Suggestions for a Temperature Sensor - Page 1
Forum section: electronics / projects

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 06:33:27 am
content: I am looking for suggestions for a temperature sensor. Currently I only have some cheap thermocouple probes with a 1degC resolution and I need a lot more.I haven't bought a temperature sensor for years, so ideas would be great.The task is this. I want to test the stability of various resistors over time and temperature, and so I don't need absolute accuracy for the resistance measurement, but I want better than 1ppm relative accuracy. This is for building a Kelvin-Varley divider, and I also want to make a 9:1 and a 100:1 Hamon divider.So I am going to take the readings on a 7 1/2 digit meter using a Leeds and Northrup standard resistor as the reference.The L&N standard resistors are made from Manganin wire, so I first have to characterize it over temperature. With temperature compensation, these standard resistors are good as a reference for better than 0.5ppm for at least a week. Without compensation for temperature, the errors can be up to 50ppm. There is a standard formula for compensating, but I need to calculate two coefficients for the formula.I tried with my thermocouples, but to get a curve accurate enough to calculate the characteristic curve of the resistor, I really need 0.01degC resolution and pretty good linearity.Here is the curve I did measure with the thermocouple temp meter, and an Advantest 6871E-DC meter. It looks promising since it looks like the meter is capable of about 0.2ppm stability over the time I am measuring the standard resistor:So what I am looking for is:1 degC absolute accuracy to 35 degC if I calibrate for 0 degree point.0.01 degC resolution and stabilityGood linearity from 10 degC to 35 degCLess then 7mm diameter, but the smaller the better.Can be an IC, a probe, some kind of thermistor circuit. If it is a part like an IC, I will have to make my own sealed probe.I am not after spending big $$$ for proper instruments. Anyone got some something that has worked for them?Richard.

author: BravoV
author_rank: Super Contributor
timestamp: December 02, 2011, 06:43:14 am
content: Though I can't give you any suggestion, just want to say what a nice resistor you have there !Just curious, what is that hole in the middle for ?

author: IanB
author_rank: Super Contributor
timestamp: December 02, 2011, 06:54:06 am
content: I think the kind of temperature sensor you want would be some kind of resistance temperature detector, consisting of a coil of wire (like Manganin perhaps?) which has a variation of resistance to temperature that is accurately known in advance. You measure the resistance and from that compute the temperature.Joking aside, the reference standard would be a platinum wire sensor, since pure platinum has predictable properties obtained from standard reference tables. The question becomes, is the cost reasonable?(I suppose you might obtain some thin platinum wire from a jeweler's supply and construct your own thermometer...?)

author: IanB
author_rank: Super Contributor
timestamp: December 02, 2011, 06:54:42 am
content: Quote from: BravoV on December 02, 2011, 06:43:14 amI'm guessing that's where you put the temperature sensor.

author: IanB
author_rank: Super Contributor
timestamp: December 02, 2011, 06:58:59 am
content: A mercury in glass thermometer might be sufficient. Can you obtain one of those from a scientific supply house?

author: grenert
author_rank: Frequent Contributor
timestamp: December 02, 2011, 07:04:04 am
content: Just about anything you would want to know about your three basic options of RTD, thermocouple and thermistor (for electrical measurement of temperature, anyway) can be found at Omega:http://www.omega.com/temperature/Z/zsection.aspOne of their starter documents is here:http://www.omega.com/temperature/Z/PracticalGuidelinesforTemperatureMeasurement.htmlYou should get one of those Vishay bulk metal foil resistors with the super-low tempco to compare with

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 07:10:06 am
content: Quote from: IanB on December 02, 2011, 06:58:59 amA glass thermometer. What a weird idea.I will check out how much a good one costs.Richard

author: mobbarley
author_rank: Regular Contributor
timestamp: December 02, 2011, 07:11:14 am
content: Not sure if these fit the bill:Analog:Quoteor Digital:Quote

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 07:13:49 am
content: Quote from: BravoV on December 02, 2011, 06:43:14 amThat is the temperature well to measure the resistor. Brass lines and 100mm deep.With these resistors, you have to know the temperature accurately for them to be much use for really accurate measurements.

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 07:16:32 am
content: Quote from: mobbarley on December 02, 2011, 07:11:14 amThey both sound great. I will check them out to see the smallest package size.Richard

author: mobbarley
author_rank: Regular Contributor
timestamp: December 02, 2011, 07:18:58 am
content: The digital one is in something like a DFN so you might have to sling some wires on it and whack it in a small tube full of goop?

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 07:23:14 am
content: Quote from: grenert on December 02, 2011, 07:04:04 amBelieve it or not, one of those Vishay resistors would cost me more then I paid for the L&N standard resistor. The beauty of the standard resistor is it is old - it will probably not change much. Once I have the curves and a suitable temperature measuring device, it will be better as a reference then a Vishay foil resistor. If I heat it to 28C +/- 1C, I can use it as a relative reference to 1ppm, and I am hoping to do better.I will check out the links.Richard

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 07:27:08 am
content: Quote from: mobbarley on December 02, 2011, 07:18:58 amI probably need to make a probe that is water and oil resistant - the standard resistors do like leaking a bit of oil. So I will have to put the sensor in epoxy or something similar. Maybe play with some glass tubing. Haven't done that since I last had to touch Chemistry.

