What's a solid inexpensive 1µV multimeter? - Page 1
Forum section: electronics / beginners

author: Fritoeata
author_rank: Newbie
timestamp: December 12, 2014, 12:31:16 am
content: First post here. I've read links from Hackaday and (I think) blog.Makezine.com. I'm NOT an EE. I am a tool and die maker, and general "fire-puter-outer" (...LOL!)Today, I'm troubleshooting a high temp furnace's process...Therefore, I have a need for a decent 1µV multimeter! (ie: 0.001mV meter)!To do this, I'm trying to check the accuracy of my Type B TC at various points between 1400C-1500C(it gets pretty hot in there!)The NIST (http://srdata.nist.gov/its90/download/type_b.tab)uses a 1µV callout, but I'm unsure what's realistic here pricewise.Once I have it "nailed", the instrument will likely just sit there for a few years...=====Budget: $100-?=====My findings:So far, I've only found the higher end of the "portable" market... A bit more coin than I'd like to spend.Fluke 287 ($400ish USD)Agilent U1272A ($350ish USD)Also, there's the of yet "unproven"Extech GX900 ($440 USD) with a bunch of really cool features that I really don't need.I'm totally open to a used bench/rack model! But I have ZERO idea what to look for, and it's all a WIDE range of prices.I did find a few decent HP 3478A on Fleabay for <$200!... but they are 10µV.They will get me pretty close... but I'm also pretty anal (remember, I am a toolmaker by trade). I REALLY, REALLY want that last decimal of the NIST spec....Any help you guys have would be truly helpful!Thanks in advance!edit: general formatting (to keep even myself from being confused!)

author: dannyf
author_rank: Super Contributor
timestamp: December 12, 2014, 12:56:42 am
content: A few cents + an opamp will get it done for you.

author: robrenz
author_rank: Super Contributor
timestamp: December 12, 2014, 01:03:50 am
content: What are you using to accurately measure the temperature at 1500 to 1deg resolution to compare the TC output. Even then its about 11µV per degree.

author: ivan747
author_rank: Super Contributor
timestamp: December 12, 2014, 01:08:18 am
content: Quote from: Fritoeata on December 12, 2014, 12:31:16 am1µV resolution or 1µV full range? If it's only reslution, any used 5 1/2 benchtop multimeter will work, HP, Fluke or Keithley. For example the Keithley 191, 192, 197, those are the ones I am familiar with, but there are more. They're about $100 used on ebay and generally are very reliable even if they are old, provided they are in good condition.For some reason on a quick ebay search, they seem to have gone up in price, but maybe it's only the season, maybe the sellers also want their yearly bonuses in the form of pricey sales*except from the Keithley 191Right now there seems to be a shortage of Keithley 197s, those are the most popular of all the 19x series, so that's why they got the insane prics (DO NOT BUY FOR $400)The 195s seem a bit pricey tooThe 192s right now are cheapThe 191 usually are rare to find, they used to be about $99, but right now there are about a dozen of those on sale on ebay and the price dropped to $60 or $70. I own one of these and in my opinion it's a good purchase:https://www.eevblog.com/forum/testgear/scored-this-keithley-191-multimeter-on-ebay/As a bonus, the 191 haspretty damn goodaccuracy for a 5 1/2 digit multimeter, beats the 197 by quite a bit.

author: Lightages
author_rank: Unknown
timestamp: December 12, 2014, 04:43:03 am
content: The 500,000 count Brymen handhelds will resolve 1 microvolt but don't have that accuracy.

author: EEVblog
author_rank: Unknown
timestamp: December 12, 2014, 06:26:05 am
content: The Brymen 869 have 1uV resolution.

author: mzzj
author_rank: Super Contributor
timestamp: December 12, 2014, 06:38:03 am
content: Quote from: robrenz on December 12, 2014, 01:03:50 amThis was first question in my mind also.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 12, 2014, 06:54:34 am
content: Just buy a chinese temp controller that can handle that and read its display.

