Pressure measurement devices often exhibit a slight reading deviation above or below zero, even when no pressure is applied. This is known as zero offset and is usually accounted for within the device’s accuracy specifications. While small offsets are normal in new devices, experiencing drifts in the zero point over time is typical due to gradual changes in the device’s materials.
This zero offset can usually be easily corrected with a zero tare function. However, a significant zero offset may indicate underlying issues such as:
- Excessive Overpressure: Exceeding pressure limits can damage the device.
- High Temperatures: Extreme temperatures can affect measurement accuracy.
- Trapped Pressure: Clogs can prevent accurate zero readings.
- Accidental Zero Tare or Settings Reset: User errors can impact calibration.
- Orientation Changes: Sensor position may impact readings, especially in low-pressure or liquid-filled devices.
When a gauge reference pressure instrument is first installed it should read very close to zero when measuring ambient air pressure. Some devices such as analogue output sensor, may have a small zero offset because it does not have sufficient fine adjustment to set the zero reading exactly. If there is a zero offset at zero pressure on a new device, it should be relatively small compared to the full measurement range.
During the life of the pressure instrument you will observe a gradual change in the zero measurement reading. The slight drift in the zero point over time should be treated as a normal characteristic, since all pressure measuring devices exhibit some degree of long term measurement instability due to the physical changes in the construction materials due to induced stresses caused by pressure and temperature cycling during use.
This gradual change of the zero point can easily be removed from a gauge reference type pressure range by venting the pressure port to surrounding ambient air and then trimming the offset via a display readout keypad or a trimming potentiometer incorporated into the devices electronics. The process of trimming the zero is also called a zero tare.
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In some situations, the zero offset can be significant and even beyond the range of the trim adjustment. This is not a normal characteristic attributable to lifetime changes and suggests that there is a problem somewhere which needs to be resolved. The reason for a significant offset could be due to one of the following issues:
Excessive Overpressure
If the recommended maximum overpressure has been exceeded it may have deformed the diaphragm which would create a large zero offset in the positive direction. This is especially possible with metal sensing elements that are not 100% elastic to their breaking point.
Very High Process Temperature
Some pressure measuring devices, particularly pressure sensors with analogue electronics have a large temperature error band at extreme temperatures. If the process happens to be at a very high temperature this maybe the cause of a large zero pressure offset. If the sensing element includes a oil fill fluid to isolate the sensing device from the process media, an increase in temperature will cause the oil to expand which appear as a positive change in pressure.
Trapped Pressure
If the pressure connection has become clogged it may not release all the pressure when vented to the atmosphere. The trapped pressure will show as a positive pressure.
Accidental Zero Tare
Many pressure indicators include a button on the front panel for Zero Tare purposes. If the Zero Tare button is pressed by mistake at a pressure other than zero, it will shift the zero reading in the positive or negative direction depending on whether a vacuum or positive pressure was being measured at the time.
Blocked Vented Gauge Reference
The pressure reference is typically a small diameter tube which runs from the reverse side of a pressure sensing diaphragm to outside air. If the proper precautions are not implemented, It is possible for the the gauge reference to become blocked by dirt ingress, moisture ingress, kink or bend in the vent tube. Since the trapped volume inside the vent tube is relatively small, the air inside will respond quickly to changes in temperature causing the pressure to rise and fall depending on the environmental conditions. An increasing reference pressure will cause a negative zero offset and a decreasing reference pressure will cause a positive zero offset.
Accidental Reset to Factory Settings
If the pressure sensor or instrument incorporates intelligent digital technology, the factory calibration settings are most likely stored within the devices memory. Over time these calibration settings will be altered by the user as the sensor’s performance changes. Some manufacturers build in the option to reset calibration settings to the factory default. If the factory default reset has been selected unintentionally it will cause any adjustments made over the service life of the instrument to be lost causing a potentially large calibration offset to the zero. The older the instrument is, the more likely it will have a larger difference between the manufacturer’s and user’s calibration settings.
Changes in Orientation
For some sensors a change in orientation can cause the output at zero pressure reading to change. This is particularly noticeable in lower pressure devices which have thinner and larger diameter sensing diaphragms, or a liquid filled seal on once side of the sensing diaphragm. In the case of the lower pressure range device, gravity will act on the diaphragm when in the horizontal position causing it to sag slightly towards the middle. As diaphragm is rotated into the vertical position, the effect is eliminated. This movement in the diaphragm will cause a change in the zero output of the device. Likewise an oil filled device will change in in different orientations due to the liquid head pressure of the oil filling, when the oil is on top it pushes down on the sensing device creating a positive zero offset, and underneath it has the effect of pulling down creating a negative zero offset.
Repositioning of Liquid-Filled Capillaries and Diaphragm Seals
Oil filled seals and capillaries are used to isolate pressure sensing devices from harsh media and high temperatures, and often there is a length of flexible tubing or capillaries between the diaphragm seal and the sensing device. Similarly to the above example of oil filled devices affected by orientation, if a diaphragm seal is fitted in a different vertical position to the sensing device, the head pressure of the liquid filling will cause a zero offset either in the positive or negative direction depending on the relative height of the diaphragm seal. Therefore changing the vertical fitted position of a diaphragm seal after it has been calibrated in a certain position will cause a zero shift in the output reading.
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Conclusion
While a slight zero offset in pressure measuring devices is normal, a significant or persistent offset can point to underlying issues. Understanding the potential causes of zero drift is crucial for accurate measurement and ensuring your device’s long-term functionality. Remember, regular zero tares can usually correct for minor zero offsets.
- Zero offset is common in pressure measurement devices and is often accounted for in accuracy specifications.
- Gradual zero drift over time is normal due to changes in the device’s materials.
- Significant zero offset can signal problems like overpressure, temperature extremes, blockages, or accidental settings changes.
- Regular zero tare adjustments can help maintain accuracy.
- Orientation and positioning can impact readings in some devices, especially those with low-pressure ranges or liquid fillings.
Related Help Guides
- What is the difference between zero offset and zero drift?
- Problem with zero drift when installing submersible pressure sensor