Calibration of Stainless Cell and Air Flow Calorimeter
We have completed two initial calibration runs on the insulated stainless cell within the air flow calorimeter (AFC). The goal is to attempt to calibrate and characterize both the stainless cell and the AFC simultaneously. The cell looks like this installed. Here is the link to the Active Wires in the Chambers & Insulated Cell Details blog post.
The whole air flow calorimeter has been set into a cabinet of sorts to try to eliminate the effects of cooler drafts on the exterior of the unit and to isolate it slightly from the temperature variations in the room. The box is open on the top to allow for heat to escape and not build up.
The set up works for drafts, but this room still changes about 1.5 degrees over a day and the cabinet changes with it. We may need to actively control to a higher temp. Below you can see the exterior of the cabinet and the laptop monitoring the experiment from the shelf next to it.
We did 2 calibration cycles so far.
Time constants, settling times
These runs were done with 3 hour steps rising and falling at 4 power levels. It is pretty obvious from the data that the cell has a very long time constant. On a separate, single heating to the same high power level as the calibrations, we had the resulting curve indicating a settling time of approx 5 hours. Interestingly, it also shows the the top interior sensor runs cooler- presumable because of the thinner insulation around the top of the flange and the plumbing and pass through tubes sticking through the insulation.
Are the ratios of the temperature rises consistent? Are they good indicators of constant thermal resistance? The ratio across the insulation dropped at higher temperatures. This is not unusual for most insulation materials. The ratio between the Macor and the inside wall was very constant, which would seem to indicate that the thermal resistance across the hydrogen was consistent.
8 to 10 on t_mica! The thermal noise should be minimal, too with the insulation. That means we should have good resolution for future wire tests. If we ever see a wire with 15 to 20 watts output, 2 meters could be self sustaining at 2 watts/meter, 15 to 20 meters could be self sustaining!
Non uniformity of shell
As I noted above, the top interior sensor was cooler than the middle or bottom. Meanwhile, the middle exterior sensor runs the hottest while the bottom exterior runs cooler and it's not entirely clear why. The result is that this form of calorimeter is not gonna be easily approximated based on a pure physics calculation, so we’ll have to be content with an empirical fit. We will also have to watch carefully for mischievous effects caused by slight temperature changes altering the heat flow out through the gas tube and the pass through tubes. That is one reason we are running this cell in the Air Flow Calorimeter, which will at least provide a a nice, stable exterior temperature.
Offset in the middle of run
If you look closely at the data you will see that on both runs at almost the same exact time of day, the exterior temperature slowly rises a small, but noticeable amount. This was caused by the AFC changing air flow rate because the fan was set to right near minimum speed, just barely turning. Our hypothesis is that the fan changes mode slightly and reduced the air flow. We since turned up the fan and just added another filter to slow and even out the air flow back to a speed where it will rise 1 degree C for every 10 watts. Since we made that change, we have not seen another offset like that one. We will have to try to run a more precise calibration again now that the offset seems to be cured.
SS Cell Calibration Data
All in one Zip - including 1 minute raw data, and spreadsheets.
I was hoping to include a graph of the AFC output vs SS Cell input power, but the data from the AFC is still not ready. We achieved a rough calibration, but it is proving inaccurate (off by 1.6 watts at 37 W input power) after we changed the fan speed and added an additional filter. When we re-run a more thorough calibration, we will need to zero all the sensors appropriately with offsets while everything is cold and settled. If the calibration doesn't tighten down more after a more careful calibration, at least it makes a nice controlled temperature environment for the SS Cell.
Air Flow Calorimeter Document
We are in the process of making a nice reference document to explain the design and principles of the Air Flow Calorimeter. It is a draft in process, so don't expect perfection, but I wouldn't mind your questions or suggestions to help make it better.
You can find it here: HUG Air Flow Calorimeter
Besides slowly testing this set up, we are working on Version 1.3 of the glass, Celani type cell, while we continue to watch the vertical cells. Pictures and more details soon.