It is not unusual for a pressure measurement device to read slightly above or below zero with no pressure applied. The zero offset tolerance for a device is often incorporated into the accuracy statement or shown as a separate spec on the product data sheet.
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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Occasionally in some situations, the zero offset can become so large that it is beyond the range of the trim adjustment. This is not a normal characteristic and it 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 the 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.
- Zero & Span calibration adjustable signal output pressure transducers
- Zero and span adjustable 10 bar ATEX pressure transmitter
- SCU for Isolating signal from sensors and trimming zero and full span
Related Help Guides
- What is the difference between zero offset and zero drift?
- Problem with zero drift when installing submersible pressure sensor