If a digital pressure gauge does not show exactly 0.00 bar in a depressurized state, the first assumption is often: The device is defective or needs to be calibrated. In many cases, however, this is not the case. Often there is still residual pressure in the connection, the measuring connection has not been fully vented, the temperature has changed or the device has been used in a different installation position.
Zero adjustment is an important step for obtaining reliable pressure readings. At the same time, a digital pressure gauge must not simply be zeroed arbitrarily. If the zero point is set while pressure is still applied, the entire measurement is shifted. The device then shows seemingly plausible values, but is actually measuring with an incorrect reference point.
This article explains when a digital pressure gauge should be zeroed, when zero adjustment is not permissible, how to distinguish residual pressure from a real zero point error and why correct zeroing does not replace calibration.
Table of contents
- Basics: What does zeroing mean for a digital pressure gauge?
- When is zero adjustment necessary?
- When must a digital pressure gauge not be zeroed?
- Residual pressure or zero point error: The most important difference
- Procedure: Zeroing a digital pressure gauge correctly
- Influence of temperature and installation position
- Why depressurized does not always really mean depressurized
- Zero adjustment is not calibration
- Typical mistakes when zeroing digital pressure gauges
- Table: Display value, possible cause and correct action
- Practical example: Digital pressure gauge shows 0.08 bar in a depressurized state
- Which measuring instruments / products are suitable?
- Conclusion: Depressurize first, then zero
- FAQ: Frequently asked questions about zero adjustment on digital pressure gauges
Basics: What does zeroing mean for a digital pressure gauge?
When zeroing a digital pressure gauge, the current display value is set as the new zero point. The device therefore uses the currently applied pressure condition as the reference point for further measurement. With relative pressure gauges, this usually means in practice: The pressure gauge is set to 0 at atmospheric ambient pressure.
This is useful because small deviations in the zero point can be caused by temperature changes, changes in position, mechanical influences or ageing. A correctly performed zero adjustment ensures that the measurement starts cleanly at 0 again when there is actually no overpressure or vacuum applied.
However, it is important to note: Zero adjustment only corrects the reference point of the display. It does not check whether the device measures correctly over the entire measuring range. For this, calibration with a suitable reference is required.
In practice, the term “zeroing” is often also referred to as “tare” or “zero”. Depending on the device and manufacturer, the functions may be implemented slightly differently. The operating manual of the respective digital pressure gauge should therefore be observed.
When is zero adjustment necessary?
Zero adjustment is useful whenever the digital pressure gauge shows a small value other than 0 in a clearly depressurized state. Typical examples are readings such as 0.02 bar, -0.03 bar or 0.08 bar, even though the measuring connection is open to atmosphere and there is no residual pressure in the system.
Before precise comparison measurements, pressure tests, service work or calibration tasks, it should also be checked whether the device is correctly at 0. Especially with low measuring ranges, even a small zero point deviation can cause a relevant measurement error.
Zero adjustment can also be useful after major temperature changes. If a device is brought from a cold storage room into a warm workshop or vice versa, it should first reach a stable ambient temperature before zeroing and measuring.
Checking the zero point can also be useful after mechanical stress, transport or a change in installation position. This does not automatically mean that the device is damaged. It only shows that before measurement, it should be checked whether the reference point is still correct.
When must a digital pressure gauge not be zeroed?
A digital pressure gauge must not be zeroed as long as pressure is still present in the system or connection. If the zero point is set under residual pressure, this pressure is “subtracted”. The pressure gauge then shows 0, even though pressure is actually still present. During the next measurement, all values are shifted accordingly.
This is particularly critical with closed lines, hydraulic blocks, test adapters, hoses, dead volumes, valves or quick couplings. Residual pressure can remain there even though the system appears to have been switched off or relieved.
Caution is also required with absolute pressure gauges. An absolute pressure gauge measures relative to vacuum and does not show 0 bar abs at atmospheric ambient pressure. Such a device must not simply be set to 0 just because it is open to atmosphere. It must be clear which pressure type the device measures: relative pressure, absolute pressure or differential pressure.