author: Fritoeata
author_rank: Newbie
timestamp: December 12, 2014, 04:04:59 pm
content: First, thank you guys for being patient and not treating me like an idiot. I am an idiot often, and don't really need reminded....Your replies are helpful and QUICK!!Quote from: mzzj on December 12, 2014, 06:38:03 amAt this point, it appears to be running through a Eurotherm 2404 temp controller(PID?). I only have a Fluke 179 which has .1mV res (like 10C jumps).It may be a lofty goal, but I would like to achieve something close to the 0.1C accuracy that the controller shows on screen.Quote from: ivan747 on December 12, 2014, 01:08:18 amThanks ivan747.edit:I don't understand your first question: "1µV resolution or 1µV full range?"I just want to trust what I get, even if I need to use a calculator.This is exactly what I believe I'm looking for. Great price:Value ratio!I'll jump on it after a few more questions....In my mind, 1µV is about on the same level as Voodoo.(No experience= No understanding)How important is it that I get a "tested" unit?Are these units affected by atmospheric changes?Is there usually a "field/self calibration" routine on these or something?(I can also run a calculator pretty well if you know little math "self checks" that you can walk me through)About leads:Do I need $$$ leads or are they going to be pretty much the same for my needs?How much will resistance in the leads affect the 1µV resolution?Is there an offset I can compensate with? (or can I program it in?)Thanks guys! I really appreciate it!Fritoedit: I skipped ivan747's first question...oops, my bad. fixed.

author: robrenz
author_rank: Super Contributor
timestamp: December 12, 2014, 08:38:13 pm
content: Quote from: Fritoeata on December 12, 2014, 04:04:59 pmI am not trying to be a jerk but your above statement makes me wonder if you have a clear understanding of the difference between resolution and accuracy. Are you aware of the things you need besides a thermocouple and a voltmeter to measure temperature? You still have not mentioned what 1 deg or 0.1 deg accurate temperaturereferenceat 1500 deg you will be using when you measure the thermocouple output. Without that you are chasing your tail.

author: jeremy
author_rank: Super Contributor
timestamp: December 12, 2014, 08:46:05 pm
content: Yes, you need to be careful here. Accuracy vs resolution is a big trap for beginners. Seehttp://www.appmeas.co.uk/blog/index.php/2010/04/sensor-accuracy-and-resolution-explained/for an explanation.I think that you will need to state very clearly (maybe even in bold lettering for us forgetful folks) whether your goal is 0.1C accuracy or 0.1C resolution.If what you want is to "fully trust" your readout, then accuracy is what you probably want. But you'll pay for it!Edit: generally speaking you need more resolution than accuracy. So (and I'm guessing here) if you want 0.1C accuracy you might need 0.01C or 0.005C resolution.

author: AG6QR
author_rank: Frequent Contributor
timestamp: December 12, 2014, 08:50:29 pm
content: Quote from: Fritoeata on December 12, 2014, 04:04:59 pmA meter with 1µV resolution would have the ability to distinguish between two voltage sources, where one of the voltage sources is 1.999 mV, and the other is 1.998 mVA meter with 1µV full range would be able to show finer differences, for example, the difference between 1.000µV and 0.800µV. It might even be able to show the difference between 1.000µV and 0.999µV. Such a meter, at that 1µV range setting, would be overloaded by a voltage of 10µV.

author: Fritoeata
author_rank: Newbie
timestamp: December 12, 2014, 09:10:52 pm
content: Quote from: robrenz on December 12, 2014, 08:38:13 pmI am aware of the differences between resolution and accuracy as evidenced in your quote. At work, I have several sets of standards that are within .000001in and .00000001in3 and .0000001g. This is 3 completely different quantifications.When it comes to electronics... I haveNOidea how to establish this, or what's cost effective.Your being confused is confusing me...What did I miss?My goal is to read the voltmeter as the thermocouple is in use... hitting 1400C... writing down appropriate uV reading. hitting 1410C (etc., etc.)Which is what my furnace manufacturer recommends doing.I'm using NIST's Type B tables as my uV reference (see OP)... did I miss something? Do I need a reference Voltage to compare?...robrenz, I don't need "dead nuts" accuracy, I just need to know something as close asreasonably accurate(ie: determined by you guys).Quote from: AG6QR on December 12, 2014, 08:50:29 pm...Thanks for this tidbit. Understanding this,I believe I would like accuracy to within 1C. I'll trust the temp controller to subdivide any further.It seems that I cannot afford an 0.001uV accurate meter. I assumed the deviation of a 1uV would be +/-2uV at any point.That's how all my other lab equipment works (mg scale, pycnometer, micrometer, etc.)edit:bold for ease of visibility... I'm not shouting.

