Pressure is one of the most important diagnostic variables in a hydraulic system. It shows whether a pump is delivering sufficient performance, whether a pressure relief valve is operating correctly, whether a filter is blocked or whether an actuator is being supplied with the intended operating pressure.
With a conventional pressure measurement, a line would often have to be opened, a pressure gauge screwed in and the system subsequently vented again. This takes time, causes oil loss and creates a risk of contamination or installation errors.
MINIMESS measuring points provide permanently installed access to the hydraulic system. With a suitable design, a digital pressure gauge, pressure sensor or measuring hose can be connected while the system is under operating pressure. The internal check valve only opens when the matching coupling is connected and closes automatically again when it is disconnected.
However, “measurable under pressure” does not mean that every type of work on the measuring point may be carried out while the system is operating. Installing, replacing or repairing the permanently installed test point always requires a safe, depressurised system condition. Only the designated measuring coupling may be connected under pressure and only within its permissible pressure, temperature and media limits.
This article explains how MINIMESS measuring points are planned, which installation locations provide meaningful results and what must be considered when selecting the measuring hose, pressure range, pressure-spike detection and safe service procedures.
Table of contents
- How a MINIMESS measuring point works
- What “connectable under pressure” actually means
- Planning measuring points during system design
- Selecting the correct installation location
- Considering the pressure range and pressure spikes
- Materials, seals and hydraulic fluid
- Microbore hose and measurement dynamics
- Digital pressure gauge or pressure sensor?
- Safe procedure for a pressure measurement
- Preventing leaks and contamination
- Typical measuring tasks in hydraulic systems
- Typical errors during temporary pressure measurements
- Practical example: Pressure loss at a hydraulic cylinder
- Which measuring instruments / products are suitable?
- Conclusion
- Frequently asked questions about MINIMESS measuring points
How a MINIMESS measuring point works
A MINIMESS test point is a compact measuring connection permanently installed in the hydraulic system. It contains a mechanically operated check valve.
As long as no matching measuring coupling is connected, the valve remains closed. When the measuring hose is screwed on or coupled, the valve is opened mechanically. The system pressure then passes through the microbore hose to the connected pressure gauge or pressure sensor.
When the coupling is disconnected, the valve closes again. This means that the actual hydraulic line does not have to be opened for routine measurements.
Depending on the design, the measuring point can also be used for other service tasks, for example:
- venting part of a hydraulic circuit
- filling or controlled draining of small fluid volumes
- connecting a pressure sensor for temporary data recording
- taking a fluid sample using suitable accessories
- simultaneous pressure and temperature measurement using a combined test point
However, the test point does not replace a conventional shut-off valve or a large filling or draining line. Microbore hoses are primarily intended for measurement, diagnostic and control tasks.
What “connectable under pressure” actually means
The main advantage of a MINIMESS measuring point is that a suitable measuring instrument can be connected while the hydraulic system is pressurised.
However, several conditions must be met:
- The test point, measuring coupling, hose and measuring instrument are rated for the existing pressure.
- The medium, temperature and sealing materials are compatible.
- The test point and coupling belong to the same connection series.
- The components are undamaged, clean and correctly installed.
- The connection is made by trained personnel.
Normal installation work on the threaded system connection of the test point must not be carried out under pressure. If the permanently installed test point has to be replaced, tightened or removed, the affected part of the system must be safely shut down, secured against restarting and completely depressurised.
Even after disconnection, pressure may remain trapped in the measuring hose or connected instrument. This residual volume must be relieved in a controlled manner using a suitable device. Loosening an arbitrary threaded connection is not a safe method.
Planning measuring points during system design
Measuring points added at a later stage are often installed wherever space happens to be available rather than where the measured value would be most useful for diagnosis.
For new hydraulic systems, measuring points should therefore already be included in the hydraulic circuit diagram and the mechanical design.
Useful measuring points include, for example:
- at the pump outlet
- upstream and downstream of a pressure filter
- upstream and downstream of a control or throttle valve
- at the working ports of a cylinder or hydraulic motor
- at a hydraulic accumulator
- in control and pilot-pressure lines
- in the return line when back pressure is relevant
A single measuring point at the pump is often insufficient for systematic troubleshooting. Only a comparison of several points shows at which component a pressure drop occurs.
The measuring point should be easily accessible without requiring the technician to work close to hot surfaces, moving machine parts or unprotected high-pressure lines. At the same time, the test point must not be installed where it can be damaged by tools, loads or machine movements.
Selecting the correct installation location
The measured pressure always applies to the specific connection location. Significant pressure differences can occur between two measuring points due to filters, valves, pipelines, hoses and flow restrictions.
For a pump test, the test point should be located as close as possible to the pressure outlet. If the measurement is only taken downstream of several valves, it is no longer possible to determine clearly whether low pressure is caused by the pump or by an intermediate component.