author: robrenz
author_rank: Super Contributor
timestamp: December 02, 2011, 11:06:27 am
content: Platinum RTD on 8846A , you should be able to get similar on any 6.5 digit meter with 4 wire ohmsTest Current ............................................................ 1 mAAccuracy is stated as ± °C and is based on a Platinum RT100 (DIN IEC 751, 385 type) RTD with less than 10 ohms leadresistance. The accuracy listed in the table below are valid only when using the 4-wire RTD measurement function.Specifications do not include probe accuracy, which must be added.AccuracyRange Resolution 90 Days (23 ±5 °C) 1 Year (23 ±5 °C) Temperature Coefficient/ °C Outside 18 to 28 °C0 °C 0.001 °C 0.04 0.06 0.002100 °C 0.001 °C 0.05 0.08 0.002

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 12:02:43 pm
content: Robrenz, those platinum RTD's look fantastic! I thought they sounded expensive, but PT100 and PT1000 probes can be amazingly affordable.I ordered two class A (0.2%) PT1000 sensors. I though 1000 ohms sounded a bit easier to read. 1000 ohms at 0 degC and 1384 ohms at 100 degC. Linear in between. I will be able to get all the resolution I need. And no extra electronics.With these, I should be able to achieve up to 0.3 ppm resistance accuracy if I warm the Standard Resistor between 26 and 29 degC and 1ppm outside this with no extra calibration of the sensor.The two probes will cost about $20 including postage.Richard.

author: robrenz
author_rank: Super Contributor
timestamp: December 02, 2011, 12:47:56 pm
content: Quote from: amspire on December 02, 2011, 12:02:43 pmRichard, At first I thought you might be going the wrong way with the 1000 vs the 100 since Fluke picked the 100 to do their temp specs (you would think they would use whatever would give the best accuracy). But the 8846A 1000 Ohm range %of range adder is .001% versus .004% on the 100 Ohm range (more accurate). On the other hand you will generate .001W on the 1000 versus .0001W on the 100. So you might want to only take pulse temp measurement of 166milisec (10NPLC = 0% additional error) to minimize self heating of the RTD and correspondingly locally heating the oil

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 01:04:27 pm
content: Quote from: robrenz on December 02, 2011, 12:47:56 pmI can just use a higher resistance range on my meter. If I keep the PT1000 test voltage to 0.1V, then the power dissipation will be 0.00001W. That will still give me 0.003 deg C resolution. That should be fine. I will just have to remember to keep the meter off autoranging. At 7 1/2 digits resolution, my meter's autoranging would put 10V across the RTD which would be about 0.1W. The Advantest does not have any PT1000 mode, but it does have maths functions, so I can set it to calculate temperature from the RTD resistance.Richard.

author: robrenz
author_rank: Super Contributor
timestamp: December 02, 2011, 01:48:39 pm
content: Quote from: amspire on December 02, 2011, 01:04:27 pmRichard, Agreed, your meter on the 100K range uses 100microA so the .00001 Watt is correct. But your meter 180 day accuracy on that range is .006% of reading + 25 lsd (10mOhm) = +/- .3164 Ohm for a 1107 Ohm (28 deg) reading. that works out to +/- .029% (+/- .0836 deg C) accuracy at 28 degrees.But your meter 10K range 180 day accuracy on that range is .006% of reading + 25 lsd (1mOhm) = +/- .0914 Ohm for a 1107 Ohm (28 deg) reading. that works out to +/- .008% (+/- .023 deg C) accuracy at 28 degrees.EDITI forgot to add the .0004% of reading + 2lsd /degC temp coeficient errors on to the above calcualtions. I assume they are talking about temp deviation from calibration temp.IMO you don't need to continuously measure temp so use the 10K range very quickly and very infrequently.

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 02:35:27 pm
content: RobrenzWhen I get the sensors, I will try the different alternatives. I only need the high accuracy when I am plotting the R vs T curve for the standard resistor as I need a very clean curve to generate the resistor correction function that I need.Once it is done, I gather it never has to be done again for the resistor as it never changes. It doesn't matter that I do not know the exact absolute value of the resistor since the standard resistance correction function (a 2nd order polynomial) that is used metrology is purely a relative function.If you pay a metrology lab to generate this correction function for a standard resistor, they put it in a temperature bath and then cycle through a number of different temperatures, allowing the resistor to stabilize for at least 2 hours at each temperature after the bath has restabilized before measuring. It is automated, but it could take days. It sound unbelievably expensive.My method stick the resistor in the oven and heat it to 40 degC. Then I take it out and wrap it in insulation, and just let it cool down very slowly, taking readings every now and then over about 6 hours of cooling. Then to go the other way, I stick it in the fridge and then let it warm slowly. Is that good enough? I will have to find out.So you are right - if I have to measure temperature in very short bursts, that is fine as I will never have to do it again. I can just take a measurement of temp and resistance every 15 minutes, and leave the temp sensor disconnected the rest of the time.When I am using the standard resistor and need to correct for temperature, most of the time 0.2 degC accuracy is more then enough.I am really glad you made me check the voltages out of my meter. I was assuming it was more like 1V, but when you think about it, measuring anything below 1uV has all sorts of problems including thermal EMF, so 10V for 7 1/2 digits makes a lot of sense.Richard.