author: Fritoeata
author_rank: Newbie
timestamp: December 12, 2014, 09:25:40 pm
content: Just to be clear...I realize I may have sounded offended. For a second there, I think I was... but that's prob just cuz I'm an ass....It comes natural O0My apologies if I stepped on a toe with that last post.I really don't know what I'm doing in the electronics world.I can machine and build stuff to the .0001in(+/-2u), I just struggle with the abstractness of the E's.

author: robrenz
author_rank: Super Contributor
timestamp: December 12, 2014, 09:32:43 pm
content: I am also way better as a machinist than I am as a wanabe EEWhat are you using for cold junction compensation with your thermocouple measurement?

author: Fritoeata
author_rank: Newbie
timestamp: December 12, 2014, 09:47:01 pm
content: Quote from: robrenz on December 12, 2014, 09:32:43 pmLOL! Glad I've got a buddy out there.re: compensation...Honestly, I haven't had time to watch the 30min video... I really only have time to get the uV from the Tcouple.If you've got some recommended math? I was planning on making the machine OEM do the math for me anyways...That's why the boss paid him.I do know this... It's a type B, NOT a type K.

author: robrenz
author_rank: Super Contributor
timestamp: December 12, 2014, 09:54:31 pm
content: You need to get up to speed on general thermocouple use and measurement. You cant just whack a meter on the thermocouple leads.Thiswould be a start.

author: Fritoeata
author_rank: Newbie
timestamp: December 12, 2014, 11:02:26 pm
content: @robrenz...Noted. Thanks!I'll read up on this.<reading>I read this about Type B: "The emf function has a minimum around 21 °C, meaning that cold junction compensation is easily performed since the compensation voltage is essentially a constant for a reference at typical room temperatures"?I read this about "Cold junction": "The reference junction block is allowed to vary in temperature, but its temperature is measured using a secondary thermometer. This does not necessarily defeat the usefulness of the thermocouple as a thermometer: The thermocouple wires may be exposed to extreme environments, while the reference junction can simply be made in an ordinary environment with a cheap semiconductor thermometer."I read the whole "type B" paragraph, and I deduce that the average of 0 and 42= 21C therefore I need a "typical room temperature" constant..I also have another type K thermometer unit. Do I use this somehow?Wait...Do I either subtract or the add the value's deviation from the 21C on my thermometer and adjust my meter's reading (using the reference voltage of the deviating temperature amount... which will be about .001-.003uV-ish) when I'm writing down the 1400-1500C range?I think I'm starting to understand that really don't understand...But at least I'm dangerous!!

