Many people may be familiar with seeing an incorrectly high level reading on capacitance sensors when a coating or film buildup coats the probe. This often occurs in fluids of high viscosity or ones which contain some type of particulate which easily accumulates on the probe surface. A similar affect is observed in cryogenic applications but the affect is much more dynamic and less understood.
Capacitance sensors used in cryogenic applications exhibit a particular phenomenon which AMI refers to as “Air Offset”. The sensor is correctly calibrated at 100% level fully immersed in cryogenic liquid and at 0% in the cold vapor phase of the same material with the probe cold. In certain applications the vessel is allowed to run dry and air is introduced into the system. When this happens the moisture in the air quickly condenses on the cold sensor surface forming ice. Ice is not a good electrical conductor. As the system continues to warm up the ice melts and water begins to form on the sensor surfaces. Water is an electrical conductor and, depending on several factors, the sensor reading will increase dramatically even though the vessel is empty. Over time as the system warms further, the water begins to evaporate that the level reading returns to lower levels. Typically after 5 to 6 hours the water has all evaporated but the moisture in the air is enough to cause an “Air Offset” in the level reading of 10 to 25% with the vessel empty. There is also an increased capacitance effect caused by the thermal expansion of the mechanical sensor components between the calibrated cold condition and the warm empty state. The actual final level reading with the sensor in warm air is a combination of these two effects. The thermal contraction effect is most prevalent on sensors of very short length.
The graph Air Offset illustrates the phenomena in the case of two different sensors. Several factors tend to make the affect more pronounced:
- Very short active length sensors
- High humidity environments
How to deal with this affect:
- Understand what is happening
- Begin every new warm start autofill sequence by initiating a manual fill. As soon as the sensor cools down what little moisture that remains will form ice and the level reading will correctly go to zero before liquid begins collecting in the dewar. At this point switch control mode to auto.
- During shutdown conditions disable any control loops using this level input as a measured variable.
- To completely avoid this effect, keep empty vessels blanketed with dry inert gas during non-use periods.
Our experience has shown that a simple understanding is the biggest first step to be taken in properly designing any good cryogenic level measurement system.