LM399 heat loss within a vacuum - Page 1
Forum section: electronics / general

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 28, 2022, 04:52:52 pm
content: was thinking about 'portable references' that are shipped live. has anyone ever calculated the heat loss of a bare LM399 package in a vacuum?1. the LM399 package is a small (TO-46) metal cylinder, 5.5mm diameter by 3mm high, with a somewhat reflective surface. this conveniently works out at about 100mm2, or 1cm2. i suspect the plastic thermal shield would have no real function within a vacuum and so can be discarded.2. the LM399's die is heated to to 90°C when running, so assuming a worst-case environmental shipping temperature of -10°C, we end up with a deltaT of 100°C. at equilibrium this will exist not just at the die, but across the whole surface the LM399's metal package. this will be the RADIATED heat losses.3. there will be heat transferred (CONDUCTED heat losses) down the four leads of the LM399. but these leadscouldin theory be thinned down to a diameter no more than that of the bond wires running to the LM399 die. however, for mechanical strength they would need to be kept thick enough to support the TO-46 package, perhaps formed into coiled springs? i suspect these leads may constitute the major cause of heat loss.4. the vacuum chamber would be metal, a sphere in two halves (soldered together) with a polished interior and 4x feed-throughs for the LM399 wires/supports. it would not need to be very big, perhaps only 3cm across in any dimension, and evacuated via a small diameter copper side tube that is crushed then sealed with a little solder.5. only a moderate vacuum would be necessary as it is there just for thermal purposes, the equipment used to create it being quite achievable in a home-lab.i'm not sure what the final heat losses of the LM399 would be in this sort of configuration. mysuspicionis that it may be quite low, hence making for a long battery life during transport. i can see no reason why any thermal insulation outside of the metal vacuum chamber would be necessary, so the overall size of the completed design would remain small. much the same calculations, etc would apply to an LTZ1000 based portable reference.comments?cheers,rob :-)

author: bdunham7
author_rank: Super Contributor
timestamp: August 28, 2022, 05:13:01 pm
content: Quote from: robert.rozee on August 28, 2022, 04:52:52 pmI don't know about the rest of your plan, but you'll need a vacuum of about 10 microBar or 1 Pascal to achieve any meaningful 'thermal purpose'. Pressures significantly higher than this won't vary much in thermal performance from a standard atmosphere.

author: Gyro
author_rank: Super Contributor
timestamp: August 28, 2022, 06:06:31 pm
content: Remember that the LM399needsheat loss, otherwise at some point its thermal regulation will become unstable.

author: iMo
author_rank: Super Contributor
timestamp: August 28, 2022, 07:12:23 pm
content: The 399's heater current ranges from say 80-100mA when cold to some 15-20mA when hot (it depends on the voltage and the thermal isolation, though). So let us consider the heater power is 300mW in normal use.In vacuum the current will drop down further on to a certain level where the regulation will stop work properly, as Gyro wrote above. You may try to simulate that by decreasing the heater voltage such the heater current goes to say 5mA - what the regulation will do then..

author: Kleinstein
author_rank: Super Contributor
timestamp: August 28, 2022, 07:29:53 pm
content: The main purpose of the moderate grade vacuum is to reduce / essentially eliminate the heat loss by convecion in the residual gas. The thermal conductivity would essentially remain unless really good vacuum is reached, which can be tricky and expensive. Depending in the residual gas, the thermal conductivity can vary - depending on the pump / system used the residual gas may be quite different from air, like a large portion of water or helium.Quote from: Gyro on August 28, 2022, 06:06:31 pmThe regulator inside the LM399 should be essentially a proportional regulator, as there is no significant capacitor inside. The main part to worry about for dynamic stability is the relatively fast delay from the heater to the temperature sensor and not really the rather slow external time constant that is effected by the thermal isolation. So I don't see a reason to expect a dynamis stability problem from reduced heat loss.Looking at the regulation circuit, it looks like the loop gain is going down with less heater power, despite of using a transistor as a heater and thus a theoretically linear heater. So the regultaion may be a little worse with reduced power. With the effort needed for the vacuum system it may be worth the effect to select for a low TC before the regulation and thus less relyiance on the regulation.If really needed one could also add additional regulation of the outer case (e.g. some 40 C) for the time when actual accuracy is needed and keep only the LM399 own heater in transit.Due to the rather high temperature and moderate power (some 7 V * 1 mA) there is not much self heating. So one is unlikely to get the power needs that low to become an issue, even in a warm environment or good insulation.For the LM399 it is still questionable if it is worth the effort:With the relatively high working temperature there is usually not much hysteresis and the reference stabilizes relatively fast after a phase of being unpowered. When powered the high temperature can also lead to quite some aging, as opposed to the time when off. Ideally one would record the time powered on separately.For a long term stable LM399, I would more consider powering it only when actually needed, and maybe provide just some basic keep warm (e.g. avoid goind below freezing) when on transit.

author: iMo
author_rank: Super Contributor
timestamp: August 28, 2022, 07:51:46 pm
content: The Rtheta of the 399 package incl cap is something like (90C-25C)/0.3W=217C/WWith 1mA and 7V and -10C ambient the temperature of the chip will be around -8.5C then..