author: ivan747
author_rank: Super Contributor
timestamp: December 13, 2014, 03:52:54 am
content: Quote from: Fritoeata on December 12, 2014, 04:04:59 pmWell, there is a difference between resolution (which is fomally called sensitivity) and the maximum value that the instrument can display. I was asking if you needed something that can go down to 1µV (so that is can read down to 1µV) or something that's so sensible that is only goes up to 1µV (which is really rare, if not economically impossible for us enthusiasts).I assumed you want 1µVsensitivity, also known as resolution, so I got you a list of benchtop multimeters you could buy used.Okay, now, calibration.You instrument being in "cal" or calibration means, in the end, that your instrument reads the voltage/resistance/current/whatever within its margin of error. There is a LOT of more stuff to know about calibration, but I'm not an expert in this topic, so I don't want to misinform you. There are plenty of guys who do know that are watching this thread and could help me outWhen measuring 1µV resolution you might have to deal with noise. It's simple to get rid of noise. There is a Null button on practically all multimeters. If you short your probes and still see a reading (for example 000.003mV) you hit the Null button and it makes that reading zero. If you are measuring really sensitive stuff, you might also want to use a BNC cable or a twisted pair of cables. You only have to get a pair of BNC to banana plug adapters in the case of BNC. For a thermocouple I'd probably use twisted pair. The idea with twisted pair is that the wires are twisted together and are exposed to the same noise signals, so basically the noise cancels out. The idea behind BNC is to have a small wire carrying a sensitive signal in the center and have that surrounded by a tube of wire, that acts as a shield for the inner wire and is normally connected to the circuit ground. But since a thermocouple has no circuit ground and neither does the multimeter... I think twisted pair is simpler and easier to deal with.Atmospheric changes, I have no idea. Depends on the unit. If it's not specified in the manual, don't worry unless you have extreme conditions out there. (make sure you can get the manual on PDF. For Keithley and HP you can get the PDF manuals online, but you have to give them your name and email, which is not that bad). What really does affect them can be very high humidity.There is no field calibration routine for the multimeters themselves, apart from nulling. Maybe for your aplication there could be, like reading the voltage of your sensor at room themperature, but that depends on your application.One thing to keep in mind. The input impedance of these multimeters is typically more than 1 gigaohm for 2V and 200mV ranges only. That is a really high impedance. You can read voltages directly from thermocouples with that. Apart from that, the input impedance is 10 megaohm for all other ranges. Lead resistance will do nothing for voltage measurements at this impedance. Noise is the problem here. That is why i suggested BNC cables earlier, because there is a shield. Either BNC, with the negative shield grounded or twisted pair cables.Moving on to a completely different topic.Buying a used multimeter. Get one that doesn't look physically abused, if it has a calibration sticker on it, with a recent date, even better. That means someone was using it and took basic care for it and that it didn't sit on a wet basement corner.Now, technically speaking, you'll never know if an eBay multimeter is still within its accuracy specifications (within spec.), but practically speaking, these multimeter after some time, don't drift very much from their specifications, after some years they settle. So you either get a multimeter that's accurate, or you get something completely out of place.If you absolutely need to know if the thing is accurate you have a couple of options:*Send it to a calibration lab for calibration only (no adjustment). Calibration is when they check whether it meets its specifications or not. Adjustment is when they tweak it so that it meets its specifications.*Or you can buy a little home lab voltge standard like this one:http://www.voltagestandard.com/-.htmlThere are some voltage standards on that page.Me? Meh, I haven't calibrated mine or anything. The day I meet someone with a multimeter as accurate as mine I will invite him or her to compare the measurements to see if both multimeters agree.You don't need particularilly expensive leads for this multimeter, but since your aplication involves thermocouples, I don't really know what effect my suggestions will have on your measurements.For normal multimeter probes, you can solve the problem in several ways, many of them are discussed in the post I linked in my previous message.For thermocouples, I don't know enough about that, bud Dave has some good videos, I did like them. Here's a thermocouple tutorial:And I think he talked about thermocouples in one of his very early episodes (50-smonething or something like that)I hope you find a 5 and 1/2 digit multimeter for cheap, and that it wourks out for your project and for other tasks you might find later on. They're handy.-Ivan

author: ivan747
author_rank: Super Contributor
timestamp: December 13, 2014, 11:51:34 am
content: I'd like someone to check if this is correct, but I found a way to calculate thepercentagecombined error of a voltmeter (a thing with an error mostly presented in percent) and a voltage reference that it measures. Again, I got this formula through trial and error, but you can try reproducing it, of course. Goes like this:Percentage of the error on the measurement = ((1+error DMM)*(1+error Voltage reference)*(1+ yet another error) -1)*100It does look like a formula that could come from a statistics book... It can also be expressed in sigma notation if you want, with Xi, i being an index for a table of X and all that jazz.The error are expressed in fractional form, I.E. a 5% error is expressed as 0.05It should give you a figure like 1.2525% for example.

author: ivan747
author_rank: Super Contributor
timestamp: December 13, 2014, 12:27:48 pm
content: And, as you will see, at least the thermocouple I linked to is not nearly as accurate as the typical 5 and 1/2 digit multimeter, so you won't have to worry *too much* about calibration. You should check the multimeter's accuracy against a couple of other mutimeters, even if they are not as accurate, just to confirm it is within a reasonable range.http://reotemp.com/thermocoupleinfo/type-b-thermocouple.htm

author: jeremy
author_rank: Super Contributor
timestamp: December 13, 2014, 04:45:22 pm
content: Sorry, didn't mean to insult your intelligence. It's just hard to tell what people know already, and I'd rather do that than just give you a bunch of confusing and overly complex information.Basically, what you should look for is a measurement device that has a long integration time or large averaging buffer as nulling will only correct offset error. You also need to approximately know at what rate your temperature would be changing when you take this measurement. 1C per sec? less? more?Also, is your furnace electric or gas? (or something else?)Ok, well I wrote a big explanation here, but I deleted it because it was pretty mathematical. If you'd like something explained more, just ask.