Likewise, zeroing should not be performed if the display deviates strongly, drifts abruptly or does not stabilize. In such cases, there may be a technical fault, damage, moisture in the connection, an overloaded sensor or a calibration issue.
Residual pressure or zero point error: The most important difference
The most common mistake when handling digital pressure gauges is confusing residual pressure with a zero point error. A displayed value of, for example, 0.08 bar can actually be a small zero point deviation. However, it can just as easily be caused by trapped pressure in a hose, test line or valve.
Residual pressure is often recognized by the fact that the display changes as soon as a valve is opened, a coupling is released or the connection is vented. A slow drop in the measured value also indicates that pressure is still being relieved.
A real zero point error, on the other hand, also appears when the measuring connection is clearly open to atmosphere, no liquid column is acting, no pressure source is connected and the display does not settle stably at 0.
Before every zero adjustment, it should therefore first be checked whether the measuring connection is really depressurized. Only once residual pressure has been ruled out should the zero or tare function be used.
Procedure: Zeroing a digital pressure gauge correctly
Zero adjustment should be carried out calmly, carefully and in a traceable manner. The decisive factors are that the device is not under pressure and that the display is stable. The following procedure has proven itself in practice.
- Depressurize the device: The pressure source must be safely disconnected or relieved. In systems, test benches or hydraulic systems, the pressure must be reduced in a controlled manner.
- Vent the connection: The measuring connection, hose or adapter must be open to atmosphere. Trapped residual pressure must be able to escape completely.
- Wait for a stable ambient temperature: The device should not be zeroed immediately after a strong temperature change. A short acclimatization period improves measurement reliability.
- Allow the display to stabilize: Before zero adjustment, the displayed value should be steady and no longer drifting.
- Set the zero point: Only now should the zero, tare or zero adjustment function of the digital pressure gauge be used.
- Perform a control measurement: After zeroing, it should be checked whether the device shows a stable 0 in a depressurized state and responds plausibly when a known pressure is applied.
For safety-relevant pressure systems, high pressures or hazardous media, pressure relief and disassembly may only be carried out by qualified personnel. Pressure connections must not be loosened as long as dangerous residual pressure may still be present.
Influence of temperature and installation position
Temperature changes can influence the zero point of a digital pressure gauge. Although high-quality devices are temperature-compensated, small deviations can still occur in practice if the device has been strongly heated or cooled.
A typical example is a digital pressure gauge that is stored in a service vehicle and brought into a warm production hall in winter. Immediately after switching on, the display may deviate slightly. After a short adaptation period, the value often stabilizes by itself.
The installation position can also play a role. Especially with sensitive measuring ranges or devices with liquid-filled connections, changes in position, hydrostatic effects or trapped liquid columns can influence the displayed value.
For reproducible measurements, the digital pressure gauge should be used as far as possible in the same position in which it was zeroed. If the measuring task requires a specific installation position, the zero point should be checked in exactly that position.
Why depressurized does not always really mean depressurized
In practice, “depressurized” is not always unambiguous. A system can be switched off, but pressure may still remain in individual line sections. Especially with check valves, shut-off valves, hydraulic hoses, quick couplings or small test volumes, pressure often remains longer than expected.
Liquids can also play a role. If there is a liquid column between the measuring point and the pressure gauge, the height of the liquid can create a small hydrostatic pressure. In low measuring ranges, this influence can already be visible.
In pneumatic applications, trapped air can escape slowly and change the display over several seconds or minutes. In hydraulic applications, elastic hoses, seals and dead volumes can cause residual pressure to dissipate only slowly.
Therefore, the connection should not only be disconnected from the pressure source, but actually vented or depressurized. Only when the display is stable and residual pressure is no longer plausible should zeroing be performed.