author: MK
author_rank: Regular Contributor
timestamp: August 28, 2022, 08:41:06 pm
content: You would still have to regulate the temperature of the enclosure to provide an unchanging thermal loss though radiation

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2022, 09:39:16 am
content: From the datasheet the LM399 needs some 400 mW at 5 C and thus some 85 K difference to the die, which translates to some 210 K/W for the thermal resistance to ambient in air.A moderately good vaccum would still conduct as much as air, just without convenction. So the same 470 K/W would apply also for a not vey good vaccum. With the extra case the conductive heat loss to air can be higher. The poor vaccum part would still need some 2 mW/K or some 140 mW of heater power. So even in a moderate vacuum the power consumption is not very low.The remaining part is from the pins and radiation (could be low with a reflective surface). If needed part of the thermal resistance could be from the small PCB to the outside using separate drive and sense wires for the zener. This may still be a significant part of the heat loss. For the normal air operation the conductive part through the pins could still be something like 1/3 of the heat loss.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 29, 2022, 10:05:46 am
content: lets change tack (simplify things) slightly:1. assume NO PCB. the LM399 is sitting in the vacuum chamber connected to the outside world by four feed-throughs to which the existing component leads are attached. NO other electronic components. the inside of the chamber is reflective, so that we are sending back towards the LM399's package as much RADIATED energy as iseasilyachievable.2. the LM399's heaters are powered by an external 12v battery, and the device temperature is at equilibrium. if you like the zener can also be powered, although this shouldn't matter at this point.3. we will assume there will be the same heat loss through the LM399's four leads as occurs when the device is sitting within free air; at this stage we will make no attempt to mitigate this loss.NOW, compared to a similar arrangement without the surrounding vacuum chamber, and JUST the LM399's default plastic thermal shield,how much will the current draw of the heater have been cut by?10%? 50%? 90%?4. an important point is that, for the purposes of shipping, the reference need not be functioning as a reference. it merely needs to be in a state such its calibration will not degrade and the reference can be brought back 'online' (to calibrated operation) with relative ease.cheers,rob :-)

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 29, 2022, 10:09:31 am
content: Quote from: Kleinstein on August 29, 2022, 09:39:16 amthen why do we keep our coffee in a thermos flask?cheers,rob :-)

author: alm
author_rank: Super Contributor
timestamp: August 29, 2022, 10:40:49 am
content: Because it's "without convection", and convection is a major source of heat transfer.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 29, 2022, 10:51:57 am
content: In air and some 20 C one needs a heater power of some 300 mW. From what it looks like a moderate vacuum would still need some 100-140 mW for the gas part and maybe some 50-100 mW for the leads.So I would expect to stil have something like 50-70% of the power when in vacuum.Quote from: robert.rozee on August 29, 2022, 10:09:31 amFor the Coffe flasks there are cheap ones with mediocre vacuum that is mainly there to suppress convection and keep the surface clean to keep radiation low. They don't hold the coffee hot very well.The good ones have a quite good vaccum and do get the conduction in the gas down. This is however not easy and needs a good pump (e.g. diffusion pump or turbo pump) and a good seal (e.g. sealed glass of welded stainless. On the industrial scale it is not so difficult and it was used quite a lot for vacuum tubes.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 29, 2022, 11:21:29 am
content: Quote from: Kleinstein on August 29, 2022, 10:51:57 amif a 50% reduction in power consumption was achievable, that would be an extremely useful result. bear in mind that this will also include a significant reduction in overall insulation requirements outside of the chamber - one posting i read suggest 8" of insulation required around a heated reference when shipping!of interest, see:https://www.researchgate.net/publication/339111964_Influence_of_Vacuum_Level_on_Heat_Transfer_Characteristics_of_Maglev_Levitation_Electromagnet_Module->https://www.researchgate.net/publication/339111964_Influence_of_Vacuum_Level_on_Heat_Transfer_Characteristics_of_Maglev_Levitation_Electromagnet_Module/link/5e3e763b458515072d88e345/download->https://www.researchgate.net/publication/339111964_Influence_of_Vacuum_Level_on_Heat_Transfer_Characteristics_of_Maglev_Levitation_Electromagnet_Module/fulltext/5e3e763b458515072d88e345/Influence-of-Vacuum-Level-on-Heat-Transfer-Characteristics-of-Maglev-Levitation-Electromagnet-Module.pdf?origin=publication_detailwhich suggests results 'within a similar ballpark' with a reduction from 1atm to 0.1atm.cheers,rob :-)

author: iMo
author_rank: Super Contributor
timestamp: August 29, 2022, 11:24:33 am
content: And below my modelbased on my first model I made in June..

author: Gyro
author_rank: Super Contributor
timestamp: August 29, 2022, 11:49:19 am
content: Does your intended design include a gain stage to provide a 10V output? If you feel that it is necessary to maintain the LM399 continuously powered during transport then you should seriously consider temperature stabilisation for the opamp and divider resistors, particularly over the shipping temperature range (down to -10'C) that you indicated. Just a thought.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 29, 2022, 02:48:36 pm
content: Quote from: Gyro on August 29, 2022, 11:49:19 amno, a gain stage just introduces another source of error - actually a whole load of sources! it is far easier to have a cal lab measure the output of an LM399 (or any other reference) and write the measured voltage on a label attached to the top of the box.cheers,rob :-)