author: robrenz
author_rank: Super Contributor
timestamp: December 02, 2011, 02:57:03 pm
content: Richard, Sounds good, but with large temperature gradients involved you are going to want to use the the current reversal technique on your 4 wire ohms measurement to eliminate the thermal EMF errors.This would apply to both meters since you are measuring resistance in both cases.You can see the setup I made on this thread reply#15https://www.eevblog.com/forum/index.php?topic=5460.msg71401#msg71401Normally this would only be used on low Ohm measurements but because you have large thermal gradients I think it would apply at the 1107 Ohm range you will be in.Robin

author: robrenz
author_rank: Super Contributor
timestamp: December 02, 2011, 03:54:02 pm
content: Just thinking out loud: I think you might more accurate results using an oil bath. At least 1 liter, 2 would be better for more gradual temperature changes. All non-meter connections should be in the oil also. Must have a stirrer/circulator that willvery gentlycirculate the oil especially vertically to minimize the ever present vertical temperature gradient. If possible remove the protective cover from the resistor so the oil circulation actually gets to the manganin wire. This is important since you plan to take dynamic readings of temp and resistance. have a insulating sleeve to fit the oil bath ready. place the oil bath in an Ice bath and let it stabilize. You can use this temp to calibrate your RTD at 0 DegC. Then put the oil bath in the insulating sleeve and proceed with measurements as you planned. When it finally reaches room temp you then put it on a hot plate and increment the hot plate temp a little at a time so you get a nice slow temperature ramp. take your readings until you have reached your max temp. FWIW

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 02, 2011, 05:27:48 pm
content: I have a few of those resistors, though yours seems to have retained more shine than mine. They're quite good, though not as stable as some modern stuff. OTOH, mine, like yours, are old, so I'd expect whatever changes were going to happen would have happened a few decades ago. That said, I believe they have some hysterisis, so cycling them up and down may change the value a bit. Also, if you haven't changed the oil, do so. I use plain mineral oil from the drug store, but a transformer oil would be best. What happens is the oil becomes acidic and slowly changes the value of the resistor. You'll want to do a couple flushes. Leakage is normal if not stored upright. Another interesting piece of trivia- sometime back in the '50s or '60s the powers that be changed the value of the ohm by something large, maybe 0.05%. I can find this if you want. When I've had my "master" resistor measured at the cal lab, they wanted to reject it completely, as the resistance was out by about that much. I had a lot of trouble explaining to them that it was manufactured prior to the change, and that it actually hadn't changed by a gnat's whisker since the 1930s when it was made.As for sensors, one can probably get the most sensitivity from thermistors, but with a big correction curve. If you want to measure .001 degrees, they work great. RTDs are probably most accurate and, as you found, not as expensive as when they wound them from wire. I use the National Semiconductor LM3xx-something direct-to-kelvin sensors for most things these days. You can also wire two simple NPN transistors together to make a remarkably good temp sensor. Unless you find a very special one, few glass thermometers will do better than about 0.2 degrees resolution. The best will be mercury, but nobody wants to have those around anymore.The L&N (and other) resistors would originally have been suspended in an oil bath (no, you don't want direct contact with the element- leave the can on), with the arms immersed in mercury cups for the current contact. The voltage was measured on the inner terms. Naturally nobody does it that way anymore, but brand new multi-thousand dollar resistors are still built with the curved arms in just the same way.

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 11:22:49 pm
content: Quote from: Conrad Hoffman on December 02, 2011, 05:27:48 pmI have looked at some of the new ultra stable references, and one of the specs just makes unacceptable. Namely the one called "price". I think my L&N Rosa-type references will do extremely nicely.I do have five 100 ohm L&Ns as well -- they don't look as good as the 10K one. One is really old. It has no part number, but it is a pre-screwtop version. The top must just be press fit into the can I guess.QuoteI am sure some of my resistors are older then that, but none of them are 0.05% out. The really old looking one that is really old has a 1989 cal attached showing it to be 0.002% low. Curious.I will do an oil change. Thanks for the advice.QuoteThermistors will still need to be calibrated somehow. Also how stable are they? You get paranoid about everything with this metrology mania.QuoteLooks like the RTD's will do the job I need, but I think I will get a few temp sensor IC's just to have on hand. Just have to decide which.Richard

author: amspire
author_rank: Super Contributor
timestamp: December 02, 2011, 11:38:10 pm
content: Quote from: robrenz on December 02, 2011, 03:54:02 pmI know it is what I SHOULD do. I may end up having to doing it after finding my bodgy methods are not good enough.It definitely doesn't take much to mess up the measurement. To get my resistor below room temperature, I put it in a jar of 16 degC water. There is an error in my measured curve right at the point I did that - the readings are all lower then they should be, so obviously I had a thermal gradient in the resistor. The readings returned back to the proper curve when the resistor temp and the water temp got close again.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 03, 2011, 03:57:31 am
content: My info is a bit shaky, but apparently there was a difference in international units that was supposed to be rectified in 1939. Because of the war it was put off and the official change was made on 1/1/1948. For resistance, the correction factor to bring international/NBS resistance into agreement with the new absolute standard, was 1.000495, and would vary slightly between countries. This is from a table in Electrical Measurements (Harris), but I know I've seen it elsewhere.I have some L&N resistors where "abs" is stamped right below the value. No idea how long they did that. My best L&N is a 100 ohm unit in a chrome case. I have some later ones in aluminum cases, and some black painted ones of unknown vintage, probably quite old. No press fits though! The only one I've had measured at a cal lab is the chrome one. Calibrating up the decades with great accuracy is harder than one might think ;-)CH