Two measuring points are required for filter monitoring: one upstream and one downstream of the filter. The pressure loss is determined from the difference. The comparison should be made at a defined oil temperature and a comparable flow rate.
On a hydraulic cylinder, measuring points at both working ports are helpful. This allows the load pressure, back pressure and directional-valve behaviour to be assessed separately.
With pulsating pumps or rapidly switching valves, it should also be checked whether the test point is exposed to strong pressure shocks. Such loads affect not only the measured result but also the service life of the sensor, hose and coupling.
Considering the pressure range and pressure spikes
The pressure range of the measuring instrument must not be selected solely according to the normal operating pressure. Very short pressure spikes can occur in hydraulic systems when valves switch, a load is decelerated or a cylinder reaches its end position. These may be barely visible on a slowly responding pressure gauge.
The measuring instrument should display the expected working pressure with sufficient resolution while also providing enough reserve for permissible pressure spikes.
An unnecessarily large measuring range is also disadvantageous. If a 25 bar pilot pressure is measured with a 600 bar gauge, the resolution or readability is often inadequate for the actual task.
Several measuring instruments may therefore be required for different pressure levels, for example for:
- low-pressure and return-line measurements
- control or pilot pressure
- normal system pressure
- high-pressure circuits or hydraulic accumulators
A digital pressure gauge or pressure sensor with a sufficiently high measurement rate and peak, minimum/maximum or data-logging function is suitable for detecting rapid pressure spikes. A stable value on the display does not prove that no brief overpressure events are occurring in the background.
Materials, seals and hydraulic fluid
The test point, seal, hose and measuring instrument must be suitable for the fluid used. Standard versions are frequently used with conventional mineral-oil-based hydraulic fluids.
Compatibility must be checked separately for fire-resistant, biodegradable or water-containing fluids. Not only the metal body but especially the O-rings, hose material and surface coating are relevant.
At least the following information is required for selection:
- exact designation of the hydraulic fluid
- operating and ambient temperature
- maximum working pressure and expected pressure spikes
- required connection and mounting threads
- environmental conditions and corrosive exposure
For unusual or critical media, compatibility must not be derived solely from a general resistance table. The specific version must be assessed on the basis of binding product data and the application.
Microbore hose and measurement dynamics
MINIMESS hoses have a small internal diameter. This means that the volume of oil contained in the hose is low. Only a small amount of fluid is displaced during connection and disconnection, and the stored hydraulic energy remains limited compared with a large hose line.
Common versions include DN2 and DN4 microbore hoses. The permissible working pressure depends on the hose material, nominal size, fittings and temperature. The component with the lowest permissible working pressure always determines the limit.
The hose length and internal diameter also influence the dynamic behaviour of the measurement. A long, flexible microbore hose can damp or delay very rapid pressure spikes.
For a normal service check, this damping is often insignificant or even beneficial. However, if the rapid switching behaviour of a valve or a short pressure shock is to be investigated, a suitable pressure sensor should be positioned as close as possible to the measuring point.
The measuring hose should:
- not be kinked or crushed
- not be routed over sharp edges
- be kept away from hot surfaces and moving parts
- be protected against abrasion and tensile loads
- be inspected for damage before every use
Digital pressure gauge or pressure sensor?
A digital pressure gauge is particularly convenient for a quick on-site check. It displays the pressure directly and does not require an additional evaluation unit.
Important functions for hydraulic service work include:
- minimum/maximum memory
- peak-value capture
- zero adjustment
- selectable pressure units
- sufficient measurement rate
- robust and easily readable housing
An external pressure sensor is advantageous when pressure has to be recorded over a longer period or evaluated together with other measured variables. During troubleshooting on mobile machinery, pressure, temperature, speed and flow rate can, for example, be recorded simultaneously.
For stationary long-term measurements, the sensor must be compatible with the data-acquisition equipment and the available input system. Measuring range, accuracy, sampling rate and storage interval must be selected together.
A mechanical pressure gauge may be sufficient for a simple indicative check. However, very short pressure spikes or rapid pressure drops are often not fully visible because of its mechanical damping.
Safe procedure for a pressure measurement
Before the measurement, the expected pressure, medium and permissible specifications of all components must be checked. Personal protective equipment and the company’s safety requirements must be observed.
A suitable service procedure consists of the following steps:
- Identify the measuring point: Check the circuit diagram and labelling to ensure that the correct hydraulic circuit is being measured.
- Inspect the components: Check the coupling, hose and measuring instrument for damage, contamination and the correct pressure range.
- Prepare the measuring instrument: Set the zero point, unit, minimum/maximum function and, where applicable, data recording.
