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Thermocouple mux

M

Marco Trapanese

Jan 1, 1970
0
Hello!

I'm working on a simple board that uses the recommended Analog's design
to acquire a tc:

http://www.analog.com/en/circuits-from-the-lab/CN0271/vc.html

1. what do you think about this circuit? IMO, it's quiet expensive but
it should be reliable

2. I need to acquire two tc: how to mux both sensors? I mean: I want to
use the same front-end and select what tc to acquire.

Thanks!
Marco
 
R

rickman

Jan 1, 1970
0
Hello!

I'm working on a simple board that uses the recommended Analog's design
to acquire a tc:

http://www.analog.com/en/circuits-from-the-lab/CN0271/vc.html

1. what do you think about this circuit? IMO, it's quiet expensive but
it should be reliable

2. I need to acquire two tc: how to mux both sensors? I mean: I want to
use the same front-end and select what tc to acquire.

I don't know how expensive this circuit is, but I seem to recall that
you can get a decent TC chip for $5 that gives you a digital output with
cold junction compensation. Turns out the hard part of a TC measurement
is the cold junction compensation. How do you measure the temperature
of the cold junction to ±0.5­°C? The last time I looked at this I found
that ±2°C was about as good as you can get without spending more bucks.

You can mux the TC wires using an analog switch, but you need to decide
up front how you are going to handle the grounding. Some probes expect
one side of the TC to be hard grounded, others need to be floated. The
most common compromise is to ground one side with a 1 Mohm resistor like
show in the app schematic. So you may want to switch both legs so the
two probes can be different.

Rick
 
M

Marco Trapanese

Jan 1, 1970
0
Il 25/11/2012 23:29, rickman ha scritto:
I don't know how expensive this circuit is, but I seem to recall that
you can get a decent TC chip for $5 that gives you a digital output with
cold junction compensation.


Do you recall any part number?

Turns out the hard part of a TC measurement
is the cold junction compensation. How do you measure the temperature
of the cold junction to ±0.5­°C? The last time I looked at this I found
that ±2°C was about as good as you can get without spending more bucks.

You can mux the TC wires using an analog switch, but you need to decide
up front how you are going to handle the grounding. Some probes expect
one side of the TC to be hard grounded, others need to be floated. The
most common compromise is to ground one side with a 1 Mohm resistor like
show in the app schematic. So you may want to switch both legs so the
two probes can be different.


Ok thanks!
Marco
 
M

Marco Trapanese

Jan 1, 1970
0
Il 25/11/2012 22:06, John Larkin ha scritto:
They are obviously trying to sell chips. A decent delta-sigma ADC with
PGA can handle a tc input directly, without those two silly in-amps.
If you want to mux a bunch of thermocouples, use some HC4051s.


I was afraid about the change of materials, but I was wrong because all
tc will change at the same point. Very easy, indeed.

Use something like an LM45 or an LM71 to pick up the local
ref-junction temperature, ideally isothermal with all your
tc-to-copper transitions. Do the ref junction compensation in
software.


Got it.

If you only want to do two tcs, you could use a dual-channel ADC, like
AD7793 or some such, with an internal reference.


The ds proposes an hardware solution to compensate the cold-junction
using a thermistor and a precision resistor. I guess this requires a
precise selection of them (and perhaps I need to adjust values among
production batches).

In fact, you suggested to do the compensation in software.

How to handle this? I mean: I read the absolute Vtc using ADC (e.g. in
uV). Then I know the temperature of the cold-junction in °C.

In order to get the temperature at the reference junction do I need to
"invert" the tc equation? So I get the voltage a tc should exhibit at
the cold-junction temperature, I subtract it from the actual Vtc and
finally convert this into a temperature again.

Am I right?

Thanks
Marco
 
W

whit3rd

Jan 1, 1970
0
Most cold-junction compensation just adds a linear slope of
voltage-vs-temp to the tc voltage, often by putting it electrically in
series with the thermocouple.
It's impressive how bad the cjc is on commercial thermocouple
instruments. It's common for people to use the linear approximation
(with a diode or thermistor) and miss the correct cold junction slope
by 10% or so. And put the sensor only vaguely near the actual cold
junction.

It's usually the case that the cold junction is expected to be at
room temperature (like, 20C +/- 5) but the hot junction can be
at -200C to 800C... so one can get away with crude compensation.
That's a good reason not to trust the all-in-one chip solutions,
unless you really HAVE to make the output show up on a moving-needle
meter (or other panel voltmeter).
 
R

rickman

Jan 1, 1970
0
Il 25/11/2012 22:06, John Larkin ha scritto:



The ds proposes an hardware solution to compensate the cold-junction
using a thermistor and a precision resistor. I guess this requires a
precise selection of them (and perhaps I need to adjust values among
production batches).

In fact, you suggested to do the compensation in software.

How to handle this? I mean: I read the absolute Vtc using ADC (e.g. in
uV). Then I know the temperature of the cold-junction in °C.