author: amspire
author_rank: Super Contributor
timestamp: December 03, 2011, 04:24:55 am
content: The variation between my five 100 ohm resistors is 0.008 ohms which is a lot smaller then your 1.000495 change.I wonder if Australia and the US were a bit different back then, and we got resistors tailored for Australia? Australia almost certainly followed the UK standards, so we may have had no change, or perhaps a small one. Looking at my resistors, there seems to be two groups separated by about 0.006 ohms. The newer resistors are the higher ones.And I know about calibration up the decades. I made a Hamon divider with some 25ppm 0.1% smd resistors I had, and they just change way to much to be useful for accurate decade calibration. It does not take much power to make a SMD change temperature.Have attached a photo of the 4 different types of L&N resistors I have.Richard.

author: IanB
author_rank: Super Contributor
timestamp: December 03, 2011, 04:52:29 am
content: Quote from: Conrad Hoffman on December 02, 2011, 05:27:48 pmI'm puzzled by this, as mineral oil is completely stable and there is no way for it to decompose. For it to become acidic it would have to be in contact with something that contaminates it, such as moisture from the air. Oil would be able to absorb a certain amount of water, and this water would in turn be able to absorb acid gases such as carbon dioxide. Or the oil might perhaps have contained impurities that have decomposed over time...I don't know what is actually the case, but it would be a good idea to make sure the oil compartment is sealed and airtight in any event. I would guess also that food grade mineral oil is your best chance of a pure uncontaminated product at a reasonable price.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 03, 2011, 05:04:47 am
content: Those look really familiar- have you been messing around on my bench? I also have an old Guildline resistor, 10 ohms I think, but it lost its hermetic seal and thus its oil. I've never had the courage to try and fix it because I don't know exactly how it was put together. The bottom appears potted with epoxy or something. If you were sealing up an oil reservoir, naturally you'd put the cork in the bottom. Not! Last price I saw was something like $5k which seems like a lot for a resistor, but the specs are excellent. If you can find the square grey plastic General Radio Corp. resistors, they're not highly regarded, but my experience with them has been very good.As for the oil, I've read about the acid problem in various places and I think it's well established. The L&N resistors are not air tight- that may be a requirement for stability, but I'm not sure. It almost seems the opposite. Modern oils are probably better than the old ones, so maybe the problem has gone away. There's also the issue of what they constructed the resistors with- I've seen glyptol, there's fabric or silk and I suspect they couldn't have gotten every trace of flux out of there. I do know if you smell the oil from an old L&N it's pretty awful.BTW, there was a big Jim Williams article on temperature measurement that's good reading. It should be on-line somewhere, maybe EDN.CH

author: Unknown
author_rank: Unknown
timestamp: December 03, 2011, 05:15:05 am
content: You guys are slowly getting into serious metrology. I enjoy reading it.

author: IanB
author_rank: Super Contributor
timestamp: December 03, 2011, 05:16:57 am
content: Quote from: Conrad Hoffman on December 03, 2011, 05:04:47 amYes, pure oil may be stable, but there are plenty of other things that are not. My worst bugbear is the kind of soft, tactile rubber or foam that is often used for cushion grips these days. After a few years it seems to decompose into a messy, molten gloop. Ugh!

author: amspire
author_rank: Super Contributor
timestamp: December 03, 2011, 11:34:21 am
content: I bought some mineral oil today from our local Chemist. In Australia, it seems to called Paraffin Oil rather then Mineral Oil.I needed to fill the resistors all up anyway after years of seeping oil. I will post some photos of the internals when I get back home in a few days.Not sure if I can open the one on the left in the photo.Here is where I am going with all of this. I want to test to see how some ordinary 50ppm/C metal film resistors behave with temperature variation, and any other interesting resistors that are on hand. I made an "oven" out of two metals strips with a foam sheath that allows me to fill with a lot of resistors between the strips and let them cool slowly. The leads come out each side so i can easily test them all as they cool through the temperatures. The setup allows me to cool over about 3 to 5 hoursI want to see how well different resistors in a batch match for temperature curve, how much error they have after a thermal cycle, and if any resistors are worth an attempt to stabilize with temperature compensation, or is there too much variation in the temp curves.Richard

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 03, 2011, 05:55:21 pm
content: Cool! (hot?) The results should be interesting. I only did a casual investigation, but found that the inexpensive metal films I bought from Digikey ($8 for 200 back then) were quite good, but the tempco could be plus or minus and there wasn't much consistency in the bag. It's probably value dependent as well, since the materials are different. In making up my own resistors one of the better materials can be constantan. It's cheap and usually has a very low tempco. The problem is the huge thermal emf with copper, thus its use as a thermocouple. For AC or where the thermal emf cancels out, the stuff might be better than manganin. When I did my resistor tests I just immersed the resistor, clip leads and all, in a beaker of oil, warmed to the temperature desired. I've always been a big fan of instant gratification! The cheap metal films biggest weak point was the permanent change in resistance during soldering. After selecting them, and using a clip on the lead to sink the heat, I always needed to swap out one or two in a decade to bring things back to trim.CH

author: amspire
author_rank: Super Contributor
timestamp: December 03, 2011, 09:58:08 pm
content: The oil bath works great, but I want to be able to leave the resistors on the bandoliers.I will see how it goes.I did notice that Constantin seems to have something like 10x the EMF of mangagin, and I gather it really should be welded, not soldered.I have also been surprised how much difference you can get between different batches of the same wire. Definitely, there is a big difference in stability between making all resistors in a divider out of one batch of wire, and making it from different batches.The super stable resistors seem to be made of Evanohm and they use multiple resistors with different temp coefficients that all cancel out.Richard

author: IanB
author_rank: Super Contributor
timestamp: December 03, 2011, 10:40:45 pm
content: I'm sure you have this one nailed already, but when winding your own resistors it is important to use a winding pattern that avoids the wire stretching when it cools below the temperature you wound it at. Also to prevent problems if the wire warms up, when it could snag and then stretch when it cools down again.