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: August 29, 2022, 05:15:51 pm
content: One way to think about the vacuum situation is to consider a thermocouple vacuum gauge. They have a typical range of about 1000 millitorr down to about 5 millitorr, at which point there isn't much thermal conductivity change to measure. A Thermos bottle needs to be somewhat better than that. To get the greatest benefit from a vacuum system you probably need to pull something below 30 millitorr. Naturally all the problems referenced above still apply concerning the range that a 399 can operate over. Another possible idea is to just backfill the chamber with Argon. Any local welder can do that for you. Conductivity of air is 0.026 and argon is 0.018 W/m-k, so not quite a 50% improvement, but maybe friendlier to the 399 circuit.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 30, 2022, 04:25:53 am
content: Quote from: Conrad Hoffman on August 29, 2022, 05:15:51 pmexcellent! digging into thermocouple vacuum gauge operation led me (eventually) to the following graph:(from:https://www.researchgate.net/figure/Air-thermal-conductivity-as-a-function-of-pressure_fig1_230959207)the above suggests that we get a good reduction of (atmospheric) heat loss at 0.01atm (30% of original loss remains), and an excellent reduction at 0.001atm (only 8% of original loss remains). 0.001atm (a 1000:1 reduction) should be achievable with a mechanical pump.the next two questions are:1. how much original total loss is radiated (ie, will pass through the vacuum), and do we need to account for it?2.what percentage of the original total thermal loss of an LM399 is via the leads?it also interesting to note: we can likely use the current consumption of the LM399's heater as an indicator of the vacuum 'health'cheers,rob :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2022, 06:49:49 am
content: The problem with this graph is that in the low pressure region the concept of thermal conductivity breaks down. The larger the dimensions the more the curve shifts to lower pressure. So the curve (especially the pressure scale) only makes sense if they also give the length over which they measure / calculate the conductivity. For the LM399 as heat source the relevant dimension would be some 5-10 mm while for a presure gauge the rather thin wire diameter matters. The curve is for 10 µm plate spacing - so mbar instead bar for the pressure scale when used for 10 mm dimensions.P.s.:The heat loss through the leads can be calculated. The resistivity and thus thermal conductivity of the covar alloy is known good enough. The question is more if there is additional thermal resistance in the extension of the leads.Radiation could be a factor, due to the relatively high temperature. A somewhat unknown is how low the emissivity of the LM399 case is. Refelction at the other side can also effect it and if one really case a gold coating can bring the IR absorbtion way down.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 30, 2022, 09:12:44 am
content: Quote from: robert.rozee on August 29, 2022, 11:21:29 amIn my opinion, this is an overstated requirement. In an old book, I came across a paradoxical conclusion - increasing isolation does not work indefinitely. After a certain thickness, a further increase in thickness does not give anything.

author: MegaVolt
author_rank: Frequent Contributor
timestamp: August 30, 2022, 09:14:00 am
content: And also, losses cannot be brought to 0. Losses through the walls and leads must be greater than the power allocated on the zener diode itself. Otherwise it will overheat.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 30, 2022, 09:22:10 am
content: i feel the trick is for someone with a vacuum pump to set up an experiment:1. feed (say) 12 volts into the heater of an LM399 and measure the stable-state current of that particular sample;2. place the same LM399 into a vacuum chamber, bringing out the heater wires, pump down to 0.01atm or less, and check the current consumption at 12 volts again.this will give some indication as to if the idea is worth pursuing further. if there is a significant reduction (35% or better) then it may be worthwhile going further. others are right thatsomethermal loss will be necessary for the LM399 to regulate its temperature successfully. hopefully the radiated and conducted (via 4x leads) losses will suffice.cheers,rob :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2022, 11:13:46 am
content: Good foam insulation is not much worse than a poor vacuum (e.g. 0.1 mbar or worse). For shipping the foam part would at least be lower weight. So one could do a simple test also with foam. For shipping this also has the advantage of lower weight compared to the vacuum chamber.The possible level of insulation is limited as the more isolation is added the larger the surface to isolate gets. So even with infinite insulation there would be heat loss. The problem is a little like the capacitance of a sphere against infinite space.Usually there is not much hysteresis with the LM399. So it is questionable if it is worth keeping it that hot all the time. Just keeping it above 10 C may be good enough. The precision of the LM399 is anyway limited due to it's noise and long term drift when at temperature.For the convection part the vaccum is very effective. I expect more like better than linear, more like square, as the heat capacity goes down and the driving density difference. For the heat conduction the step to 4 mbar would not help, unless this is for very small dimensions.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 30, 2022, 11:21:25 am
content: Quote from: Kleinstein on August 30, 2022, 11:13:46 amthe vacuum chamber need not be any larger than a ping-pong ball. remember: it only has to withstand 15psi from the outside atmosphere.cheers,rob :-)

author: iMo
author_rank: Super Contributor
timestamp: August 30, 2022, 02:22:45 pm
content: FYI - the heat transfer via an insulating box - an excel sheet..https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg3429710/#msg3429710