Zero adjustment is not calibration
A common misconception is that a zeroed digital pressure gauge automatically measures correctly. This is not true. Zero adjustment only sets the starting point of the measurement. It says nothing about whether the device is within the permissible deviation at 10 bar, 100 bar or 700 bar.
Calibration compares the digital pressure gauge with a suitable reference at defined test points. This determines the deviation across the measuring range. Depending on the application, factory calibration or DAkkS calibration may be required.
If a digital pressure gauge clearly deviates at known pressure values despite correct zeroing, does not measure linearly or drifts strongly again after a short time, it should be checked or calibrated.
Calibration is also useful after overload, dropping, strong pressure shocks, contact with unsuitable media or prolonged use. Zero adjustment cannot reliably assess such influences.
Typical mistakes when zeroing digital pressure gauges
The most common mistake is zeroing under residual pressure. The device then shows 0, even though the measuring connection is still pressurized. This causes systematic measurement errors that are difficult to detect during later tests.
Another mistake is zeroing directly after switching on or immediately after a strong temperature change. If the display is still drifting, an unstable condition is taken over as the zero point.
Zeroing an absolute pressure gauge at ambient pressure is also problematic if the user actually requires an absolute pressure measurement. Here, the pressure type of the device must be clearly known.
In practice, it also happens that a pressure gauge is zeroed even though the display is clearly outside a plausible small zero point deviation. In the case of larger deviations, the cause should first be investigated: residual pressure, contamination, medium in the connection, overload or a possible device defect.
Table: Display value, possible cause and correct action
| Observation on the digital pressure gauge | Possible cause | Recommended action |
|---|---|---|
| Display is stable at 0.00 bar when depressurized | Zero point is correct | No correction necessary, measurement can begin |
| Display is stable at 0.02 bar when depressurized | Small zero point deviation | Check depressurization, then perform zero adjustment |
| Display is at 0.08 bar, connection is still closed | Possible residual pressure in connection or hose | Vent connection in a controlled manner, do not zero immediately |
| Display slowly drops toward 0 | Pressure is still being relieved | Wait, vent and evaluate only once the display is stable |
| Display jumps or drifts strongly | Unstable pressure, temperature change or device fault | Check cause, do not zero while drifting |
| Absolute pressure device shows approximately ambient pressure at atmosphere | Normal behavior of an absolute pressure sensor | Do not zero to 0 bar abs, observe pressure type |
| Device shows an incorrect value at a known test pressure after zeroing | Possible calibration error or measuring range issue | Perform calibration or check with reference |
| Display does not return reproducibly after pressure relief | Overload, contamination, sensor problem or mechanical influence | Have device checked, do not use measured values without verification |
Practical example: Digital pressure gauge shows 0.08 bar in a depressurized state
A maintenance technician uses a digital pressure gauge to test a hydraulic system. After relieving the system, the device still shows 0.08 bar. At first glance, it appears as if the zero point has shifted. The question now is: Is the device defective, does it need to be calibrated or is zero adjustment sufficient?
First, it is checked whether the pressure gauge is really depressurized. The measuring hose is still connected, and there is a small trapped volume between the quick coupling and the pressure gauge. After careful venting, the display drops to 0.03 bar and stabilizes there.
This shows: Part of the reading was indeed residual pressure. Only after the connection is open to atmosphere and the display has stabilized is the zero point set. Afterwards, the digital pressure gauge shows 0.00 bar in the depressurized state.
For verification, a known test pressure is then applied. The display responds plausibly and returns to 0 again after renewed pressure relief. In this case, the device was not defective. Calibration would only be necessary if the measured values at known test points were outside the permissible deviation or if the zero point repeatedly drifted strongly.
Which measuring instruments / products are suitable?
High-quality digital pressure gauges with an easy-to-read display, suitable accuracy and appropriate pressure range are suitable for precise pressure measurements, service work and test tasks. A typical example is the DPI104 and DPI104-IS digital pressure gauge. It is suitable for demanding pressure measurements on site, test tasks and applications where a reliable digital display is required.
