Do Thermcouples need to be physically touching? 1

[spggodd]

Hi,

I have an application where I am using K Type Thermocouple probes to measure bearing temperatures in a gearbox, the probes are fed down straight bores into the gearbox to the bearings.

However.. I was a bit of a numpty, the depth I required was 142mm but once I include the glands the 150mm long probes I ordered shrink to a useful depth of 129mm.

Now, with a 13mm gap between the probe end and the bearing will this enable me to make sufficient measurements of the bearing temperature?
Bear in mind that once the gland is tightened the probe will be essentially in a sealed chamber.

Thanks in advance.

Cheers
Steve

 

[Sparweb]

Half an inch away from the bearing it won't detect a thing about its temperature.
Congratulations on your new oil temperature gauge installation.

STF

 

[racookpe1978]

Well, yes, it does become a sealed system, but the thermal lag between bearing material -> thermocouple wall -> 13 mm air gap -> thermocouple sensor will create unacceptable delays in your reading.

Can you force a 14mm long thermosetting-but-thermo-conducting epoxy or material firmly and with very, very good contact into the base of the thermowell as a plug or filler?

 

[MikeHalloran]

Filling the air grap with grease is better than nothing, but not better than ordering the longer, correct probe.


Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

Your thermocouple will measure the temperature at the junction, which is usually at the probe tip. If the thermocouple's metal sheath is in contact or has a very close fit with the bore it is installed into, then the thermocouple will probably measure the temperature in the metal structure near the probe tip. In reality, if the bearing race is not too small and has a good fit in the housing, and the housing is made from a metal with good thermal conductivity, then the temperature measured at the probe tip in a close fitting bore 13mm away from the bearing race surface may not be too far off.

As to whether this thermocouple arrangement is adequate, it depends on what you intend to use these temp measurements for. If all you require is a record of the relative change in temperature for the test conditions, then your existing setup may work OK. But if you require precise temperature measurements at the bearing race surface, then you'll want to purchase a correct length thermocouple.

Just one additional comment about measuring bearing temperatures at the outside race surface, if possible take 2 measurements with one located in the direction of load on the race, and one located 180deg opposite. With a fixed outer race and a fixed load direction in respect to the outer race, the loaded sector of the outer race will normally run at higher temperatures.

 

[spggodd]

Thanks for the advice, for the relatively low cost I've decided to buy some replacement probes from RS in the correct size.

Cheers
Steve

 

[Tmoose]

there are some that are axially spring loaded to push the tips into contact with the bottom of the well, or is that what yours are like?

The bearing manufacturers want good protection for their bearings, so would likely recommend the tip touch the outside of the bearing race directly for best response and accuracy