- Connect the coupling: Align it straight and connect it completely according to the series being used. Do not force or cross-thread it using tools.
- Position the hose safely: Maintain sufficient distance from moving, sharp or hot parts.
- Establish the operating condition: Bring the machine into the load and switching condition to be tested in a controlled manner.
- Document the measured value: Record the pressure together with temperature, speed, load and valve position.
- End the measurement: Bring the system into a safe condition according to the operating procedure and disconnect the measuring coupling in a controlled manner.
- Observe residual pressure: Relieve trapped pressure in the measuring hose only using a designated pressure-relief device.
Hands must not be used to search for leaks during the measurement. A fine high-pressure jet can penetrate the skin and cause serious injury, even if the external injury initially appears minor.
Preventing leaks and contamination
A correctly selected and installed MINIMESS measuring point provides very clean access to the hydraulic system. This requires the sealing surfaces and valves to remain free from dirt.
The protective cap should be refitted after every measurement. It protects the connection against dust, moisture and mechanical damage. Depending on the design, it may also provide an additional sealing function.
Before connection, the test point and mating coupling should be cleaned using a suitable lint-free cloth. Open connections must not be placed on contaminated machine surfaces.
Recurring drops after disconnection may indicate a damaged seal, contamination or a worn check valve. Such a test point should not simply be tightened further or repaired using sealing compound. It must be inspected correctly and, where necessary, replaced while the system is depressurised.
Typical measuring tasks in hydraulic systems
| Measuring task | Suitable measuring points | Possible finding |
|---|---|---|
| Check pump performance | Directly at the pump outlet | Achievable pressure and behaviour under load |
| Assess filter condition | Upstream and downstream of the filter | Differential pressure and increasing contamination |
| Check the pressure relief valve | Upstream of the valve or in the main pressure circuit | Actual opening and limiting pressure |
| Investigate cylinder force | At both cylinder ports | Load pressure, back pressure and internal losses |
| Determine return-line back pressure | In the return line upstream of critical components | Restrictions, filter loading or undersized lines |
| Detect pressure spikes | As close as possible to the valve or actuator causing them | Switching shocks, end stops and dynamic overloads |
A pressure value should always be documented together with the operating condition. Without information about oil temperature, speed and load, measurements from different service visits are often only comparable to a limited extent.
Typical errors during temporary pressure measurements
| Error | Possible consequence | Better approach |
|---|---|---|
| Measuring instrument with an excessively large pressure range | Small pressure deviations are difficult to detect | Select a measuring range suited to the actual pressure level |
| Pressure spikes not considered | Overloading of the instrument or incorrect diagnosis | Provide sufficient reserve and peak-value capture |
| Incorrect coupling series used | Damage, leakage or incomplete valve opening | Clearly match the test point and mating coupling |
| Measurement taken only at the pump outlet | Local pressure loss remains undetected | Compare several strategically positioned measuring points |
| Long measuring hose used for a dynamic test | Short pressure spikes are damped | Install the sensor close to the test point |
| Measured values compared at different oil temperatures | The influence of viscosity is interpreted as a component fault | Document the operating conditions as well |
| Protective cap not fitted | Dirt enters the coupling and valve | Close the test point after every measurement |
| Residual pressure in the hose underestimated | Uncontrolled oil discharge when loosening a connection | Relieve residual pressure using the designated relief device |
| Test point tightened while pressurised | Serious risk of injury and leakage | Work on permanently installed parts only in a safe, depressurised condition |
Practical example: Pressure loss at a hydraulic cylinder
On a mobile machine, a hydraulic cylinder extends significantly more slowly under load than when unloaded. According to the existing display, the pump still generates the intended system pressure.
Three MINIMESS measuring points are used for diagnosis: at the pump outlet, upstream of the directional valve and directly at the cylinder port.
The pressure values are normal during no-load operation. Under load, the pump pressure rises significantly. Almost the same value is present upstream of the directional valve, but a considerably lower pressure reaches the cylinder.
This initially rules out the pump as the primary cause. The pressure loss occurs between the valve inlet and the cylinder.
An additional measurement at the second working port reveals unusually high back pressure. Further inspection identifies a damaged return-line coupling that restricts free flow.
After the coupling is replaced, the back pressure decreases and the cylinder returns to its normal speed.
Without several measuring points, the high pump pressure could incorrectly have been regarded as sufficient evidence for the entire hydraulic circuit. Only the comparison along the pressure path revealed the section in which hydraulic power was being lost.
Which measuring instruments / products are suitable?
The MINIMESS couplings and hoses category includes test points, microbore hoses, adapters and accessories for diagnostic, filling, venting and service tasks.
MINIMESS couplings from the 1620 series
The MINIMESS couplings from the 1620 series have an M16 × 2 measuring connection and are available with numerous mounting and connection variants.
