Only a version whose manufacturer has approved the wetted materials and seals for R717 should be used.
Why does a pressure gauge fog up on a cold pipe?
If the temperature falls below the dew point, moisture can condense on or inside an inadequately protected instrument. A suitable instrument design and case filling can improve operational reliability and readability.
Is the temperature scale on a refrigerant pressure gauge an actual temperature measurement?
No. It derives a saturation temperature from the measured pressure for a specific refrigerant. The actual pipe or gas temperature must be measured separately.
Can a sensor be statically correct but still provide incorrect values during operation?
Yes. A static calibration may be perfectly correct while the installed measuring point is affected by pulsations, pressure spikes, vibration or temperature.
Should I install the measuring point directly at the compressor?
Only if the pressure actually needs to be measured there. Vibration and pulsations can be particularly strong directly at the compressor. For a simple process pressure measurement, a more suitable measuring location or appropriate mechanical decoupling may be preferable.