author: maat
author_rank: Regular Contributor
timestamp: August 30, 2022, 02:26:37 pm
content: I don't want to rain on your parade, but there are a couple of issues with this design:1. Why spend $$$ and time on building a vacuum chamber for a mediocre LM399? The LM399 will typically return to a value within specs even after a longer time off. Just don't cool it down to very cold temperatures. Rather build a tiny 'oven' around it to keep it above lets say 15 °C (Kleinstein already suggested this).2. The vaccum feedthrough will likely be kovar in a glass seal. Since the vacuum chamber obviously needs to be bigger than the diode and is likely hand-made, the temperature gradient in the feedthrough will most likely be bigger than from the diode substrate to the the PCB. So the thermal EMF will be a lot worse in this configuration.If you want to build something around the LM399, I would recommend you buy a batch of LM399 (like 5-10) and bring about 6 months of time during which you monitor those diodes in a burn-in circuit (or the final circuit if you just want 7V out as the BOM cost ist relatively low). Burn them in for like 1000-2000 h and toss out any stinkers, that do crazy stuff like excessive popcorn noise or sudden 1-2 ppm jumps. Then subject the rest (probably half of them will get to this point), to a test, where you power cycle them to see how they behave. Again, toss those, that show unpredictable behaviour.This is probably the best you can get with the LM399, maybe low single digit ppm uncertainty over a year. By the time you done with that, the market value of that diode would likely be around $300, due to the equipment and man-hours spent. At that point, you may consider going for the LTZ1000, ADR1000 or even wait for the ADR1001, which is about the same price range (dunno about the ADR1001, though).

author: iMo
author_rank: Super Contributor
timestamp: August 30, 2022, 02:47:02 pm
content: Even the ADR1399 (also in smd, no kovar) would be a better choice, imho. Not sure on 1399's popcorning, but you can hardly find an LM399 without popcorn.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2022, 03:44:18 pm
content: Even when kept hot the performance of the LM399 is limited. A practical problem are the typical popcorn type noise jumps or some 0.5 ppm that essentially all lm399 show. Even those where they are less obvious may have them or start to show them after some time. The LM399 is mainly an easy to use, moderate cost reference good for something like a 6 digit meter. With the internal heater loop and high temperature it has its limitations and is a bit unflexible.For a transfer standard one could consider other alternative references instead of the extra effort of a vaccum and keep hot circuit. An LTZ1000 even without a keep hot circuit could be less effort and likely better performance. If in doubt have more than just 1 reference, possible different types.The LM1399 is rather new - so the longer term performance is less proven. It's lower noise, but not necessary lower drift.

author: iMo
author_rank: Super Contributor
timestamp: August 30, 2022, 05:26:32 pm
content: ADI offers "small portable standards" for hobbyists based on the ADR1399 classic, ADR1399 smd, ADR1000 and ADR1001. A small pcb powered by 5V from an USB, the reference is floating (via isolating switcher)..

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 31, 2022, 01:25:16 pm
content: Quote from: maat on August 30, 2022, 02:26:37 pmi am afraid you are missing the point, as are several other posters.the LM399 is agoodchoice to experiment with because it isCHEAPandOBTAINABLE. i really couldn't care less about the zener diode properties: snap, crackle, pop and drift. all i am interested in is an ovenized TO-46, and exploring how it can be transported 'live' for a usefully extended period of time.the ultimate solution - if indeed the vacuum idea does work - would likely end up being no more complicated or bulky than this:... a technology dating from the WW2 era, that is relatively cheap and easy to achieve. if it works, then someone else may care to invest in an LTZ1000, ADR1000 or ADR1399 and carry the experiment further. i'm just interested in the thermals and energy budget, nothing more.cheers,rob :-)

author: iMo
author_rank: Super Contributor
timestamp: August 31, 2022, 03:12:07 pm
content: Added a simple naive prop reg into the LM399.Assumptions - min heater current 5mA, max heater current 130mA, sensor temp 10C lower than the heater, prop delay between sensor and heater such it heats up to 90C in aprox 25secs, heater voltage 15V, heater max 300mW, vacuum 4700C/W, Volts=Celsius.. Has been made for fun only..

author: maat
author_rank: Regular Contributor
timestamp: August 31, 2022, 07:15:01 pm
content: Quote from: robert.rozee on August 31, 2022, 01:25:16 pmAh ic. Jolly good fun them

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 01, 2022, 12:44:24 am
content: Quote from: imo on August 31, 2022, 03:12:07 pmam i right in saying that your model suggests a decrease in power consumption from 250mW at 1atm (from LT's LM399 datasheet, pg3: 8.5mA @ 30v) down to less than 30mW when operating in a vacuum? ie, close to a 10:1 reduction in power consumption.if so, this isveryimpressive and good grounds to experiment some morecheers,rob :-)

author: iMo
author_rank: Super Contributor
timestamp: September 01, 2022, 06:19:22 am
content: Nope, that simplistic model shows the system in vacuum, at the beginning at 25C, then you switch on the LM399 heater, there is a short spike 300mW as the system is cold, and the the heat propagates from chip/package/legs into the pcb. The 4700C/W (??) is the thermal resistance of vacuum I took from a previous post here. You may see the temperature of the hole system/pcb grows slowly up to some 80C in vacuum. With something like 200C/W (??) for the air the pcb temperature will go to some 40C.The heater power goes down, but still there is an issue with the regulator model as the regulator does not include changes of the current (??), thus it is just a joke, imho. In reality the power loss of the heater at 25C air in equilibrium is something like 300mW (15V*20mA), this model shows rather BS in the air. Mind that is a pretty primitive model (everything there is just a guess), I made it while watching news in TV..It is intended for somebody here as a motivation to improve it..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2022, 06:34:27 am
content: Usually the thermal resistance from the PCB to the environment is way lower than 4700 K/W. There are at least the leads to the supply that have more heat loss.Another point for the dynamic part is that the thermal capacitance is allways to ground - so no series connection of the heat capacitance part.According to the datasheet 300 mW is the normal power needed in steady state air. For transients there can be more power.