author: amspire
author_rank: Super Contributor
timestamp: December 08, 2011, 12:32:28 pm
content: I have replaced the oil in some of my Leeds and Northrup Rosa/NBS Standard resistors, following Conrad's advice. The construction of the resistor element in different vintages of resistors is very consistent. They stuck to the formula very closely.I have added some photos.This design has been around since the early 1900's, and metrology labs were still using them in the 2000's. I am sure they are still in use today. They are not the best standard resistors, but their behavior is extremely well known. It all comes down to confidence, and the labs are slow to trust new resistor designs.Here is the insides of the newest resistor - the 10K one. The central brass tube is the temperature well so the resistor temperature can be measured. The resistor is wound with a cloth covered Manganin wire on a laquered brass former. Brass and Manganin have a very similar temperature coefficient of expansion.For full accuracy, the maximum dissipation is 0.1 watt but if you only need 0.02% accuracy, you can go all the way up to 1 watt.The covering is important . Manganin is badly affected by any stress on the wire caused by winding, and silk-cotton blend was the most successful material they could find to minimize stress in the wire as they were winding it..To calibrate the 10K resistor, they added a small 2.5 ohm resistor wound on one of the terminals.The 100 ohm resistor is pretty similar, but no extra calibration resistor. They just stripped the insulation off each end of the manganan wire calibrated it by adjusting the brazed joint to the copper terminating wires.If you look at the closeup, you can see the wire was wound as a pair of wires, so it will be a Bifilar winding to minimize inductance.Richard

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 08, 2011, 03:53:41 pm
content: Good photos! I've never seen the extra calibration resistor before, but Guildline added a parallel resistor to fine tune my 10 ohm unit.Just to give an idea of how badly stress can affect precision resistors, I once wound a 100 ohm resistor on a brass form. I kept the winding slightly loose and made many measurements. The thing was dead stable. I then sprayed it with a coating of Krylon Electronic Clear. The performance was terrible! It immediately developed an offset from the original value, plus a large temperature coefficient. Baking and ageing it didn't help. I washed off the Krylon in solvent and the value and temperature stability came right back to normal. IMO, Krylon is not a very rigid coating, probably more flexible than Glyptol. When Julie Research was winding precision resistors they always left the wire completely loose on the bobbin inside the plastic shell. You can saw one apart today and easily reuse the wire- if you can spot weld it.Did you know that manganin is used as a sensor for the pressure waves from an explosion?CH

author: Unknown
author_rank: Unknown
timestamp: December 08, 2011, 04:05:54 pm
content: Thanks for the photos Richard, very interesting indeed.The cloth cover means that these were not annealed after winding. Apperently this has no significant impact even at these levels of accuracy.If one wanted to redesign this thing to be able to withstand annealing that would increase cost at least tenfold.Making the thermal well out of brass neatly prevents thermall stress but adds quite a lot thermal mass. Another curious point is the capacitor formed between this core and the winding and its effects on the resistance.

author: robrenz
author_rank: Super Contributor
timestamp: December 08, 2011, 05:44:38 pm
content: Quote from: HAL-42b on December 08, 2011, 04:05:54 pmIn looking at evanohm material the exact tempco is achieved by exacting heat treatment after cold working. see page 2 of this hpmetals.com/download/Evanohm-R.pdf so annealing above 475degF woud be a very bad thing. I dont know if this applies to Manganin but I bet Conrad does.

author: Unknown
author_rank: Unknown
timestamp: December 08, 2011, 06:19:56 pm
content: Quote from: robrenz on December 08, 2011, 05:44:38 pmWell the document says 475 degrees Celsius which is 887 degrees Fahrenheit. Also it does not say it is a bad thing, on the contrary it says it is more or less mandatory.Quote

author: robrenz
author_rank: Super Contributor
timestamp: December 08, 2011, 06:38:55 pm
content: Quote from: HAL-42b on December 08, 2011, 06:19:56 pmMy bad, The point I was making is the anneal sets the tempco and is unique to each melt of material. so the heat treat curve they mentioned will have to be very precise to achieve a very low tempco. Interesting side note Evanohm is made by Carpenter technology and they are about 25 miles from my home and the Hamilton company the pdf is from is only about 10 miles from my home.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 08, 2011, 07:41:43 pm
content: I too can be approximated by a sphere, but I digress... In a past life I did a lot of work with Invar and Super Invar, so I'm a big fan of Carpenter Technology. They also make some of the nicest machining stainless steel around, Project 70 I think. Though I've read all the same stuff about the heat treatment, I've zero experience trying it. Most of my efforts have been to wind very stable standards without heat treating, though I've been known to do a mild bake just by applying a suitable voltage for an hour or so.BTW, here "paraffin oil" is the stuff used in odor free kerosene lamps when you don't want to use kerosene. It's definitely not what we call mineral oil, and I doubt you'd want it in a resistor. I'd do some searching and confirm what's what. There was a big discussion of this topic on one of the machinists forums a while back, and it's amazing what different substances you can get under a given name, depending on what country you're in.CH

