Hi Phil
I've seen plenty of thermocouples connected over several tens of metres of unscreened cable in *very* noisy industrial environments working well.
Of course long integration times help, but what is often overlooked is that while their output is small in voltage terms, the source impedance is very low.
Cheers
-- Syd
A low source impedance helps with capacitive pickup, but not so much with radiated EMI and inductive coupling.
The EMI issue is in the amplifiers rather than in the sensors. A bit of rectification in the front end will cause surprisingly large offset voltages, which are really bad news with a low sensitivity device like a thermocouple.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
Thermocouples aren't sensitive to EMI!
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
l.
fHmm, interesting I hadn't thought of that. Hence John L's 'trick' of adding some ferrite beads where the TC wires enter the box. Maybe even a few turns on a toroid?
George H.
t -
Tell that to the OP27 in the front end amplifier. ;)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
The temperature differences are typically small in relation to the "hot" junction vs. ambient, so the errors from extension wire are minimal. For base metal thermocouples, it's the same alloys that are used (the limits of error specs are very very old and conservative). Not so for precious metal thermocouples, which are another beast (very low output, not necessarily even monotonic output with temperature).
With proper design it's not a problem unless you're going for silly resolutions. This isn't like your high bandwidth high Z low noise systems- we typically can LPF the heck out of it. You can't and have any useful signal left.
RTDs are more sensitive to EMI. You can get substantial self-heating from a Ge RTD when RFI gets into it.
right, and if the cabinet is 75F+ hotter than outside.. it changes. I know, we deal with that all the time, if the cabinet cooling isn't working, in other words, running hot inside verses out side where the J cables and probes are. there is a differential that creates and error.
There is a need in the summer to calibrate the process controllers when temps are high in the cabinets, mostly due to the season and working conditions. All the techs do is simply bring a unit on a roll cart out and get the offset correction needed for each unit in the panel. You must remember that much of the couple lead wire is also in this panel getting heated up.
Yes, we know the cabinets should be cooled, but its not always an option.
Jamie
No, but the front ends are and they carry RF very nicely.
Jamie
RTD's are ok, but don't do well well in high temp apps. At least they don't suffer from the offset of temp on the two ends.
Jamie
Well, it needs good circuit design!
-- John Larkin Highland Technology, Inc jlarkin at highlandtechnology dot com http://www.highlandtechnology.com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation
That's why they pay us the big bucks!
r K.
How accurate? What sort of specification did you have in mind?
Some thermocouples do.
But you can't be bothered to say how accurate.
I've interacted with Ken S. Tucker before, and I know that he isn't going to be ambitious about accuracy. It's useful information in this context. You may see it as an insult - you do seem to see a lot of insults.
-- Bill Sloman, Nijmegen
r K.
Jamie doesn't seem to understand the Peltier and Seebeck effects.
As long as the two individual wires - of two different materials - are pure and homogeneous, the only thing that determines the voltage difference around the loop are the temperatures at the two points where the two different wires are joined together - the hot junction and the cold junction.
Cold junction compensation is just a system to compensate for the fact that cold junction may not always be at a convenient and stable temperature. Obviously, you have to know the actual temperature at the actual cold junction if you are going to compensate it out.
It's all elementary thermodynamics - in as far as thermodynamics is ever elementary.
-- Bill Sloman, Nijmegen
Wouldn't it be more sensible to toss out the poorly performing controllers?
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
close enough to be problematic, BP of water is 100°C and with by distillation you can't get the water to below
4% in the distillate.-- ?? 100% natural --- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net
it's not the controllers, its the environment.. All controllers have the same issue of this type.
In one group of controllers they have PTCs on the board for the local temp to work with the Seebeck calculations for the offset.
THen you have the Seebeck (thermocouple) chips that come with the Cj with in them. Careful design much be made to make sure this "chip" is seeing the ambient environment, not some near by hot spot on the board, in which case,coefficient tables must be tailored per chip, depending where it sits. Of course, you generally need tables to get very accurate readings of a standard J/K type any way and this is also due to the reference not being exact per device.
Good cabinet design dictates that you have balanced air temp moving through the area of the controllers that matches the out side environment as close as possible. Most of the time, these types of controllers are installed away from high heat zones or temperature zones that are hard to match with the ambient temperature of the cabinet.
Most of the time we don't worry about it because the difference at the machine may mean 10F offset at the actual probe with temperatures of
750f range. This happens in the summer when cabinets get hot that house both high power electronics and the process controllers with insufficient air balancing.In cases of machines that generate military special products, these machines must be checked and cataloged on a regular bases and data attached to the final paper work per job that comes off that machine. For that, out board process controllers of the same models are used to plug into the couple, get the real reading, and adjust the offset on the in board controller to match, the data is them logged.
I guess you have to live it to believe it.
You should try and use your hot air wand some time and heat up a J/K chip or the PTC, what ever they are using and watch your readings.
Jamie
Cj
gThis is badly expressed. There's heat being dissipated inside the cabinet, so it's necessarily warmer than it's environment. By blowing outside air through the cabinet, you transfer that heat out into the surrounding space.
The air blown into the cabinet has to warm up to absorb that heat; the more air you blow through, the less it has to warm up to absorb the heat and shift it outside, but the inside of the cabinet always has to be at least slightly warming than its surroundings.
Again, poorly expressed.
the
It isn't "insufficient air balancing". It's inadequate ventilation - there isn't enough air being blown through the cabinet to absorb the heat being generated there without getting too hot to meet the cooling specification.
eUnderstanding what was going on would be even better. That seems to take a bit of thermodynamic insight that Jamie seems to have failed to pick up.
Always informative, more so if you understand what you are doing.
-- Bill Sloman, Nijmegen
No. Some controllers have significantly better compensation than others. There's more than a 20:1 range, just in industrial class instruments. Lab stuff can do better again, but it sucks for industrial use for other reasons.
I've never tried to use that kind of gadget in a controller design, precisely because of that problem (well, they're also usually too expensive and single-sourced).
That's horrible. It's easy to get 10:1 better with an industrial plug-in construction, and much better in lab-type instrumentation.
Okay, it's easy to measure temperature to 0.1°C, the reasons you are seeing _changing_ errors is that the two cold junctions and the compensation sensor are not isothermal and/or the sensor is not tracking changes accurately. That's a sensor and thermal/mechanical design trade-off.
Of course it's also possible the controllers are fine and that someone made an error wiring up the panels! I've seen that at least half a dozen times. I'd be happy to look at the reason for the errors if you want.
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Guess you don't get it. What ever, I had the same argument with QC at work over this, me and about 10 others joining in. We had to actually show them the errors of their way.
They too, could not believe what they were seeing, all they could throw at you was, what they learned in college and to them, that was enough, but in the real world that does not apply because college does not teach you about reality. I too learned the same thing at college years ago, however, experience has served the better way and I do fully understand why this happens.
There are reasons why abs devices like R type RTD's are used over the seebeck method when possible. It's obvious to me.
Jamie
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