In order to get the temperature at the reference junction do I need to
"invert" the tc equation? So I get the voltage a tc should exhibit at
the cold-junction temperature, I subtract it from the actual Vtc and
finally convert this into a temperature again.

Am I right?

I'm not sure this is the right question. It is easy to deal with the
cold junction if you know its temperature. How do you plan to get the
cold junction temperature? That is the hard part, at least if you want
to get an accurate reading of temperature (better than ±1°C). Getting
the local temperature accurately is not so easy to do inexpensively and
accurately. That is why the TC specific devices seem a little pricey.

Rick
 
S

Spehro Pefhany

Jan 1, 1970
0
Il 25/11/2012 22:06, John Larkin ha scritto:



I was afraid about the change of materials, but I was wrong because all
tc will change at the same point. Very easy, indeed.




Got it.




The ds proposes an hardware solution to compensate the cold-junction
using a thermistor and a precision resistor. I guess this requires a
precise selection of them (and perhaps I need to adjust values among
production batches).

In fact, you suggested to do the compensation in software.

How to handle this? I mean: I read the absolute Vtc using ADC (e.g. in
uV). Then I know the temperature of the cold-junction in °C.

In order to get the temperature at the reference junction do I need to
"invert" the tc equation? So I get the voltage a tc should exhibit at
the cold-junction temperature, I subtract it from the actual Vtc and
finally convert this into a temperature again.

Am I right?

Thanks
Marco

Careful, that's not quite correct.

To be a bit more explicit, the thermocouple transfer function or
tables assume a certain reference (cold-junction) temperature, usually
(but not always) 0°C (the temperature of an distilled ice-water
slurry, generally close enough anyway).

The inverse transfer function gives you the voltage you would read if
the "hot" junction was at the terminal block temperature and the cold
junction was at the reference temperature (say 0°C). For terminal
block temperature > 0°C that will be a positive voltage.

You then ADD that voltage to the measured voltage from the
thermocouple, and turn the crank attached to the transfer function to
get the temperature at the hot junction.

For example, suppose the terminal block is at 20°C we measure 4.360mV
from a type R (Platinum-Rhodium) thermocouple.

For 20°C. the corresponding voltage is +111uV

Add 4.360mV + 0.111mV = 4.471mV

Turn the crank, and get +500°C for the hot junction temperature.
 
G

George Herold

Jan 1, 1970
0
Right.

Most cold-junction compensation just adds a linear slope of
voltage-vs-temp to the tc voltage, often by putting it electrically in
series with the thermocouple. That creates a linear approximation of
the actual tc curve, maybe good enough in a small range of expected
ref junction temps. It's more accurate to measure cj temperature, run
the tc curve "in reverse" (use the actual tc equations or lookup table
to convert ref junction temp to millivolts) and add that to the
measured tc voltage. I usually have two lookup tables, the main table
that converts millivolts to temperature over the full range, and a
smaller reverse table to convert ref junction temp to offset voltage,
over the expected ref junction range. I usually lookup/interpolate
both tables, and use a thinfilm platinum RTD to pick up the cold
junction temp.

What do you pay (approx.) for a platimun RTD? And where do you get
them.

What about a 'regular' ~10k thermistor?
(Yeah I know, the 1 degree ones cost more.)

Or just a diode, the slope is (pretty much) known it's the 'offset'
that is different piece to piece. So maybe a single point 'zero'
where you stick the TC onto the cold junction would work.

George H.
 
S

Spehro Pefhany

Jan 1, 1970
0
What do you pay (approx.) for a platimun RTD? And where do you get
them.

The thin film ones are cheap and plentiful these days- even Digikey
has them for a couple dollars. They don't have much hysteresis (their
big failing) if you keep them around room temperature. Wirewound ones
are relatively expensive (and slow).
What about a 'regular' ~10k thermistor?
(Yeah I know, the 1 degree ones cost more.)

The slope is more important than the absolute accuracy, because you
want to avoid having to test each instance of the product in a thermal
chamber- too slow and expensive. Platinum RTDs are pretty good.
Cheapie "accurate" thermistors seem to be trimmed for the zero, but
have rather poorly controlled slope.
Or just a diode, the slope is (pretty much) known it's the 'offset'
that is different piece to piece. So maybe a single point 'zero'
where you stick the TC onto the cold junction would work.

George H.

Same thing applies to the RTD, in spades. Just zero it out in an ice
bath and Robert's your uncle. You don't need really high precision
parts.

If you really want to use a junction, a transistor-connected diode is
better than a diode, and transistor-connected diode measured at two or
three currents is even better (but the signal level goes down, so the
measurements have to be better).

For most food and paint baking and most plastics machinery, few care
about a few degrees C. They're good enough, and the whole BOM
including power supply, case and output stage has to come in at less
than those boutique chips cost. It's hard to make the device that
isothermal without adding cost and subtracting ease of replacement and
multi-sensor type ability, so the accuracy of measurement is a moot
point in many cases.