author: robrenz
author_rank: Super Contributor
timestamp: December 08, 2011, 08:15:56 pm
content: link to Carpenter Evanohm data.Evanohm Rhttp://cartech.ides.com/datasheet.aspx?I=101&TAB=DV_DS&E=27Evanohm Shttp://cartech.ides.com/datasheet.aspx?I=101&TAB=DV_DS&E=26looks like the better tcr

author: IanB
author_rank: Super Contributor
timestamp: December 08, 2011, 08:18:13 pm
content: Quote from: Conrad Hoffman on December 08, 2011, 07:41:43 pmAll of these substances are related, and practically speaking it comes down to the purity and the viscosity of the stuff you are examining.As a chemical term paraffin refers to the family of straight chain unsaturated hydrocarbons (the alkanes). The small molecules are gases and liquids like propane, butane and (components of) gasoline, the heavier molecules are found in kerosene, and heavier still in oils and waxes.Paraffin oil, also called mineral oil, is the kind used medicinally as a laxative and in food production machinery as a lubricant. If the stuff you have comes from a chemist or pharmacy, is safe for internal consumption, and is thick and viscous, then it is going to be pure good stuff. When the molecules get even bigger you have paraffin wax, which is solid at room temperature. Paraffin wax is inert and is a very good electrical insulator.If what you have is thin and pours easily, and is sold as a fuel, then it is not the right stuff, although chemically it will be rather similar. If it is odorless it probably is pure, but it will have the wrong physical and mechanical properties.

author: Unknown
author_rank: Unknown
timestamp: December 08, 2011, 08:47:56 pm
content: What I know aboout heat treatment of metals comes from a book calledMetallurgy of Steel for Bladesmiths & Otherswho Heat Treat and Forge Steelby Em. Prof. Dr. John D. Verhoevena highly recommended read and it is available for download.As far as I understand it, annealing stress relieving etc. are always fairly exacting processes. Interestingly it is not about the temperature of the metal per se, but rather the rate of change of the temperature between two specific temperature levels.In our case the annealign temperature is around 475 degC. This means that the metal has to be heated to a temperature somewhat above this and soaked at that temperature for certain amount of time. Then the real annealing happens by allowing the metal to cool at a rate not exceeding X degrees per minute until reaching the lower limit say around 350 degC, this is the annealing process proper. The cooling rate below the lower limit is of no consequence.A possible fly in the ointment is that high temperatures cause metals to oxidize and this is something we definitely want to avoid in a resistor which is meant to be stable. So the annealing of manganin or evanohm has to be carried ideally under vacuum.P.S: The spherical cow has to do with our tendency to simplify things which we can not comprehend. It is not about surface curvature

author: Unknown
author_rank: Unknown
timestamp: December 08, 2011, 09:52:36 pm
content: Using an inert gas like nitrogen or argon would be much easier than vacuum, and has less issues with leakage since it can be positive pressure. These gases may not be readily available in a home lab, but neither is the ability to evacuate almost all of the air.

author: amspire
author_rank: Super Contributor
timestamp: December 09, 2011, 12:36:42 am
content: Quote from: Conrad Hoffman on December 08, 2011, 03:53:41 pmThe main windings on the L&N resistors are definitely not varnished. They are actually wound fairly firmly, but they can probably do that with the silk/cotten cover as it probably has enough give to prevent any pressure spots in the wire.Quote from: HAL-42b on December 08, 2011, 04:05:54 pmThe 1 ohm Thomas resistors have manganin wire that is annealed at about 500 degC, but they can do that as the windings are self supporting - they do not need a former. It is hard to do with any multilayer wound resistor as insulation cannot survive the temperature. Also at that temperature, the wire oxidizes, and it then has to be pickled to remove the oxide. Much easier to do with a free standing Thomas resistor then a resistor wound on a former.Edit: Or as Hal-42b suggests, heat in a vacuum or an inert gas.I have read that the standard way to stabilize a new Manganin wound resistor is to age it at 120 degC for 24 hours, so I would guess that L&N did that with these resistors. Not as good as annealing after winding though.QuoteThese resistors are usually need about 2 hours in a temp controlled oil bath to stabilize before measuring. The newer ultra stable standard resistors have built in temperature sensing often by a second slightly less stable standard resistor thermally connected to the main resistor. By measuring the difference in resistance between the two resistors, you can use a chart to compensate for temperature errors. It means the newer resistors can often be used immediately without waiting for anything to stabilize.All the information about Evanohm is great. I was wondering how the TCR was controlled. The IET Labs standard resistors are made using multiple Evanohm resistors that have TCR's that cancel out. So it looks like you can give different batches of wire different heat treatments to give different TCR curves, and then you can carefully mix them together to get the most stable blend.Richard.

author: amspire
author_rank: Super Contributor
timestamp: December 09, 2011, 12:53:44 am
content: Reading the Evanohm data sheet, it looks like it does not have one of the chronic problems of Manganin. With Manganin, if you heat up the wire to 100 DegC +, the resistance will almost certainly change significantly and permanently.Evanohm does not seem to change significantly at temperatures up to 200 DegC.Very impressive!

