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**posted on**

- ceramics

December 12, 2005, 12:02 am

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?

Re: Non-linear DC amplifier?

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

the offset.

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

Re: Non-linear DC amplifier?

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...

Bye.

Jasen

Re: Non-linear DC amplifier?

On Tue, 13 Dec 2005 09:01:19 -0000, Jasen Betts

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

--

Please remove one 'i' from my address when replying by email.

Please remove one 'i' from my address when replying by email.

Re: Non-linear DC amplifier?

"Franc Dakar"

******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

Re: Non-linear DC amplifier?

On Wed, 14 Dec 2005 19:31:18 +1100, Franc Zabkar

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

--

Please remove one 'i' from my address when replying by email.

Please remove one 'i' from my address when replying by email.

Re: Non-linear DC amplifier?

Franc Zabkar wrote:

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

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