EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: David_AVD on May 21, 2014, 02:47:19 am
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I'm using a MAX31855 (K-type) thermocouple chip and am having issues with the returned readings being approx 25% low. eg. It reads 77°C when the probe is actually 100°C (boiling water).
I've plugged the same thermocouple into my Fluke DMM adapter and it reads the correct 100°C. The percentage error seems to be fairly constant over a range of temperatures.
I did have issues with erratic (random) readings before I added more PSU decoupling. I already have the recommended 10n cap across T+ and T- and am using a K-type socket.
Any ideas? There has to be something very simple (silly) going on.
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can it be backwards?
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The thermocouple? No, I triple checked the connections. I did try backwards to see what happens and you just get a negative reading (as expected).
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If it was backwards it would read lower, I mean, as the temperature goes up in the sensor tip, the chip will read lower and lower, been there done that, was really confused thinking that the thermo-couple was a chinese special
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I've always found this Omega Engineering data page useful in identifing the various thermocouple types (J,K,etc) and their polarity, it's not always intuitive.
http://www.omega.com/techref/colorcodes.html (http://www.omega.com/techref/colorcodes.html)
Also are you wiring the TC all the way directly to the MAX31855 where the cold reference junction compensation is applied? If you transition to copper wire prior to the MAX chip and their is a temp difference between the transition point and the MAX chip that can introduce a measurement compensation error.
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Yeah, I actually meant "negative going reading" (with increasing temperature) when I said negative above. :)
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Is it possible you need the ferrite beads like in the eval board schematic?
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Here's the relevant portion of the PCB:
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Is it possible you need the ferrite beads like in the eval board schematic?
I had not seen an eval board for it, it should be easy to cut the Vcc track and insert a bead as a test.
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can you splice that connector in circuit with your fluke meter to see if it is good?
like thermocouple wire - silver solder - connector- thermocouple wire. I had weird stuff like this happen when I ordered a K connector on ebay and got a J or something like that.
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Your cold junction compensation is off? What does it read when short circuited with no TC in the socket, and when the TC is at room temerature along with rom temperature from another thermometer.
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Here is the link to the data sheet of the eval board. http://datasheets.maximintegrated.com/en/ds/MAX31855EVKIT.pdf (http://datasheets.maximintegrated.com/en/ds/MAX31855EVKIT.pdf)
If that fixes the offset problem, I will need to add that to my board i'm working on before I order it. Winging it and learning as you go gets expensive! :-DD
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Is it possible you need the ferrite beads like in the eval board schematic?
I had not seen an eval board for it, it should be easy to cut the Vcc track and insert a bead as a test.
shematic of Eval kit
http://datasheets.maximintegrated.com/en/ds/MAX31855EVKIT.pdf (http://datasheets.maximintegrated.com/en/ds/MAX31855EVKIT.pdf)
Are you 100% sure that you're using proper MAX31855K type chip?
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Your cold junction compensation is off? What does it read when short circuited with no TC in the socket, and when the TC is at room temerature along with rom temperature from another thermometer.
My take on it is that the MAX chip does the cold junction compensation and applies it to the thermocouple portion of the digital reading. You can read the comp reading separately of course, but as far as I know you don't have to do anything with it.
Hmmm... if I disconnect the TC and short the input, the reading is 26°C which would be about right for my office at the moment.
EDIT: Adding the bead to the Vcc (before the cap on the MAX) made no difference.
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could you measure the voltage across the IC input,
and calculate the value using K type chart, and compare with the reading from the IC.
1. If the voltage reading match the actual temp and chart, then something wrong with the IC
2. If the voltage reading didn't match with the actual temp and chart, then probably something wrong with the soldering/ metal type on the cold junction.
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Your cold junction compensation is off? What does it read when short circuited with no TC in the socket, and when the TC is at room temerature along with rom temperature from another thermometer.
My take on it is that the MAX chip does the cold junction compensation and applies it to the thermocouple portion of the digital reading. You can read the comp reading separately of course, but as far as I know you don't have to do anything with it.
Hmmm... if I disconnect the TC and short the input, the reading is 26°C which would be about right for my office at the moment.
EDIT: Adding the bead to the Vcc (before the cap on the MAX) made no difference.
On the eval board the bead is on the t+ and t- trace. I'm pretty new to all this so not sure if it matters.
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It might be a J type chip.
100 deg is around 4127.6 uV on a K Thermocouple.
4127.6 uV on a J chip is around 71 deg.
Also check what the measured voltage is at the chip T- and T+ pins is.
And check what the error bits show.
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Or T would be around 79 deg.
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I just tried another K-type probe. Since it had spade terminals on it, I soldered them to the back of the existing K-type socket. With that one I get a reading of 80°C in boiling water. Grrr...
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Time for the right Max chip then. Yours is either the wrong type of has accidentally been programmed to a different thermocouple type.
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Just checked the chip (again) and it's marked as a M31855K type.
I read approx 3.4mV at the MAX T+ and T- pins which seems to correspond to 80°C.
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Make sure T- is not grounded.
Try measuring the TC voltage without it connected to anything.
Post schematic.
