Thermocouple Temperature using DPM or DMM
Temperature measurement and control uses the
Thermocouple and the Seebeck Effect for measurement.
The most common is J Type as it is affordable. K is
better and works well even at higher temperatures
Control Simulation - ITC Leipzig
If wires of two dissimilar metals are joined at both
the ends and the junction formed at one of the ends,
is heated more than the other junction, a current
flows in the circuit due to Seebeck thermal emf.
This effect is used in thermocouple temperature
The Peltier effect is the converse of above Seebeck
effect, which means that if a current is forced
through junctions of dissimilar metals, the junction
will generate heat or absorb heat (cooling)
depending on direction of the applied emf. This
effect is used to make portable and small
Comparison of Thermocouple and RTD
Going to practical temperature measurement, we know
that one of the junctions is the sensing or hot
junction (Tmes) and the other junction is the
terminating or cold junction (Tref), the voltage
between terminals 'a' and 'b' is proportional to
Tmes - Tref (and given in the Table 1) . The formula
being Vab = alpha x (Tmes - Tref), where 'alpha' is
the Seebeck coefficient of the thermocouple.
|Temperature in Deg C
cold junction is not zero but is at room
temperature (RT) add RT to temperature.
Feed 10.777 mV to the TC+ and TC- terminal
if RT then is 30 Deg C reading on 2V DPM
Will be 230 counts - 230mV.
|Reference junction or cold
junction at 0 deg C.
In the circuit, use only metal film resistors (MFR)
of 1 per cent tolerance, as this is an
instrumentation application. Power supply should be
a stable +5V, -5V supply, for which one can use 7805
and 7905 regulators.
The inputs TC+ and TC- terminals should go to a
4-way barrier terminal block, the 2 extra terminals
are used to mount TH1 Cu thermistor. This forms an
isothermal block, which is good enough.
A simple way to make a TH1 Cu thermistor, is to take
a 1 Meg-ohm 2W resistor as a former and wind 2
meters of 46 SWG enameled copper (Cu) wire (5.91
ohm/meter) over it. This gives a 12-ohm value.
Terminate wire ends on resistor leads.
Circuit Diagram -
Circuit - PDF version of above, more details
and easily printable.
Test and Calibration -
For calibration, you will need a DMM-DPM and a
milli-volt source (as shown in the Fig.). First
connect source to terminals TC+ and TC-, then set
source to 0.00 mV (verify with DMM for zero). The
output across +out and -out (use DMM) terminals must
be mV representing the room temperature (RT). For
example, if RT is 30° C (use a glass thermometer)
then +out should be 30mV at 0mV input. Adjust VR1
till 30mV is read at +out terminal. This is 'zero
Now increase mV input to 21.85 (corresponding to
400° C). Now vary VR2 till +out terminal is at 430mV
(temp. +RT). This is 'gain cal'. Now as VR1 and VR2
are interdependent you may have to repeat 'zero cal'
and 'gain cal' a few times till you get the above
Properties of J thermocouple and design aspects
of gain block used in the temperature measurement
instrument are summarized below:
J Thermocouple Ansi Symbol 'J' -
- J is a thermocouple made of iron + VE and
- Constantan is an alloy of copper and nickel.
- Full range of use is from -200° to +700°C
- Practical to use only from 0°C to 400°C.
- Useful in reducing and Alkaline atmosphere.
- Corrodes-rusts in acidic and oxidizing
- Color code of wires negative-red and
- J type is popular because of Low price and
high mV output.
- J type TC used in rubber-plastic forming and
general purpose use.
Design of Gain Block -
- Minimum input from thermocouple is very low
like 1-2 mV. Hence ultra low offset (100uV opamp
is required - OP07 used).
- Inputs may be subjected to wrong connections
or high voltage. Use of R1 limits current and
Zener ZD1 clamps voltage to safe level. (low
leakage zener or use diode).
- Gain required is 400mV - 21.8mV that is approx
18 at 400° C. Gain Av = ( Rf + Ri ) / Ri here Rf
is R7 and Ri = R5 + R6 + VR2 (in circuit value).
Design of TH1 cold junction compensation copper
J Type TC output changes by 0.052mV per deg C
as per Table 1. Copper has a temperature coefficient
of 0.0042 ohm per ohm/deg C. eg. for a copper wire
of 12 ohms, it is 12 x 0.0042 = 0.05 ohm/deg C.
For R1 of 5K current Thru TH1 =5V / 5K = 1mA. Change
of voltage across TH1 with temperature is
0.05 x 1mA = 0.05mV / deg. This rate is same as J
type TC hence it simulates cold junction