Monitoring dewar performance - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: December 04, 2014, 07:51:00 pm
content: Has anyone had experience in monitoring performance (i.e. failure or impending failure) of dewars? More specifically relatively small (10-50 L) stainless steel liquid nitrogen dewars. First line of defense is daily monitoring of weight loss (or other measurement of fill level) and comparison with historical losses. However, if the dewar is not in a temperature-controlled room (often the case), there will be seasonal and even day-to-day variation in weight loss. In addition, adding or removing items to/from the tank or moving the tank will affect weight loss. Rate of weight loss in some tanks is affected by LN fill level. So rate of weight loss is not as precise a measurement of performance as you might think at first. Weight is most useful for determining when the tank needs to be topped off.Dewar failure is due to loss of vacuum, which can be slow or sudden. Monitoring vacuum is not feasible because they are factory-sealed. With failure or impending failure, the external of the tank becomes relatively cooler and eventually will have condensation or possibly frost. It would be best to detect the process prior to the formation of condensate or frost. Problem is the external temperature of the dewar is also determined by ambient temperature.My thought is to have a thermocouple or thermistor on the surface of the dewar (low on the side) with a thin layer 2-3 mm of foam insulation over it. Then another temperature sensor about 10 mm away and a small resistance heating element 10-15 mm further away with a foam insulation cavity (25 mm thick walls) covering the whole works. The apparatus would protrude about 50 mm from the wall of the dewar and be held in place by an elastic strap. Heating element would be controlled to provide temp of 45 C in the cavity so as to have a constant thermal background. Cavity diameter would be 10-15 mm. A decrease in the temp sensed on the surface of the dewar would indicate impending failure of the dewar.Concept is that for monitoring surface temperature, if the whole dewar can't be in a temperature controlled room, then one tiny piece of the surface can be.Comments and ideas?Mike in California

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 04, 2014, 08:44:31 pm
content: 45C seems pretty warm to me. If you're adding any sort of heat to the neighborhood of the dewar, some of that heat is going to leak inside to the contents, and increase the rate of boiloff. So the heat costs you twice; once for the energy to make the heat, and a second time for what it does to your dewar contents.Maybe that cost is negligible, so I could be off base here, but it seems more efficient and elegant just to do this passively, without adding any unwanted heat. Putting the dewar in a lightly insulated box, and monitoring the inside vs outside temperature of the box should give you some idea of the rate of heat transfer into the dewar. A crude algorithm might be to simply sound the alarm anytime the delta between the inside and outside exceeded historical norms. As you accumulate data, you might eventually be able to refine this, giving the expected delta between inside and outside as a function of the outside ambient temp.

author: calzap
author_rank: Frequent Contributor
timestamp: December 04, 2014, 10:11:42 pm
content: The portion that would be exposed to 45 C is less than 2 sq cm. Evan a dewar as small as 20 L will have a surface area of more than 6000 sq cm, so it's less than 0.03% of the area. On a warm summer day, ambient may be 40 C over the whole surface anyway. The reason the "oven" temperature would be 45 C is because ambient temperature is unlikely to ever exceed that value. Concept is similar to that of an OCXO.Putting a dewar in any kind of box is inconvenient for users especially with larger dewars. They have to be moved around from time to time and inspected. Even if it is in a box, there is still the uncontrolled ambient temperature to deal with which will affect the surface temperature. Even with unrealistically thick external insulation, there will be significant seasonal and diurnal swings in surface temperature. Perhaps with a lot of experimentation, a complex algorithm could be developed to relate surface temperature under a patch of insulation to ambient temperature. This would complicate things for users as they would have to put surface temperature and ambient into a computer and see if the program signals a problem. Much better that there is a simple temperature readout with a sign on it stating that if the temperature is less than a particular value, there's a problem.Mike in California

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 04, 2014, 10:17:06 pm
content: I've seen it done with a flow gage on the vent. It was a ball in a glass tube and could detect very small flow variations, probably expanded plastic or a pith ball of some sort. Between the dewar and the flow gage was a coil of copper tubing to get the gas to room temperature. If your boil-off rate goes up, it indicates trouble.

author: calzap
author_rank: Frequent Contributor
timestamp: December 04, 2014, 10:51:15 pm
content: Venting rate, which is equivalent weight loss, would be subject to the same problems as the latter: seasonal and diurnal variation, stimulation by moving the tank, taking things in and out, etc. In addition, the dewars I deal with are used for tissue (semen, tissue culture, embryos, stem cells, etc.) and have holder cans on long metal rods. There is a relatively wide opening, and the cans are immersed in the LN with hooks on the rods securing them to the rim of the opening. There is a slightly loose styrofoam plug in the middle and a slightly loose, hinged plastic or metal cap over the whole works. Nitrogen gas escapes around the plug and under the bottom of the cap; there is no specific vent that can be tapped.See the picture. The blue circle is the top of the styrofoam plug. The spokes around it are the hooks on the rods. The yellow hinged lid is to the left. The small yellow cap covers the vacuum seal.Mike in California

