An oil-filled sensor is non-toxic and can therefore be used for many medical, food or other critical applications. However, the organic oil fill in the oil-filled system can degrade over time at high temperatures.
Some long chain molecules in the oil may degrade over a period of months of continuous operation at elevated temperatures (greater than 275 oC/525 oF). This process generates gas molecules which, whilst in continued operations, remain in the oil and do not affect the performance of the sensor.
If/when the sensor is returned to zero pressure and room temperature however, the gas may come out of solution or “outgas” over a short period of minutes to hours. This creates a bubble or bubbles in a closed filled system. The gas exerts a small additional pressure (about 50-100 psi) at zero pressure and non-operating room temperature and this causes an apparent increase in the zero output.
The gas offset error represents a higher percentage effect in lower pressure range systems compared to at higher pressure. The magnitude depends on the percentage of oil that has degraded over time at temperature.
The gas bubble(s) will go back into solution when the sensor is returned to operation with increased temperature and pressure. Tests have shown that typically, the gas is reabsorbed at temperatures over 100 oC/212 oF.
Once the gas is reabsorbed, the sensor reading is the true pressure at specified accuracies. The sensor can be re-zeroed without any impact on the calibration accuracy once it has been installed and brought to operating temperature and before applying pressure.
Recommended Zero Adjustment
It is recommended that the system is installed and the equipment brought to process temperature greater than 100 oC, or at near zero pressure, prior to performing a zero adjustment. This allows any gas to be absorbed into the oil and eliminates offset effects.
If zero adjustment is conducted at a temperature below 100 oC (e.g. bench calibration test port), the system should be briefly pressurized to ≥150 psi/10 bar, then depressurized before the zero adjustment is performed.
The pressurization followed by depressurization temporarily allows any gas to absorb into the oil and will eliminate offset effects. Reapplying pressure to ≥150 psi/10 bar followed by depressurizing will verify that the zero output value will be repeated.
This information has been sourced, reviewed and adapted from materials provided by Dynisco.
For more information on this source, please visit Dynisco.