Responsivity of Various Thermocouples

Jan. 9. 2014 We believe that if our fine thermocouple, 25 & 50 or 13 & 25, is used by car manufacturers, they will succeed in improving fuel consumption and it is our great pleasure if we can contribute some to the restraint of global warming.

We are going to measure the responsivity of the fine thermocouples with a lamp in near future. Please wait for it.

(Part 5)Comparison of characteristics of various thermocouples

Insert the special thermocouple (25 & 50μm) into the engine as above photograph, then the pressure and temperature inside the 4-stroke engine are
measured with the thermocouple.
This engine is 4-stroke engine having below steps in 1 cycle as below:

Suction in     50 msec.
Compression in  50 msec.
Explosion in    50 msec.
Exhaust in     50 msec.
Above cycle is repeated

Horizontal axis : 50 msec./1 scale
Red line : temperature indicated by 25μm tip thermocouple
Blue line : temperature indicated by 50μm tip thermocouple
Green line : pressure indicated as mV

600 rpm (4 cycles/ 1round)
800 rpm (4cycles/ 1round)

The following results were obtained from the measurement of temperature inside the engine.
Min temp.
ΔT25μ Max temp
Min temp
ΔT50μ time for 4steps ΔP(圧力)
600rpm 750℃ 360℃ 390℃ 720℃ 480℃ 240℃ 200msec 253mV
800rpm 1260℃ 150℃ 1110℃ 940℃ 320℃ 620℃ 150msec 741mV

It is understood that 25μm size thermocouple can detect the temperature difference within 1 cycle more accurately.
In this manner, our thermocouple is capable to measure the temperature closer to actual temperature inside of the engine.
From the above, it is expected that increasing revolution number of engine will result in higher energy efficiency rather than simply increasing engine capacity.
We believe that many car manufacturers will succeed in reducing fuel consumption of their cars by adopting our finest thermocouple
such as 13 & 25 or 25 & 50 especially in Japan.
I am proud that our fine thermocouples may be able to contribute to prevention of the global warming.
(Part 4)Comparison of characteristics of various thermocouples

Stirling engine is primarily sold as a teaching tool for education.
The piston of this engine moves at high speed when a candle at bottom of the cylinder is burning.
Our very fine 25μm thermocouple was embedded inside the cylinder of the Stirling engine and temperature profile was taken.
The experimental results are shown as below graphs.

We got temperature profile as above. 1 time scale is 50 msec.
1 time scale : 20 msec.
On the graph, we can see multiple small waves within one cycle. This is the proof that the extra fine thermocouple is sensing subtle changes of temperature.
The speed of response seems to have some more margin still when judging the data.

(Part 3)Comparison of characteristics of various thermocouples
We developed extra-fine thermocouple and thinner thermocouple. Our thermocouples were compared with conventional type.

Type of thermocouple experiment

Conventional type
200μm diameter   500~1000μm
The thermocouples which we developed
Fine thermocouple



Sheet thermocouple


We performed following experiment in 1997. Sorry for introducing too old experiment but it is important in the meaning of basic theory.
We fixed various type of thermocouples with CAPTON tape
on the outside wall of stainless steel beaker at 40mm height as shown on the photograph.
To my regret, Capton tape was not strong enough to hold thermocouples particularly in case of vertical side.
Then some data were not stable.

Experimental result

Comparison of response and accuracy

Black line is of conventional type, 200μm wire, tip size 0.5mmφ.
its response was the worst due to minimum contact area and its large heat capacity.
Red line is of 25μm fine type with the best response, able to catch even faint temperature change.

Green line is of 50μm fine type with better response.
Blue line is for 40~50μm thick sheet type with fairly good response and stable due to its large contact area.


Type of thermocouple Robustness Fine spot evaluation Surface usage Response Main usage
conventional type × × × Robust&solder bond
Fine type × Fine &Surface
Sheet type × Surface

But sheet type thermocouple is not suitable for measuring very small spot like electronic parts of course.
(Part2)Immersion experiments to hot water using various thermocouples
1)Method of the experiment
Ultra-fine thermocouple product Code KFG-25-200-200, 25μm tip diameter and a conventional thermocouple with ball shape tip were instantaneously immersed into hot water in a vessel.
Vertical axis is of temperature, 1 scale is about 12.5℃.
Horizontal axis is for time, 1 scale is 100msec., dip time about 600msec..
Ultra fine thermocouple is warmed 300 msec. before immersing into hot water.
It may be due to the warming effect of vapor rising from the hot water.
The temperature rise of the ultra-fine thermocouple is saturated immediately and uniform upon the immersion of tips into the hot water.
But the conventional spherical tip thermocouple is not sensitive to the vapor, it can be seen that even at about 600msec. after hot water immersion, tip type is becoming asymptotic heated slowly.
But the conventional thermocouple with ball like tip is not sensitive to the vapor.
The temperature of the ball tip thermocouple is changing asymptotically even 600 msec. after hot water immersing.
Sensitive after removal also say the same.
The sensitivity after removal of the thermocouple from the hot water also show the same movements. That is to say heating and cooling of fine thermocouple is quick while ball tip type is slow.
In the experiment, I made an effort not to pick the temperature from hot steam as much as possible then the time constant of above extra-fine thermocouple was found to be below 10msec. .

(Part 1) Experiment of various thermocouples under adiabatic compression
1)Method of experiment
Insert ultra-fine thermocouple (13μmφ tip) and conventional thermocouple (1mm φtip ball) to a syringe. The piston moves vertically continuously.
The adiabatic temperature change inside the syringe was measured.
While the conventional thermocouple cannot capture the temperature change, the ultra-fine thermocouple can detect it.
The ultra-fine thermocouple could detect the change of temperature to about 90℃ in 0.1 sec. or less as recorded on the graph.

Left : conventional thermocouple
Right : ultra-fine thermocouple (13μm φ)

Red line : 13μm thermocouple
Black line : conventional thermocouple (1mmφtip ball)

See below temperature comparison data on the graph for more detail about difference of the temperature change in 1 cycle, which were caused by adiabatic compression and slight natural adiabatic expansion. Compare the data of various thermocouples.

Red line  : 13 μm
Blue line : 25 μm
Green line : 50 μm
Black line : conventional ball dia. 1mm

This data is telling that the adoption of these extra-fine thermocouples is ideal and very effective to measure delicately changing gas temperature.
In case of the finest 13μm type, its temperature rise is fast and naturally it reaches the highest peak temperature among all. In addition, this thermocouple can capture the lowering temperature of the gas due to expansion in reaction.
In comparison the responsivity of conventional ball tip type thermocouple is far slower than that of fine thermocouple.
Although high speed temperature measuring become possible by fast response of thermocouple, the thing was at data logger side of which sampling period, about 20msec. is too slow.
So we developed a data logger of which sampling speed is 0.02msec. or shorter.
(Exactly say, the data logger is the device consisted of thermocouple amplifier and voltmeter).
Below is the temperature data of adiabatic compression temperature collected by this data logger.
Fig-1 shows for whole picture. Fig-2 shows primarily the portion where the temperature rises. The smallest scale of horizontal axis is 0.2msec.
For example, the measurement of the air intake temperature change in an engine turning at several thousand rpm was carried out with the extra-fine thermocouple and ultra-high-speed data logger.
Would you help to improve fuel consumption of cars?



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Yoshinobu Anbe, President

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