Non-linear DC amplifier?

[[email protected]]

I'm looking for a circuit to linearise a thermocouple mV signal, so I
can display temperature on a cheap digital multimeter. In particular,
I'd like to convert the output of an R-type thermocouple. The
conversion need not be very accurate - an error of 10 degrees would not
be noticeable and 20 degrees would be acceptable.

Any suggestions?

 

[Phil Allison]

I'm looking for a circuit to linearise a thermocouple mV signal, so I
can display temperature on a cheap digital multimeter. In particular,
I'd like to convert the output of an R-type thermocouple. The
conversion need not be very accurate - an error of 10 degrees would not
be noticeable and 20 degrees would be acceptable.

Any suggestions?

** Yeah - why not use a type K instead.

Plugs direct into some DMMs or use a cheap adaptor.

What sort of temps are you trying to measure?



......... Phil

 

[[email protected]]

** Yeah - why not use a type K instead.

Plugs direct into some DMMs or use a cheap adaptor.

What sort of temps are you trying to measure?

Fair comment Phil, but we need to use R-type for the higher temperature
(working in the range 700 to 1400 deg C) and longer life.

One solution might be to take the R-type mV signal, add a bit and
amplify a bit so it looks like a K-type signal!

A

 

[Spajky]

I'm looking for a circuit to linearise a thermocouple mV signal, so I
can display temperature on a cheap digital multimeter.

maybe you would get some kind of idea looking to my CpuDiode
measuring converter under Electronics on my site ...

 

[Phil Allison]

"Jasen Betts" <[email protected]

Thermocouples are linear devices,

** But only in New Zealand - Jasen.

On the rest of the planet they are non linear.

http://www.wici.com/oem/interest/tcinput.htm

all you need to do is adjust the gain and
the offset.

** Baaaaaaaahhhhhhhhhh

I don't have the specifications for an R-type thermocouple, how many
microvolts per degree?

** No Google in NZ ???




............ Phil

 

[The Real Andy]

Thermocouples are linear devices, all you need to do is adjust the gain and
the offset.
Wrong.


I don't have the specifications for an R-type thermocouple, how many
microvolts per degree?

what range do you want to measure

do you want output in 1mV per degree Kelvin, Celcius, or fahrenheit? or
something else?

Bye.
Jasen

 

[Franc Zabkar]

that doc.

.Thermocouples are made by welding two pieces of dissimilar wire together and
.using the welded junction as a temperature sensor. A voltage is created that
.is proportional to the difference in temperature between the welded sensing
.end and the other end where the measurement is being made.

sounds pretty linear...

.Actual thermocouples have Seebeck coefficients that vary widely over their
.temperature range.

Bugger!

Only "mostly" linear over small ranges then...

In fact it *is* linear over the range (700 - 1400 deg C) and accuracy
(+/-20 deg C) specified by the OP.

Here is a "Thermocouple Reference Table Type R (Platinum 13%
Rhodium/Platinum)":
http://www.nutechengineers.com/rchart.pdf

Using two roughly equispaced points (950C and 1200C), I came up with
this relationship:

T = 74V + 221 where V = mV, T = deg C

The following table compares the temperature, T', predicted by the
formula for various values of V (as in the above chart).

V T T'
---------------------
6.743 700 720
7.950 800 809
9.205 900 902
10.506 1000 998
11.850 1100 1094
13.228 1200 1200
14.629 1300 1304
16.040 1400 1408

If the OP cares to do a proper linear regression over the desired
range, the resulting line of best fit should be somewhat closer than
my guesstimate.

- Franc Zabkar

 

[Phil Allison]

"Franc Dakar"

In fact it *is* linear over the range (700 - 1400 deg CO) and accuracy
(+/-20 deg CO) specified by the OP.

** It is NOT linear from 20C upwards - making calibration in the range
700C to 1400C a major problem.

** IF ** the OP had a reliable, independent means of calibration in the
desired range, then a simple correction chart is all that is needed for his
type R probe.




.......... Phil

 

[Franc Zabkar]

Here is a "Thermocouple Reference Table Type R (Platinum 13%
Rhodium/Platinum)":
http://www.nutechengineers.com/rchart.pdf

Using two roughly equispaced points (950C and 1200C), I came up with
this relationship:

T = 74V + 221 where V = mV, T = deg C

BTW, I believe you should be able to realise this formula using an
instrumentation op amp with a gain of 74 and an offset (in mV) given
by ...

221 + 74 x Vcj

.... where Vcj is the cold junction compensation voltage.

For simplicity you might prefer a fixed voltage at a fixed ambient
temperature of 25C, say, or you might like to use a device such as the
LM335 to sense the actual ambient temperature.

If a fixed offset is good enough, then I believe you would need to add
0.141mV to the measured values. The formula then becomes ...

T = 74 x (Vmeasured + 0.141) + 221
= 74V + 10.4 + 221

The multimeter will display the temperature in millivolts.

- Franc Zabkar

 

[[email protected]]

BTW, I believe you should be able to realise this formula using an
instrumentation op amp with a gain of 74 and an offset (in mV) given
by ...

221 + 74 x Vcj

... where Vcj is the cold junction compensation voltage.

For simplicity you might prefer a fixed voltage at a fixed ambient
temperature of 25C, say, or you might like to use a device such as the
LM335 to sense the actual ambient temperature.

If a fixed offset is good enough, then I believe you would need to add
0.141mV to the measured values. The formula then becomes ...

T = 74 x (Vmeasured + 0.141) + 221
= 74V + 10.4 + 221

The multimeter will display the temperature in millivolts.

Thank you Franc. That is the sort of thing I am looking for. Can you
send me a little more information. I only know enough electronics to
have an idea of what's possible, but not enough to know how to do it.
I'd be very grateful if you could send something to ceramics AT cof DOT
com DOT au

Cheers

AW