author: mzzj
author_rank: Super Contributor
timestamp: December 13, 2014, 05:00:34 pm
content: Quote from: Fritoeata on December 12, 2014, 11:02:26 pmB-type thermocouple is the speciality case as the sensitivity uV/K around room temperature is so low that you can ignore it. With any other thermocouple type you need cold juction temperature/compensation but not with B-type.Are you trying to verify the accuracy of you thermocouple, controller or what? Is there a reason to make you suspect your temperature readings or are you doing this just cause?

author: johansen
author_rank: Super Contributor
timestamp: December 13, 2014, 05:01:04 pm
content: in the USA, you can buy a digital scale on eBay for literally $2.99 including shipping.they are, internally, approximately 2.4mV full scale, usually 10,000 counts. input resolution is at least 1Mohmspressing the tare button resets the voltage offset to zero of course, and the voltage reading should be at least 1 diode drop away from the rails, but it may still work across a wider range.because the chopper opamp is a fixed gain, and the adc uses the battery as a reference, you will need to know the battery voltage to get an accurate measurement.however because of this fact, you can use two resistors as a current source and two kelvin clips will make a 100 milliohm ohm meter with 10uOhm resolution.. that's as far as i would try to go before burning out the transistor that normally turns on the 750 ohm strain gauge. 1000 milliohms full scale and 100uohm resolution is a good compromise imo.

author: Fritoeata
author_rank: Newbie
timestamp: December 15, 2014, 05:12:20 pm
content: Quote from: jeremy on December 13, 2014, 04:45:22 pm...No worries man! I'm the newbie!Quote from: jeremy on December 13, 2014, 04:45:22 pmIt's an electric element furnace. First purged for an hour with nitrogen(cheap gas, to remove oxidizers), but then flushed into a hydrogen atmosphere(for clean heat with small particle size).More on that: Autoignition of H2 in normal air is something like only 500C; which means... WOOOF!! (haha!) The nitrogen is really necessary to keep the furnace components from oxidizing, too.Quote from: jeremy on December 13, 2014, 04:45:22 pmThanks. At this point, I'm pretty much finding what I believe to be "practical" for accuracy.Quote from: ivan747 on December 13, 2014, 03:52:54 amThanks! I'll get some decent leads, and see how bad they are compared to the tables. If the reading's way off, I'll check those first.I'll check out Dave's tut.Thanks alot Ivan!Thanks for the tips guys! I think I'll be able to get the measurements I need!Best Regards,Frito

author: Fritoeata
author_rank: Newbie
timestamp: December 15, 2014, 11:55:16 pm
content: Quote from: ivan747 on December 13, 2014, 03:52:54 amPer ivan747's reccomendation I bought a Keithley 191 (eBay auction: #361143295771) for $79 shipped. (thanks for the recomendation!)I now need leads... 12ft of leads!prerequisites: I'm running my leads within 12in of some 60amp/240ac heater coils that will kick on and off, I assume. I don't want any noise.Can I simply take a decent set(ex:eBay auction: #151502764931), cut 'em in half, and solder in an 8ft chunk of twisted wire?...It sounds to me like a ridiculously longshot, but so far it fits in with shielding as a concept, and you didn't say not to!(XLR audio is pretty similar iirc... and low noise over really long distances)...heck... I actually think I have some XLR mic cable in my garage that I could hack into this if you guys give me the greenlight.Thanks for the recommendations!-Frito

author: ivan747
author_rank: Super Contributor
timestamp: December 18, 2014, 04:43:00 pm
content: If you don't need that cable... you could try. If I rememeber correctly XLR has a twisted pair and a shield inside. The best of both worlds. You could try using the twisted pair for the thermocouple and then ground the shield. I don't want to recommend a method for grounding because I haven't seen what's your intended setup or anything.Will that cable be exposed to high temepratures, by the way?You just have to be wary that the copper and the thermocouple's metal will form a new thermocouple as Dave says in his tutorial. If the ambient temperature of the multimeter and the wire is kept constant, yeah, you can compensate that offset with the null button.The 191 has binding posts, so you can hook up wire directly to it with no issues. One of its plus sides. The negative side of that is that you can't plug in shrouded leads unless you made an adapter (I should make an adapter for mine, really...).Good luck!