When starting up an autofill system from a hot or warm condition the following information is helpful. Regardless of the dewar size 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. On very short capacitance sensors the combined effect may be as great as 100%. Once the sensor is cold it will read zero and begin functioning as normal.

Begin every new warm start autofill sequence by initiating a manual fill cycle. 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. To automate this process the following routine can be used to communicate with the Model 286 via RS232 during the initial filling of the tank. This routine can be saved in the customers higher level system memory as an "Initial Fill" or "Warm Dewar" routine and recalled as needed at the beginning of a warm start cycle.

(Refer to Section 5, Remote Interface Reference, in the Model 286 manual). 

On very small dewars, < 0.5 liters in volume, the initial fill process is further complicated. The cross sectional area in the neck region of such dewars is so small that the expansion of liquid to gas during the initial hot fill will force liquid back up through the vent tube until the dewar is cool. It is not uncommon to see a small dewar begin to fill and then "burp" liquid nitrogen out the vent on the first fill cycle. Subsequent fill cycles do not exhibit this effect. Initially cooling these small dewars manually by pouring nitrogen in with a styrofoam cup is one way to avoid this effect. Most customers choose to ignore the minor inconvenience of some nitrogen spitting during the first cycle and are well satisfied with the long term automated system performance.

How to deal with this affect:

• Understand what is happening
• Begin every new warm start autofill sequence by initiating a manual fill cycle. 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.

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.