My test section is stainless steel microtube with 16 cm length, 500, 600 or 900 micron inner diameter and 100 micron thickness heating by alligator clips. The used thermocouples are K-type or T-type omega brand as below with gauge 40 or 36 wire size.
http://www.omega.com/pptst/5TC.html
Question: Any method for better attaching thermocouples on the surface of microtube for less temperature fluctuation? Or even any new reliable measurement method?
Thanks in advance...
Dear Abdul aziz Jaafar.
1) Heat flux sensor is typically using for flat or curved plates not circular tube as far as I know, also it can be seen in the specification. But it can be a good method, I can try it,
2) Thermocouples is installed on the outer wall surface of tube then it does not affect the entry region of fluid.
Thanks for your kindness...
Dear Abdul aziz Jaafar.
Thanks for your creative and constructive response.
Best...
@Mehrdad, surface temperature measurement is ALWAYS a problem and error prone, even in normal sizes and normal materials. I do not know details of your system, so I am unable to give a better answer than the following.
Since you mentioned alligator clips, I presume you are using Joule heating, i.e. autothermal heating by using the tube wall as a heating element. If you were using a condensing vapor (such as steam) then the wall temperature can be taken to be equal to the saturation temperature of the vapor. Its pressure can be measured far more accurately than wall temperature measurement from which the saturation temperature can be got from property tables.
If electrical heating and direct measurement of the wall temperature are inevitable, you can try spot welding microthermoelements. They used to be available with SS sheath from Philips. Degussa or Detriv may also have it. The overall dimensions will be 200 micron outer dia, with the individual wires of 5 microns, packed in MgO2. Buy it with the connectors pre-joined to the wires.
Run the thermocouple for 2 to 3 cm along the tube to minimize conduction errors. This is a very delicate (and expensive) thermocouple. Therefore, before spot welding (by spark discharge), practice the welding with a fine (0.1 mm) stainless steel wire on a plain stainless steel plate of the same wall thickness as your tube. The discharge current is important because an excess current will burn the thermocouple and a hole through the plate. Too low a current will result in poor contact.
Try spot welding the power lead to your tube. Alligator clips have excessive contact resistance for your application.
Good luck.
Dear Prof. Raghavan.
Thanks for your comprehensive response.
Yes temperature measurement problem takes my time during every set of experiment for getting reliable and repeatable data.
Your guess about the joule heating is perfectly correct. Inside the tube in fluid in my experiments. I tried welding of the thermocouples before but unfortunately it was not successful because of their thickness as you mentioned. This time I should try spot welding of microthermoelements. Thanks for suggesting the new method.
Best regards...
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