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 01, 2022, 06:56:41 am
content: I must agree with Kleinstein. Forget about vacuum and use foam.Vacuum is hard to handle. Some remarks:every material is porous - the glass was usually used for vacuum chambers in the past and now it is used stainless steelideal chamber will be smooth to have small amount of adsorbed air on the walls as possible (you can help with heat to speed up evacuation, but most of the time you pump the air adsorbed on the chamber itself)smaller chamber works against you - quickly fills with gascreation of vacuum tight chambervacuum tight feedthrough is even bigger challenge than the chamber - as already recommended use Kovar and correct glassvacuum tight valve - how do you want to connect and disconnect your chamber from the pump?all apparatuses need to be constantly pumped - thus getter in tubes, mostly the gas get inside around the legsIn the picture that you have posted the tube itself may not be evacuated, resp. it can be that the vacuum is not essential (I do not see the Getter), it can be filled with inert atmosphere or small overpressure if needed. It was common way to use tube socket and tube housing for exchangeable parts. It can be easily filled with inert atmosphere or small overpressure if needed. It also has the good isolation properties etc. Except the fact that the tubes are fragile.If you don't have the access to the tube or bulb production line (ideally big double filament lamp like P21/5W, W21/5W or similar), than it will be hard for you to evacuate and air tightly seal the chamber after.In home conditions I think that the glass chamber is easiest solution. Take the W21/5W, cut the head off, install the reference instead of the filaments, weld the glass prolongation (you must be careful not damage the reference with the heat), connect to vacuum and collapse the tube with torch. Example how it can be done in "home" conditions when you have better access to correct materials is for examplehere.

author: iMo
author_rank: Super Contributor
timestamp: September 01, 2022, 07:06:03 am
content: This is with thermal capacities towards ground (ground is virtual ENV ambient), and in the "air" at 25C. There is still the issue with the heater power, we need a better model, indeed..The current model isP=V(Vcc)*1.5*(20mA+130mA*(100C-V(chip_T)))P=V(Vcc)*(5mA+130mA*(100-V(chip_T))/90)where Vcc is 15V and the chip_T is the temperature measured by the on-chip sensor (delayed by some prop delay, targeted 90C with 100C at the heater),20mA5mA is min current (obviously wrong) and 130mA is max current (the max could be ok, but there is something missing here), and 100C is max heater temperature (perhaps could be higher, like 150C ??).PS: I've found a problem in the model - there should be a ratio deltaT/T90 in the model, as well as the scaling of the Power graph was wrong.Now a little bit better model, imho, below..

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 01, 2022, 08:23:18 am
content: Quote from: DavidKo on September 01, 2022, 06:56:41 amas you have pointed out, stainless-steel and 'the right' glass are not porous. copper is also an option. and a non-porous small chamber will not fill with gas as quickly or slowly as a large non-porous chamber: that is, both will not.feed-throughs are available, and there is an epoxy called 'Torr Seal' (see:https://www.lesker.com/newweb/fluids/sealants-epoxy/torrseal/) that i have used in the past for temporary (<12 month) fixtures.the below tube is evacuated, and has likely remained so for at least 40 years without "need to be constantly pumped". thegetteryou refer to in a vaccum tube functions only ONCE when heated/activated with an external microwave heater after the tube has been initially manufactured. the purpose is to burn off any traces of contaminant not removed when the tube was initially evacuated. thereafter, the getter does nothing more for the remaining life of the tube.alas, everyone seems quite intent on making the problem far bigger than it actually is. at this stage it is just the THERMALS and ENERGY BUDGET being considered. IF these fall into line, and a copper vacuum chamber verifies that there are useful real-world results, then there is a fellow in europe who makes nixie tubes who would no doubt be more than happy to pop LM399's, LTZ1000's, etc into evacuated glass tubes for anyone interested.cheers,rob :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2022, 08:43:28 am
content: A getter helps to keep the vacuum for a longer time and reduce the need on the pumping time. Especially the feed through part can be expensive or still a bit leaky with epoxy. The epoxy solution can be OK for a system that is constantly pumped, but I would not trust it for a level good enough to make a significant improvement on the heat loss (e.g. < 0.1 mbar ).I would consider foam isolation and than also reducing the heat loss through the wires the better option. Faom is not that much worse than a vacuum im the 1 mbar range.The loss directly to air is also only one part (maybe 2/3) of the heat loss.Another option would be to use reduced heating when on transit. The LM399 does not show much hysteresis for cooling to normal room temperature. So it may be enough to keep it at above 15 C on transit with a different heater. For this it may be enough to keep a few mA of zener current all the time.