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 05, 2014, 03:51:50 pm
content: It seems like knowing the wall temperature and the ambient temperature should be sufficient, given the right algorithm.

author: G7PSK
author_rank: Super Contributor
timestamp: December 05, 2014, 04:17:45 pm
content: You could try an ultrasonic transducer, should be no reflection from the inner wall with a vacuum if air or gas gets into the void you would get a reflection from the inner wall.

author: calzap
author_rank: Frequent Contributor
timestamp: December 05, 2014, 06:14:37 pm
content: Quote from: G7PSK on December 05, 2014, 04:17:45 pmGreat idea! I'll have to investigate it further. Should be independent of ambient temperature and probably more compact. May cost more though.Mike in California

author: SeanB
author_rank: Super Contributor
timestamp: December 05, 2014, 08:18:09 pm
content: Standard piezo transducer will work there as well, you just need to measure the resonant frequency of the sensor/dewar combination and if it drifts more than a small amount ( will need to sacrifice one that you deliberately dump the vacuum to get the change) then trigger an alarm. Basically you bond the transducer to the dewar with an epoxy that bonds to glass and use it that way.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 05, 2014, 10:13:41 pm
content: Quote from: Conrad Hoffman on December 05, 2014, 03:51:50 pmNo, because with the outer wall exposed to air, you don't know the rate of heat flow from the environment. It will change depending on air currents and such.But I do think it can be done passively.For a given area of the dewar outer wall, there are are two sources of heat flow:- conduction along the metal sheet of the wall, from other areas of the tank.- conduction via the air in contact with the outside of the tank.The unknown variable, is the rate of heatflow from the general environment.For a spot on the tank wall that variable can be almost eliminated by:- Having a relatively long distance of wall, from the environment-affected wall areas, to the wall temp measurement point. This can be done by placing a good insulator sheet on the wall, around the measurement point. How far is 'enough' depends on the wall material. For thin stainless steel, maybe a foot radius? For aluminium, probably more.Needs to be airtight sealed at the periphery, to eliminate air convection in any gap. Actually best if the insulator sheet is glued to the metal all over.- For an area around the measurement point, a constant 'thermal resistance' to another metal plate at room temp. So, the outer dewar wall, then a fixed thickness of a stable insulator sheet, then a metal plate with fins on the room-air side. That plate to have a second temp sensor on it.Then you need to measure the thermal conductivity of the insulating layer between the external plate and the dewar wall. You might have to make a mock-up, since for calorimetry you'd need access to the dewar wall side. For eg, put a litre of water in contact with the inner wall, measure temp after stabilized, then heat the outer plate quickly to +10 deg, and see how long it takes the water temp to equalize.That gives you a figure in units like calories per degree gradient, per time.Then to monitor the real dewar, you measure the temp of the outside plate, and the dewar wall. That gives you a differential, and you know the 'thermal resistance' of the barrier between them, so you can work out the rate of heat flow through the known resistance.Any inaccuracy will be due to lateral heatflow through the deware outer wall from areas outside the measuring insulator pads. That's like a higher impedance resistance from your 'wall temp' node, to another node whose temp you don't know. It should be close to the room temp (same as the finned metal plate) but you don't know and it will vary depending on air currents etc.Effectively, thermally isolating an area of the dewar wall, and having a known conductivity barrier between it and 'room temp', is equivalent to inserting a current measurement shunt resistor in a circuit. You don't care about the absolute voltages, only the difference in volts (temp) across the resistor (barrier). And since the liquid nitrogen is so much lower than room temp, if there's any heatflow through the supposedly vacuum gap of the dewar, the heat flow signal will be significant. No need to heat the outer reference plate.

author: amyk
author_rank: Super Contributor
timestamp: December 05, 2014, 10:16:26 pm
content: Quote from: calzap on December 04, 2014, 10:11:42 pmWhere are these being stored, outside in the sun?

author: calzap
author_rank: Frequent Contributor
timestamp: December 05, 2014, 11:49:42 pm
content: Quote from: amyk on December 05, 2014, 10:16:26 pmCentral Valley in Northern California. In un-airconditioned buildings. And, yes, it can get that hot in the shade on some summer days. But it's 15% humidity, and after a 40 C day, temperature at night can be 15 C.Mike in California