author: amspire
author_rank: Super Contributor
timestamp: December 09, 2011, 04:10:43 am
content: It sounds like "Mineral oil" can have an extremely variable composition, so it is hard to know if it is corrosive or not.The oil recommended by Tinsley Precision Instruments in the UK for their standard resistors is:Castrol Whitemor WOM14http://www.tds.castrol.com.au/pdf/10021_whitemor_wom_14_2010_10.pdfThis is a highly refined food-grade mineral oil.I have used "Paraffin Oil B.P." from the chemist in some of my resistors, I will try it and see how it goes. The "B.P." means it passes the British Pharmacopoeia 2009 specification which includes testing for presence of any acidity or alkalinity, Sulphur compounds and Polycyclic Aromatic Hydrocarbons. I suspect it will be OK. The neutrality test is to boil a 50/50 mixture of oil and water. Then add a drop of phenolphthalein TS and it must not turn a shade of pink. Then add a drop of very diluted Sodium Hydroxide solution and it should turn a slight pink. I have the exact procedure if anyone needs to test their oil.The question I have is at what temperature does it solidify? I suspect it is something like -9 degrees, but I have put some of the oil in the freezer to see if it stays liquid. So far, it has thickned, but no solids.Richard

author: IanB
author_rank: Super Contributor
timestamp: December 09, 2011, 04:21:11 am
content: If mineral oil is food grade you can assume it will not be corrosiveThe basic constituent of mineral oil, long chain paraffinic hydrocarbons, is pretty much inert. Any problems will come from additives or impurities. For instance, lubricating or industrial oils may contain various additives to improve their properties for their intended use.Acidity, alkalinity, sulphur compounds or aromatic hydrocarbons are all things that should not be in pure paraffin oil.It probably won't solidify in a domestic freezer. The freezing point will be somewhere below -22 C most likely. But does it matter to you when it solidifies? Will this make a difference to any use you have for it? (I suspect not.)

author: amspire
author_rank: Super Contributor
timestamp: December 09, 2011, 05:05:48 am
content: Quote from: IanB on December 09, 2011, 04:21:11 amI just wanted to make sure it was still fluid enough to circulate heat at all practical temperatures for the resistor, and it looks like it does. I am sure it wouldn't cause any damage if it did solidify.

author: BravoV
author_rank: Super Contributor
timestamp: December 09, 2011, 07:53:57 am
content: Thanks for the great photos of that resistor's internal, really learned a lot here.Have been watching closely this thread, and your other metrology project threads too.

author: IanB
author_rank: Super Contributor
timestamp: December 09, 2011, 08:54:58 am
content: Quote from: amspire on December 09, 2011, 05:05:48 amBefore it solidifies it will reach the pour point, which is the temperature below which it becomes so thick it can no longer be stirred or poured. The pour point will be much higher than the solidification point. If you are cooling down a bottle of oil you can observe the behaviour by tipping the bottle on its side. Below a certain temperature you will notice the liquid becomes syrupy and stops flowing.

author: amspire
author_rank: Super Contributor
timestamp: December 09, 2011, 12:36:25 pm
content: Quote from: IanB on December 09, 2011, 08:54:58 amYou are absolutely right. I got the oil down to -11 C and it was getting towards the thickness of honey - not quite there. I thought I might get components solidifying as out but it didn't happen.

author: amspire
author_rank: Super Contributor
timestamp: December 09, 2011, 01:25:44 pm
content: Quote from: BravoV on December 09, 2011, 07:53:57 amI wasn't sure if anyone would be interested in the photo's. I am waiting on some stuff to come from China so i can start some proper testing, but in the mean time I have been learning heaps too thanks to Conrad, robrenz, HAL-42B, alm and others. Nothing beats experience.I will have to one day get my hands on some Evanohm wire. It is really amazing stuff, if you can get the top grade wire at a good price. So far, I have seen prices like $50 for 50 feet - too much!

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 09, 2011, 03:45:53 pm
content: I've gotten wire from the H.P. Reid Company, now a division of Kanthal, in Palm Coast, Florida USA. I bought several spools of manganin, plus one of Rediohm 800. See if you can still get Rediohm 800, as it's probably as good as anything out there. It was some years ago, but I remember buying many hundreds of feet of wire for about $200. One trick was that I asked them to quote me whatever short spools were on the shelf, rather than making them wind specific lengths of specific gages.http://www.kanthal.com/products/material-datasheets/wire/resistance-heating-wire-and-resistance-wire/redi-ohm-800/

author: robrenz
author_rank: Super Contributor
timestamp: December 09, 2011, 06:18:59 pm
content: Shows evanohm in the listing, reasonable shipping and pricehttp://www.ebay.com/itm/Fine-Nichrome-Wire-YOUR-CHOICE-28-46ga-Lot-30-/310363518424?pt=LH_DefaultDomain_0&hash=item48431b75d8Anotherhttp://www.ebay.com/itm/PRECISION-RESISTOR-WIRE-MANGANIN-176g-NICHROME-80g-/300623716451?pt=LH_DefaultDomain_0&hash=item45fe91e063I have bought from this vendor several times with good success. You shoud ask them for what they may have that is not listed.http://stores.ebay.com/Wires-Store?_trksid=p4340.l2563