author: Gyro
author_rank: Super Contributor
timestamp: September 01, 2022, 09:03:28 am
content: Quote from: robert.rozee on September 01, 2022, 08:23:18 amJust a couple of corrections...The ring shaped getter is actually activated, or flashed, by a relatively low frequency RF loop inducion heater that is passed over the tube, not Microwave energy. This causes it to burst open and evaporate the Barium metal flash onto the inside of the glass. The getter structure itself is merely a 'sealed container' to get the barium compound into the tube without exposure to atmospheric air during construction. The RF coil can also be used to do a final outgas of the anode by heating to a high temperature.Regarding your definition of "functions only once": The Barium flashcontinuesto work as a getter throughout the life of the tube, reacting with stray gasses that make it past the pin seals. The exception being Helium which of course is inert and can slowly build up. Indication of its continued operation is the slow fading around the edges of the flash in weak tubes, and of course going white in the case of a seal failure.I regularly use former Soviet tubes that have been stored in cold damp warehouses for 40 - 60 years. The poor storage means that they (large volume power tubes in particular) sometimes need warming in a low oven for a while to speed up the chemical reaction rate of the getter flash and mop up any accumulated gasses before application of high anode voltages. Luckily the Mil spec tubes have several getters and lots of active flash.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 01, 2022, 09:20:17 am
content: Quote from: Gyro on September 01, 2022, 09:03:28 ami stand correctedit was 30 years ago when i last worked with vacuum tubes/valves. at the time, i recall being told by my supervisor (university) that the external heater we were using operated at microwave frequencies, but my memory (it seems) was faulty.for anyone interested in a refresher, see:http://www.thevalvepage.com/valvetek/getter/getter.htmin the present (LM399/LTZ1000/ADR1399) application, i'm only suggesting a vacuum of 0.001atm, at which level i believe a getter would contribute little. the pressurejustneeds to be lowered enough to (meaningfully) reduce convention heat transfer.cheers,rob :-)

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 01, 2022, 11:23:23 am
content: You can tryDalibor Farny(he did the video which I have posted and it is the guy who is producing the nixie tubes). Hard to say how tough will be connect wire to the legs. Probably some spacer will be needed to prevent shorting. To speed up the pump process I recommend to remove the termal isolation of LM399. Probably a lot of the gas will be trapped inside of the TO-46 case which will later leak and increase the pressure, but Getter should help there.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 01, 2022, 12:32:40 pm
content: Quote from: DavidKo on September 01, 2022, 11:23:23 amDalibor is likely the fellow i am thinking of, i saw a youtube video showing nixies being built around a year ago. another outfit doing this is Millclock (https://millclock.com/), or perhaps even the chemistry department of a local university may be able to fabricate something.here is an interesting video showing feed-throughs being formed, suggesting this part of the process is not too difficult for someone skilled in glassblowing:https://youtu.be/uHv1f4_tDv4?t=205the LM399 (et al) wires can likely just be spot-welded, although there may be some thermoelectric issues to be resolved. i don't see supporting the weight of the part being a problem. the plastic thermal shield would definitely have to go as it would likely outgas for an extended period of time - and wouldn't do anything much useful in a vacuum anyway.the problem of gasses trapped within the TO-46 metal case is an interesting one, although these gasses would only be at 15psi and hopefully the hermetic sealing of the metal case would provide adequate containment. and even if not, the internal volume of the TO-46 is relatively small. an extreme solution would be to carefully drill a tiny hole in the side of the metal case! a Getter is definitely NOT an option - firing it would likely destroy the LM399 (et al). but (hopefully) we don't need anything beyond 0.001atm anyway.the above all said, at the moment i feel the questions of (1) thermals and (2) energy budget need to be addressed FIRST. this can most easily be achieved by placing an LM399 into a small copper chamber (1" diameter copper pipe, 6" length, with  sealed ends) and pumping down to a modest 0.01atm (10mbar). then the behavior when powered up can be checked, to see if weactuallyget a drop in the heater current, and if the on-die temperature is stable enough for the zener/reference to perform within specs.cheers,rob :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2022, 01:27:54 pm
content: A pressure in the 0.001 atmophere range would not help much for the thermal conduction. The curve shown earlier is for a very short distance of 10 µm. With a length scale of 10 mm instead one would habe mbar instead of bar. So to get a significant reduction one would need to reach the 0.01 mbar range.At that level one would not use a metal getter (barium or titanium) like with vacuum tubes, but more like a some very porous (e.g. silica gel, perlite) material as an absobant that is heated for some time in vaccum. This effectively acts like a larger buffer so absorb at least water, but to some degrees also oxigen and nitrogen. For a single unit it woudl not be so bad to pump for something like 1 hour before sealing.The hermetic seal of the TO46 can should be gas tight. However the vacuum would be at least some mechanical stress to the chip. With some parts in a metal case one can see an effect of atomspheric pressure.Anway: vaccum is way more effort than foam insulation. The accuracy of an LM399 is also limited and it is not al all clear that cooling to some 0 C in between has a significant effect at all. At least just a normal power one - power off - power on cycle has not much effect (e.g. < 0.2 ppm , which is about what can be measured with the usual noise level). It is more that an extended power one time can lead to some drift.