We used a small coil of copper wire as well as special resistors as
RTDs. Worked pretty well. Platinum for the rich customers. These days,
they just have to be well-heeled.
 
G

George Herold

Jan 1, 1970
0
The thin film ones are cheap and plentiful these days- even Digikey
has them for a couple dollars. They don't have much hysteresis (their
big failing) if you keep them around room temperature. Wirewound ones
are relatively expensive (and slow).

OK dat's nice. I'm use to the sire wound variety ~$10.
The slope is more important than the absolute accuracy, because you
want to avoid having to test each instance of the product in a thermal
chamber- too slow and expensive. Platinum RTDs are pretty good.
Cheapie "accurate" thermistors seem to be trimmed for the zero, but
have rather poorly controlled slope.



Same thing applies to the RTD, in spades. Just zero it out in an ice
bath and Robert's your uncle. You don't need really high precision
parts.

OK, I've
If you really want to use a junction, a transistor-connected diode is
better than a diode, and transistor-connected diode measured at two or
three currents is even better (but the signal level goes down, so the
measurements have to be better).

Grin, Yeah I'm using pnp's in to-220 pac's as T sensor's (positive
current source) Using the decade current difference measurement gives
the absolute T to something better than 1% (a degree or two).

But mostly I'd like to put the TC and cjc inside a temperature control
loop.

An all analog circuit would make things simple for me,


For most food and paint baking and most plastics machinery, few care
about a few degrees C. They're good enough, and the whole BOM
including power supply, case and output stage has to come in at less
than those boutique chips cost. It's hard to make the device that
isothermal without adding cost and subtracting ease of replacement and
multi-sensor type ability, so the accuracy of measurement is a moot
point in many cases.

We used a small coil of copper wire as well as special resistors as
RTDs. Worked pretty well. Platinum for the rich customers. These days,
they just have to be well-heeled.

For my control loop application I need stability ...
but no accuracy.

George H.


- Hide quoted text -
 
G

George Herold

Jan 1, 1970
0
Enercorp 1206 100r  $2.22

Minco ceramic slab 1K $4 in small quantities.

Much thanks,

(It's seems weird paying more for some special R's and C's
than for opamps floating around 'em.)

George H.
 
M

Marco Trapanese

Jan 1, 1970
0
Il 26/11/2012 21:02, rickman ha scritto:
I'm not sure this is the right question. It is easy to deal with the
cold junction if you know its temperature. How do you plan to get the
cold junction temperature? That is the hard part, at least if you want
to get an accurate reading of temperature (better than ±1°C). Getting
the local temperature accurately is not so easy to do inexpensively and
accurately. That is why the TC specific devices seem a little pricey.


Good point.
In my application the "cold" junction (by the way about 50 °C) and the
board are enclosed in a small thermally-isolated box.
I need to know the internal temperature for other purposes and I'm going
to use a quite good temperature sensor to get a measure better than ±1 °C.

I will put the sensor very close to the tc connector. Due to the small
box I may assume the temperature will not change a lot in few centimeters.

Marco
 
On Mon, 26 Nov 2012 13:01:16 -0800 (PST), George Herold



Same thing applies to the RTD, in spades. Just zero it out in an ice
bath and Robert's your uncle. You don't need really high precision
parts.

If you really want to use a junction, a transistor-connected diode is
better than a diode,

:)

James
 
A BFT25 is a far better picoamp diode than a PAD-1.

Yep. If you dig wayyy back in sed(*), I think you'll find I predicted
that, you suspected it, then you quickly tested, confirmed, and put it
to use.

But, that's a diode-connected transistor. All my transistor-connected
diodes(**) used two diodes, and had gains less than one.

(*) 5/2/09 "PAD5", 6/12/09 "Collector-base diode..."
(**) inventions of my youth.
 
S

Spehro Pefhany

Jan 1, 1970
0
Yep. If you dig wayyy back in sed(*), I think you'll find I predicted
that, you suspected it, then you quickly tested, confirmed, and put it
to use.

But, that's a diode-connected transistor. All my transistor-connected
diodes(**) used two diodes, and had gains less than one.

(*) 5/2/09 "PAD5", 6/12/09 "Collector-base diode..."
(**) inventions of my youth.

Yeah, I bet a lot of us tried that one. Two 1N60* diodes don't have
much gain.

* dating myself, but the equipment was old enough to be scrapped even
then.
 
F

Fred Bartoli

Jan 1, 1970
0
Spehro Pefhany a écrit :
Easier said than done!

When I was a kid (11 or 12) when I read how a transistor was done I had
to try a transistor-connected-diodes-pair...

Do I need to say how that worked? :)

If I were that same kid nowadays I guess I'd be even more confused.
MMBT3906 vs BAV70: same package, same multimeter test result but not
same working...
 
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