Operation of the LN2 Autofill System
Jim Gunn, 29-November-2005
The following text describes the operation of the LN2 autofill system for the imaging camera and spectrographs.
A main (camera) fill is initiated in one of two ways. EITHER a timer, which is set so that the fill is normally initiated BEFORE a camera dewar runs dry, times out, OR (as a failsafe) the camera dewar goes empty as indicated by the LN2 can temperature. There are no liquid level sensors in the camera dewars in either instrument. A timer controls both spectrograph cameras on one spectrograph (the two spectrographs are independent) and ALL the dewars on the camera; likewise, if a dewar goes empty, all the dewars on that instrument fill. The fill is stopped for each dewar independently as registered by its full sensor seeing liquid.
If a dewar goes empty, which should never happen in the course of normal operation, a LN2 EMPTY critical error is triggered. This is cleared the next interrogation cycle around after the fill (which it initiates immediately), so is an evanescent condition; it is almost never still there when you query the instrument. But the error is real, and indicates that that camera dewar has run out of nitrogen before it should have. Common causes for both instruments are some malfunction in the full sensor, so that the dewar did not fill properly on the last fill, ie did not get completely full, or, for the spectrographs, a bad vacuum. If a fill is not completed as indicated by the full sensor in a timely fashion, it is terminated by another timer, and a critical error called a FILLFAULT is triggered. A fillfault is normally the result either of insufficient pressure on the 10-liter--if it is low ENOUGH, a secondary LN2 pressure critical error is generated--or a bad valve.
Note that the fills of the camera dewars are controlled by valves on the EXHAUST only. Liquid is kept out because the gaseous N2 from the liquid in the dewar is made to exhaust out the fill line. When the exhaust line valve is opened, the pressure drops and liquid can flow into the dewar.
So much for the PRIMARY system.
The SECONDARY 10-liter system is similar but differs in detail.
There are no timers; a 10-liter fill is initiated when the level sensor in a 10-liter goes dry; this is set to happen when there is still of order a liter and a half of liquid in the dewar. This sensor is touchy in the sense that its signal is not large and one must watch for drifts. They have been, however, very reliable--they change temperature by only a few degrees in normal operation.
When the empty sensor goes dry, a fill is initiated for both secondary dewars. This system has both exhaust and inlet (liquid) valves, both of which must operate properly. The pressure in the secondary/camera system is much lower (~10PSI) than the supply pressure in the 180s (~30 PSI). If the liquid valve sticks, the pressure goes away and nothing happens--primary fills cannot happen either. If the exhaust valve sticks, the fill will proceed with the pressure venting out the 10-liter's relief valve, and disaster will ensue--the fill will not terminate, because the full sensor is AFTER the exhaust valve.
The fill is terminated when the full sensor, which is located in the line immediately after the exhaust valve, sees liquid. This is done independently for each 10-liter.
The manual controls for the secondary system are pretty crude--the FILL button simply shorts one of the empty sensors, simulating an empty event, and the STOP button raises the level on both full sensor lines, simulating both getting wet. The MANUAL switch on the spectrographs just asserts the FILL logical level, which keeps both systems filling until you turn it off.