author: amspire
author_rank: Super Contributor
timestamp: December 22, 2011, 02:19:00 am
content: I finally received the two Platinum Resistance Thermometers I had ordered:http://www.ebay.com.au/itm/400244691889$8 each including postage. They claimed to be Grade A. They have a 1000 ohm resistance at 0 °C and 1384 ohms at 100 °C with a linear slope in between. The curve of platinum is not exactly linear but close - the exact formula is at the end of this post. The sensor is 30mm long and 5mm diameter. Sealed with epoxy, so not useful for extremely high temperatures. Should be OK up to 200 °C at least, but I am mainly interested in under 100 °C temperatures. Platinum sensors are usually available at 100 and 1000 ohms, and the 1000 ohms are a bit easier to use with 2 wire resistance measurements.So how good are they?The definition of Grade A isTolerance = ±[0.13 +0.0017 *|t|] °C . (With t measured in Centigrade)At room temperature, the two matched resistance to 0.02%. That corresponds to a temperature difference reading between the sensors of 0.05 °C. Not bad.I then checked the 0 Deg °C. The water temperature of a glass full of chilled water and ice is a very accurate 0 °C and it is not affected by factors such as air pressure. I was using rain water (non urban - almost no dissolved minerals or pollutants) so the water temperature between the ice should be 0 °C to an accuracy better then 0.01 °C.Both sensors measured -0.02 °C. Well within the Grade A spec of +/- 0.13 °C.Boiling point is very hard to check accurately, so I will not bother checking that. For one thing, I am at 1100 meters altitude, so our boiling point is well below 100 °C anyway.I have to say I am extremely impressed. Looks like they are very comfortably within the Grade A specs, so for $8 a real bargain. They are perfect for use with my standard resistors, and I have also received my metal film resistors that I was wanting to do temperature and stability tests with. At 100mV test voltage, the self-heating is not a problem (much less then 1 °C error) , but if I want to be paranoid, I can test at 10mV which is 0.1uW of heating.RichardEdit: If you want the true Temp Curve:ASTM Standards E 1137 for Industrial Platinum Resistance Thermometers specifies that the resistance-temperature relationship for such devices for the range 0 °C to 650°C, to within the tolerance mentioned above, will be described by the equation:R(t) = R(0)[1 + At +Bt2]Where:t = temperature (to ITS-90), °C,R(t) = resistance at temperature t, (= 1000 ohms for my sensors)R(0) = resistance at 0°CA = 3.9083 * 10-3(°C), and,B = -5.775 * 10-7(°C2).The European DIN standard that has now become IEC Standard 60751 uses slightly different numbers, but the end result is basically the same. For a fourth order accurate curve, see this document:http://www.uniteksys.com/Graphics/CalVan.pdf

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 22, 2011, 03:04:22 pm
content: Nice!There was an article on how to build a triple point cell in Scientific American magazine some years back. You can probably find the article on-line. It wasn't terribly difficult and gives you an excellent temperature reference. The high end is more of a problem because there isn't much that has a handy melting/freezing point in the region where you need it.

author: ejeffrey
author_rank: Super Contributor
timestamp: December 22, 2011, 03:27:32 pm
content: You could measure the temperature of eutectic solder

author: amspire
author_rank: Super Contributor
timestamp: December 22, 2011, 04:00:57 pm
content: Apparently using the Callendar - van Dusen method, you can fully calibrate a platinum RTD with just 4 common, everyday temperatures:Boiling point of Oxygen (-182.96 °C)Freezing point of water (0 °C)Boiling point of Water (100 °C)Freezing point of Zinc (419.53 °C)Sounds really simple, but I don't think I will bother.I can check 0 °C as precisely as I want, and the Class A platinum RTD's are accurate enough for anything I need. Rather then absolute accuracy, I need linearity and repeatability and I think I now have that.Richard.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 22, 2011, 04:15:25 pm
content: Harder than it looks, but fractional degree calibrations aren't necessary for much. FWIW, here's the triple point cell link-http://www.scientificamerican.com/article.cfm?id=tackling-the-triple-point

author: amspire
author_rank: Super Contributor
timestamp: December 22, 2011, 04:26:04 pm
content: A very interesting article. 0.0001 deg C accuracy is impressive.But simply freezing water gives better then 0.01 deg C, and that is good enough.Richard

author: grenert
author_rank: Frequent Contributor
timestamp: December 23, 2011, 02:48:30 am
content: The repeated warnings about shattered glass spook me too much to try it. I just use the crushed ice/water method in an insulated bottle.

author: robrenz
author_rank: Super Contributor
timestamp: December 23, 2011, 04:32:51 am
content: Quote from: amspire on December 22, 2011, 02:19:00 amRichard,I noticed these are 2 wire probes. Most RTD meter temp specs are based on 4 wire ohms (4 wire RTD). You may want to add two wires right at the potting or use a 4 wire connection right at the potting for maximum accuracy. Please keep us posted on your project progress, I love learning about the finer details of metrology.robrenz

author: lowimpedance
author_rank: Super Contributor
timestamp: December 23, 2011, 05:13:12 am
content: Quote from: amspire on December 03, 2011, 11:34:21 amRichardFor the resistor oil baths at work we use a Mobil product called WHITEREX 307 , white mineral oil.Looking through the data when you get to freezing point it states it has not been established !!A newer product being used is called Pionier 2076P (H&R group)John