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With a hot junction at 100 C and a cold junction at 20 C, the thermo-electric voltage should be around 3.4 mV. So your reading is correct for a K-type thermocouple in boiling water if your meter is at "standard" room temperature and there are no serous temperature gradients except near the tip of the thermocouple. That pretty much narrows the suspicion down to the chip. Either the cold junction compensation is wrong or the chip is mislabeled or otherwise has the wrong calibration dataset loaded.
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It really does look like the chip is the wrong curve.
I bought the chips from Digikey; 2 pieces originally then another 2 a month later. The two prototypes have one from each purchase, but they are probably the same batch.
Unfortunately neither RS or Farnell have stock, so can't easily get another chip to see if that's the issue. :(
I'll see if I can get one somewhere else and go from there.
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Sample from Maxim?
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Sample from Maxim?
That would take weeks. I'll get a breakout board from Wiltronics in the mean time as I need to get this prototype running ASAP.
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It really does look like the chip is the wrong curve.
I bought the chips from Digikey; 2 pieces originally then another 2 a month later. The two prototypes have one from each purchase, but they are probably the same batch.
Unfortunately neither RS or Farnell have stock, so can't easily get another chip to see if that's the issue. :(
I'll see if I can get one somewhere else and go from there.
I have heard that Digikey will substitute 'equivalent' parts when they can if what they sold isnt actually in stock(and without telling you; happened to my company a few times). Are you sure the chip you ordered and the chip you got are the same?(obvious question, I know)
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boiling water ? water is conductive.... make sure you have a thermcouple that is fully electrically isolated ...
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Sample from Maxim?
That would take weeks. I'll get a breakout board from Wiltronics in the mean time as I need to get this prototype running ASAP.
This is how I got my 2 chips for my project. They shipped from the philippines to the USA in about a week. Much faster than what I was expecting!
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Hi David_AVD,
I have been using the MAX31855 in a project and have produced a number of prototype boards using it and have had no problems up until this point. I have just however produced a new batch of these boards and am seeing the exact same problem you have described on 75% of them (i.e. reading ~78°C at 100°C input from an Omega thermocouple calibration unit). The new board has some very minor alterations from previous versions, none of which should be relevant to this behaviour from the MAX31855.
If you make any progress with finding out whats going on, please post it up as it would be hugely helpful to me too, but from my perspective it definitely looks like a problem with the component itself.
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We now have 2 reports of MAX31855s behaving wrongly in the same way, so a mismarked batch seems likely.
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boiling water ? water is conductive.... make sure you have a thermcouple that is fully electrically isolated ...
Yes, the thermocouple is fully isolated and suitable for liquid immersion. :)
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I have been using the MAX31855 in a project and have produced a number of prototype boards using it and have had no problems up until this point. I have just however produced a new batch of these boards and am seeing the exact same problem you have described on 75% of them (i.e. reading ~78°C at 100°C input from an Omega thermocouple calibration unit). The new board has some very minor alterations from previous versions, none of which should be relevant to this behaviour from the MAX31855.
Interesting. I've wasted way too much time on what should have been such a simple circuit. I'll update here once I get another chip.
EDIT: I've emailed Maxim support to see what they say.
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Have just had confirmation from MAXIM that it was a faulty batch, the affect chips are:
Date Code: 1352A2
Lot Code: 253AL
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That batch is what two of the chips I have are. I *think* the other two were different, but will double check when I get into work.
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Just checked the other prototype and it has batch:
Date code: 1435A2
Lot code: 253AK
Both batches of chips are giving me the same result, so maybe there is more than one bad batch ?
Maxim did reply to my enquiry, but only to say they are looking into it and will reply again ASAP.
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Just got word from Maxim:
Replacement parts should not be from lots ending in either "253" or "107"
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Well, Digikey sent me 2 replacement chips as soon as I verified when I purchased them. They arrived today and they are batch code 107AE.
I wasn't confident that they would be any better than the 253xx batch, but just replaced the chip in both prototypes and the temperature is spot on. Whoo!
So, not entirely sure what to make of Maxim's email saying the 107xx batch are also bad. The 253xx batch do seem to have been proven bad of course.
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not entirely sure what to make of Maxim's email saying the 107xx batch are also bad.
Maxim has poor documentation of its production process? That's scary.
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They probably mean that some of the 107xx batch could be bad, and they're not sure exactly which ones, so to be safe they're saying to avoid the whole batch.
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Still somewhat annoyed at the (unchargeable) wasted hours on this project due to something out of my control and should never happened. >:(
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they're not sure exactly which ones,
That is not tolerable for production purposes.
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Hi guys,
I experienced the same problem of the same batch of the MAX31855KASA+ bought from Mouser.
It's painful because I already shipped a handful of them to customer. |O
In my case, the differences of reading started to show more when the temperature goes up higher.
I got a toasted PCB as a result. Now, I'm wondering whether Maxim did a random test on each batch.
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So it looks like the trim issue is only on MAX31855KASA+ units with lot codes 253AL and 253AK with date code 1352A2. I also notice that data code 1345 is also with the same issue. other data code should be ok.
That is the reply from Maxim.
No apologies given by them on top of that.