For a broader selection of different devices, the category digital pressure gauges / test pressure gauges is suitable. Devices can be selected there according to pressure range, accuracy, design, connection and application.
If pressure profiles are to be documented over a longer period of time, digital pressure gauges or measuring systems with data logger function are also of interest. For such applications, the category pressure data loggers can be helpful.
It is always important that the pressure range, pressure type, connection, medium, accuracy, overpressure resistance and operating environment match the measuring task. In hazardous areas, oxygen applications, aggressive media or very high pressures, device selection must be carried out particularly carefully.
Conclusion: Depressurize first, then zero
A digital pressure gauge should not automatically be zeroed just because it shows a small value other than 0 in the resting state. First, it must be checked whether the connection is really depressurized. Residual pressure, trapped volumes, liquid columns, temperature changes or the pressure type of the device can influence the display.
The correct procedure is clear: safely relieve pressure, vent the connection, wait for a stable temperature, allow the display to stabilize, set the zero point and then perform a control measurement. This prevents existing residual pressure from accidentally being adopted as the new zero point.
Zero adjustment improves the starting point of the measurement, but does not replace calibration. If measured values deviate at known test points, the zero point drifts strongly or the device has been mechanically stressed, the digital pressure gauge should be checked or calibrated.
FAQ: Frequently asked questions about zero adjustment on digital pressure gauges
Why does my digital pressure gauge not show 0?
This may be due to a small zero point deviation, but also to residual pressure, trapped air or liquid, temperature change, installation position or an incorrect pressure type.
When should I zero a digital pressure gauge?
A digital pressure gauge should be zeroed if it shows a stable small value other than 0 in a clearly depressurized state and residual pressure has been ruled out.
May I zero a pressure gauge under pressure?
No. If zeroing is performed under pressure, the applied pressure is adopted as the new zero point. This shifts all subsequent measured values.
What is the difference between zeroing and calibration?
Zeroing only sets the current reference point of the display. During calibration, the device is compared with a reference at defined test points.
Is a value of 0.08 bar in a depressurized state a defect?
Not automatically. First, it should be checked whether there is really no residual pressure and whether the connection is vented. Only then can it be assessed whether zero adjustment or inspection is necessary.
Why is residual pressure so problematic?
Residual pressure can be mistakenly interpreted as a zero point deviation. If the device is then zeroed, it will subsequently measure with a shifted zero point.
How do I recognize residual pressure in the connection?
Residual pressure often appears as slowly falling values, a change when opening a valve or a display that moves closer to 0 only after venting.
Does a digital pressure gauge have to be zeroed after every switch-on?
Not necessarily. However, before important measurements, it should be checked whether the display is stable at 0 in a depressurized state.
Can temperature influence the zero point?
Yes. Strong temperature changes can cause small zero point changes. The device should therefore reach a stable ambient temperature before precise measurements.
Why must an absolute pressure gauge not simply be set to 0?
An absolute pressure gauge measures relative to vacuum. At atmospheric ambient pressure, it therefore does not show 0 bar abs. The pressure type must be clearly known before zeroing.
What does tare mean on a digital pressure gauge?
A tare function sets the current value as the reference point. It can act similarly to a zero function, but must also be used consciously and appropriately for the measuring task.
When is calibration necessary?
Calibration is useful if the device deviates at known test points, has been used after overload or dropping, drifts strongly or is used for quality-relevant measurements.
Can I simply zero away any zero point deviation?
No. Small deviations can be zeroed if the device is truly depressurized. Larger or unstable deviations should first be investigated.
What role does the installation position play?
In sensitive measuring ranges, the installation position can influence the zero point. For reproducible measurements, the device should be zeroed and used in the same position whenever possible.
What should be checked after zeroing?
After zeroing, the device should show a stable 0 in a depressurized state. If possible, a known test pressure should also be applied to check the plausibility of the display.