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 01, 2022, 04:11:11 pm
content: Quote from: Kleinstein on September 01, 2022, 01:27:54 pmMaterial that we have used for cryosorption was zeolite. I suspect that it is the similar stuff as it is used for the cats. The one that we have used on university have had gray color. If I have not mixed that up, to use it with liquid nitrogen, you should heat it above 300°C to open air (mainly to get water out of it) for few hours. For complete out gassing to open air >600°C should be used. For sure if are able to out gas it into vacuum it is even better, but what I remember from lessons, the heating was essential due to its big surface (pores are hard to pump, especially water). RemarkDO NOT storezeolite in glasses withGround glass jointyou will not be able to open them later without destroying the bottle (after out gassing, but it is also risky to do it with material which have been previously on air).Vacuum is pain in the a.. and if you do not have access to it at work or university, than it will cost you a lot of money. If you have no experience with vacuum than it is long way to get familiar with it. I liked to work with vacuum (nothing better to see than liquid nitrogen stops boiling and freezes), but I do not want to pay for it even second hand.

author: iMo
author_rank: Super Contributor
timestamp: September 01, 2022, 04:28:35 pm
content: It depends on what you want to achieve with the "vacuum". My first job was in a lab with ultra vacuum and sure for that level you would need several stages - like mechanical pump, turbo-molecular one and that zeolite one. Btw, a couple weeks back I had short discussion with DaliborF on the exactly same topic and basically it is doable, but there are some issues when one wants to get a pcb with electronics put into the tube - like pretty high temperatures during production, those wires coming out of the tube are not easy to solder, and the pcb and the components on it should not produce any gases, of course.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 01, 2022, 04:57:23 pm
content: Lots of DIY vacuum info here-http://www.belljar.net/articles.html

author: MegaVolt
author_rank: Frequent Contributor
timestamp: September 01, 2022, 07:18:52 pm
content: Replace vacuum with airgel.

author: iMo
author_rank: Super Contributor
timestamp: September 01, 2022, 08:13:16 pm
content: Now it gets interesting - the targeted temperature 90C with heater at 92C (on-chip temp difference), and all in vacuum and starting at -10C ambient. The heater power is dropping down to 100mW and goes even less while the pcb with components goes towards 80C after 500secs .. So the simple model starts to show some signs of life..

author: MK
author_rank: Regular Contributor
timestamp: September 01, 2022, 08:42:59 pm
content: To get the epoxy to stop outgassing later you would need several weeks of pumpdown before sealing. You will need to draw all the moisture and volatiles out of the pcb or the vacuum will quite quickly get worse. I prefer the mickle approach of putting it in a dewar flask to help it stay at constant warm.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: September 02, 2022, 05:56:18 am
content: Quote from: MK on September 01, 2022, 08:42:59 pmwhy put the passive components within the vacuum chamber? they will not share the same temperature stability that is afforded the die of the LM399/LTZ1000. it makes far more sense to JUST place the reference (TO-46 can) within the vacuum chamber with leads brought out for attachment of any remaining components.can ANYONE explain the results in the attached paper? not wanting to put too fine a point on it, but the paper discusses heat dissipation from afiretruckrunning TRAIN when placed in a mild (0.1atm) vacuum..."Abstract: The vacuum tube transportation (VTT) system has been a promising direction of futuretransportation. Within this system, a high-speed maglev travels in a low-vacuum environmentto reduce aerodynamic drag. However, theheat dissipationof on-board heating devices will becompromised under low-vacuum conditions, and the device performance may thus be lowered. Thisstudy investigates the low-vacuum conjugate heat transfer characteristic of a levitation electromagnetmodule of a maglev using an experimentally verified numerical method. During the heating process,the surface temperature distribution of the levitation electromagnet, and the temperature and velocitycharacteristics of the flow field are examined. It is found that, as the vacuum level increases from1.0 atm to0.1 atm, the total heat dissipating from the levitation electromagnet module is decreasedby49%at 60 min, the contribution ofconvection heat fluxover the total heat flux is decreased from49% to 17%, and the convection heat transfer coefficient of the levitation electromagnet is decreasedby 89%. This study can provide an efficient numerical model for low-vacuum heat transfer studyon a VTT system as well as help the evaluation and optimization of low-vacuum maglev thermalmanagement systems."(my bolding added for emphasis, original from here:https://www.researchgate.net/publication/339111964_Influence_of_Vacuum_Level_on_Heat_Transfer_Characteristics_of_Maglev_Levitation_Electromagnet_Module, linked in reply #13 above)cheers,rob :-)

author: Kleinstein
author_rank: Super Contributor
timestamp: September 02, 2022, 07:40:02 am
content: For many reference circuits the parts around the LM399 can also contribute quite a bit to the drift. Once you take the effort of a vaccum, secondary oven or similar it can make sense to use it for more than just the reference chip.The VTT case is about getting rid of large amounts of heat and this is mainly by convection and forces air cooling (fan). Convection and forced air cooling get less effective quite fast. So it is easy to get rid of the convection part, but this does not apply to the conductivity part.The surprising but still true point is than thermal conductivity is not depending on the gas pressure. The reduced heat capacity per volume is compensated by the longer distance the molecules can travel. The concept of thermal conductiviy breaks down if the gas pressure gets so low that the distance of free travel for the gas molecules gets comparable to the dimensions. At a lower pressure the heat transfer no longer follows the same rules and the parameter thermal conductivty makes less sense, as the equation it is ued in no longer applies.