Depending on the version, working pressures up to 630 bar are possible. The test point, seal and mounting thread must be selected for the actual operating pressure, medium and temperature.
MINIMESS microbore hoses
MINIMESS hoses are available in DN2 and DN4, among other sizes. They connect the measuring point to a digital pressure gauge, pressure sensor or portable measuring system.
The required pressure range, length, hose material, fittings and coupling series must be specified for selection. During dynamic measurements, the possible damping caused by long hose lines should be considered.
MINIMESS p/T 1620 combined test point
The MINIMESS p/T 1620 combined test point allows pressure and temperature to be measured at one common measuring point.
This is helpful when pressure values must be assessed as a function of temperature or when only limited installation space is available for separate test points.
Digital pressure gauges and test gauges
The digital pressure gauges and test gauges category includes instruments ranging from simple service checks to precise reference measurements.
Minimum/maximum memory, a high measurement rate, robust design and a measuring range suited to the pressure level are particularly relevant for hydraulic diagnostics.
The HySense digital pressure gauge is suitable for battery-powered on-site measurements and can document pressure spikes using its minimum/maximum function.
The WIKA CPG1500 precision digital pressure gauge is suitable for applications with higher accuracy and documentation requirements.
Selecting the complete measuring chain
At least the coupling series, mounting thread, maximum pressure, medium, temperature, hose length and required measuring instrument are needed to create a suitable combination.
ICS Schneider Messtechnik assists with the design of individual measuring points and complete service kits containing test points, measuring hoses, adapters, digital pressure gauges and pressure sensors.
Conclusion: Planned MINIMESS measuring points accelerate hydraulic diagnostics
MINIMESS measuring points allow pressure gauges and sensors to be connected to suitable hydraulic systems under operating pressure without opening a main line each time.
The greatest benefit is achieved when the measuring points are already positioned at diagnostically useful locations during system design. Measuring points upstream and downstream of filters, valves and actuators allow much more accurate troubleshooting than a single display at the pump outlet.
The pressure range, medium, temperature, coupling series and hose must be considered as a complete measuring chain. The permissible load is always determined by the component with the lowest limit.
Rapid pressure spikes require a sufficiently high measurement rate and, where necessary, a sensor installed directly at the measuring point. Long microbore hoses can damp dynamic events.
Only the designated measuring connection may be coupled under pressure. Installation, replacement or tightening of the permanently installed test point must only be carried out on a safely shut down and depressurised system.
Clearly labelled measuring points, suitable digital pressure gauges and a documented service procedure can significantly reduce downtime, oil loss and incorrect diagnoses.
Frequently asked questions about MINIMESS measuring points
What is a MINIMESS measuring point?
It is a permanently installed hydraulic test point with a mechanically operated check valve. Suitable couplings can be used to connect pressure gauges or pressure sensors.
Can a MINIMESS coupling be connected under pressure?
Yes, provided that the specific series is designed for this purpose and the pressure, temperature, medium and all components used remain within their specifications.
Can the test point itself be installed under pressure?
No. Installing, replacing or tightening the permanently installed test point requires a safely depressurised section of the system.
What is the difference between a measuring coupling and a measuring hose?
The measuring coupling connects the hose to the test point. The microbore hose then transfers the pressure to the pressure gauge or sensor.
Which connection does the MINIMESS 1620 series use?
The measuring side of the 1620 series uses an M16 × 2 thread. The system-side mounting thread is available in different versions.
Up to which pressure can the 1620 series be used?
Depending on the specific version, working pressures up to 630 bar are possible. The specifications of the complete test point, hose and measuring instrument are decisive.
Which measuring hose is better: DN2 or DN4?
This depends on the pressure, required flow rate, measurement dynamics and connections. DN2 has a particularly small internal volume, while DN4 allows a higher flow rate for certain service tasks.
Can a long measuring hose distort pressure spikes?
Yes. The hose length, elasticity and small internal volume can damp very rapid pressure changes. For dynamic measurements, the sensor should be positioned as close as possible to the measuring point.
Why does a digital pressure gauge indicate a higher value than an analogue gauge?
A digital pressure gauge with a high measurement rate and minimum/maximum memory can detect short pressure spikes to which a mechanical pressure gauge responds only slowly or not at all.
Where should a pressure measuring point be installed on a pump?
As close as possible to the pump outlet. Additional measuring points downstream of valves or filters help to identify local pressure losses.
How can a contaminated filter be checked using MINIMESS?
The pressure is measured upstream and downstream of the filter under comparable operating conditions. An increasing differential pressure may indicate increasing filter contamination.
Which information is required for selection?
The maximum pressure, medium, temperature, coupling series, mounting thread, hose length, required measuring instrument and the measuring task to be investigated are required.