Log entry time 17:49:11 on March 27, 2014

Entry number 387823

Yesterday we were puzzled by these blips every 2 minutes on the heater. They were about 50 W blips on a nominal 100 W heater value, and sometimes they caused alarms.

They blips disappeared when the fan speed was raised from 30 Hz to 60 Hz to test a hypothesis by Josh that the heater was boiling the liquid. The heater is a new design with smaller diameter resistive wire than was used before. If it boils the fluid in some weird way such that the bubbles get released every 2 minutes, or collect somewhere and then flush out every 2 minutes, it would explain the effect we saw.

The blips were seen on the thermometers, but the behaviour of the thermometer just upstream of the heater differed from that of the other two thermometers downstream of the heater. Points to the heater.

The blips persisted on the thermometers but disappeared on the heater when we put the heater on manual mode at 100 W (ie we turned off the PID for this short test). So it's not the heater power supply, it's something real happening in the loop which the heater was responding to through the PID.

In any case the blips were eliminated by going to a higher fan speed and that's worth remembering so I'm making this halog about it.

Note that normally with the beam on, the heater idles around 50-100 W with the fan at 60 Hz and so it's not an issue that affects the experiments. The heater power goes way up only when the beam is off and, obviously, we're not acquiring data then. One might worry that then the heater could burn out if there is enough of a vapor barrier around the wire which prevents heat xfer to the LH2, but I'm assured this was looked at and the heater was over designed.

Figure 1