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 02, 2022, 08:23:41 am
content: VTT is still the same case as what was mentioned before. You can block convection with isolation - mechanical or with lowering the pressure. Thus they have the issues with cooling, nothing new there, one of the standard things when you work with vacuum. It simply does not matter for your project the way you go. With modern materials there is no need to use vacuum in your application, since you do not have any issues with drag coefficient. Vacuum use is more or less obsolete, due to the challenges it comes with, except the areas where it is unavoidable or cheap to use.Vacuum apparatuses - heavy, bulky, need to be constantly pumped, shape is more or less given due to the outer pressureInsulation materials - depends on the depth of your wallet (cheap=bulky) and it will be mostly light even when if it is bulky, easy to work with, a lot of types ...From what you have described, I expect that you want to have the insulation of the reference. You can go for sure with vacuum, but I expect that at the end you will not be able to achieve it (meaning vacuum long term stability), pay for it or it will not fit your application. Definitely it is interesting idea like VTT, someone simply checks the blind ways the hard way.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 02, 2022, 04:56:05 pm
content: A bit OT, but I just received an ADR1399KEZ in the LCC package. My mistake as I really wanted the leaded device. Datasheet says they're similar but the LCC has potentially more hysterisis. I'm thinking of dead-bugging it, which will change the thermal conditions substantially compared to a PCB layout. Not vacuum, but probably less heat loss than the leaded package, especially if I use very fine leads. I might even use fine manganin, as that package has force and sense leads for the diode, making lead resistance a (hopefully) non-issue. Manganin is 22 W/m-k vs 398 for copper. Thoughts?

author: Andreas
author_rank: Super Contributor
timestamp: September 02, 2022, 05:48:54 pm
content: Quote from: Conrad Hoffman on September 02, 2022, 04:56:05 pmThey need more heater current due to lacking thermal isolation (the hat is missing).I would use additional thermal isolation around the device.with best regardsAndreas

author: iMo
author_rank: Super Contributor
timestamp: September 02, 2022, 06:35:26 pm
content: It has got sense leads, but with rather large current (like 2.2mA)..

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 05, 2022, 05:34:42 am
content: Quote from: Conrad Hoffman on September 02, 2022, 04:56:05 pmWhat about putting it on flex board and create a sandwich between 2 insulators. I'm still thinking about how to not over insulate the reference. You still need some heat loss for correct operation. Probably measurement will be necessary - checking the heater current at maximum expected ambient temperature and tuning the insulation thickness or adding some artificial loss (screw or metal sheet which can move and influence the heat loss).

author: Kleinstein
author_rank: Super Contributor
timestamp: September 05, 2022, 06:46:38 am
content: The LM399 / ADR1399 runs quite hot at some 90-95C and the reference part only needs some 1 or 2.5 mA and thus not much minimum power. So it would be hard to over-insulate. The thermal resistance no longer goes up linear with insulation thickness once the thickness is larger than the source size - it turn to more like a log law (and even slower for a point source) and thus deminisching returns of thicker insulation. The wires alone give plenty of heat loss unless really thin.A small flex board for the LCC version could make sense , though one still needs to somehow hold the reference in place.

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 05, 2022, 07:59:14 am
content: Quote from: Kleinstein on September 05, 2022, 06:46:38 amFixing should be done by the insulation material itself (thick foam insulation from packaging material that is quite rigid and easily available), maybe glass wool in small cut out for direct contact. Put it between two flat parts and done. I suggest that EPS will not be suitable due to the high temperature of reference itself.Such an approach can be used for whole family (LMx99, ADR1399 both). For classic version the flex board is not necessary since the legs can be easily insulated or 4 holes can be "drilled" directly to the insulation material. Maybe even the in the case of death bug approach mentioned by Conrad, the sandwich can help to fix the legs.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 08:07:01 am
content: ADR1399 LCC in 1cm thick expanded polystyrene with 6x 0.1mm dia copper leads..

author: DavidKo
author_rank: Frequent Contributor
timestamp: September 05, 2022, 08:43:13 am
content: Are you able to calculate that the isolation of the wires will be longer? It will increase both resistances, but maybe it make sense to make the thermally insulation of wires longer.Sorry for the dummy question, but I'm not to much familiar with thermal simulations. In case of 1 cm insulation the heat transfer of wires already dominates and the total loss will be cca 0.23W cca 15mA@15V.

author: iMo
author_rank: Super Contributor
timestamp: September 05, 2022, 10:23:08 am
content: The wires in that example go straight through the polystyrene wall (here 1cm thick), thus the length of the wires is 1cm. If the walls of the box will be thicker, the wires will be longer. With 2cm wall the Q_wires will be half. There are two shape factors for edges and corners, which depend a bit on the ratio of the wall thickness and the size of the box.Below a table with the results, with params as above, you may see for 20mm wall one needs a better shape factors.. Mind the inner box size is the size of the LCC package (5x5x1.5mm).Also note the Q depends on the temperature difference between LCC surface and outer box temperature (I set to 70C), not on the ADR's Vcc*Icc..