Thermocouples clamped on gas tubing for reasonable-cost, continuous temperature monitoring of stage outlet temps?

[-JD-]

(Some of you may have seen that while I'm SLOWLY going over and cleaning up my 20+ year old (seemingly, almost-pristine) RIX SA-6AG and preparing to fire it up, I'm planning for the future. While I will probably create a specific unit/build thread, I have some more generic questions that are not "type", much less unit-specific, so I think deserving of their own threads ...)

Based on suggestions made in response to my pressure monitoring point thread (Question - Hydraulic Test Ports for local or remote gauge-points and possibly controls?), I am considering how to implement practical (for me) continuous temperature monitoring. Hopefully it will be helpful to others if it results in a relatively easy, low-cost, and reasonably accurate way to monitor compressor temperatures.

At first I was considering mounting ring-lugged thermocouple probes under the head bolts. In doing a deep-dive into this, I ran across a thread where this was being discussed for long-duration, high-oxygen % use. Some relevant quotes for reference w/ highlighting by me:

At least removing the cooling fan will make it run quieter I guess LOL
Now from memory the SA-6 has used at least six (6) different cooling fan designs over the years that I can remember
And only four main types are available as stock fan designs that I can photograph as examples.

The current SA-6 for sports diving uses a moulded generic white six (6) bladed fan
The Mil-spec SA-6 uses a hand built black eight (8) bladed zero degree pitch
The SA-6 scientific Pure Air SA-6 uses as above but with a 30 degree pitch angle
For low RPM using a paddle shaped blade would be a consideration designed for generating fanned pressure
while the wing tip trailing edge reduces noise and generates a lower threshold of wake turbulance

In a nutshel for Mil-spec use its peddle to the metal and compete with an F-15 in rattling the roof while for recrational use its as quite as it needs to be to pass noise limits and not interfere with your Iphone ringtones.
I should look up the stall speed graphs for the mil-spec units as we built them if l get time

1. The fan inside the specially designed SA-6 shroud work together to incease air pressure from the fan blade tips over to the back of the shrould housing while the increased air pressure blowing around the cooling coils allow for a pretty decent amount of heat exchange between the two.
On cooling coil one off the 1st stage hot side this is to reduce the "Approach Temperature" into stage 2 inlet side.
But critically not so much to the point at which it would induce water vapour to condence out over the 2nd stage head. Then out of the 2nd stage hot side the cooling coil 2nd stage is when most of the water is "knocked out" of the gas stream and condenses out into the collection separator tower due to the sudden loss in temperature along the cooling coil and again another advantage of going oil free over oil lubricated is manifest as water cooler faster and more efficiantly than an oil film.

On the final stage this is to reduce the approach temperature into the filter (if fitted) and bring the temperatures down to within hand hold limits allow.

Now the big question here is:
1. With your required temperature limit (400F)
2 Where and how are you measuring this temperature.

Reason I ask is for consideration that what you are measuring may not be what you need to know
For example measuring the head temperature is not the same as measuring the gas temperature no way close.
And the position or point of measurement is critical for a accurate reading but more important to achieve a relivant reading.
Bauer for example to asses filer life measure the skin temperture of the cool filter shell externally with the B-Timer
Making you totally unaware of the real temperature that makes the real marked effect as to filter life expectancy. Measure the gas temperature not the filter shell.
Others measure the skin temperature of the compressor stage head again with no idea of the real gas temperature.
Others know the temp of a bolt screwed into the metal head. Why?
All are not accurate readings of the real need to measure actual gas temperature inside the gas stream at the point it comes off the cylinder head discharge valve side with a suitable pressure proof RTD
That is the point you dont want to see 350F IMHO

Frankly I think you were ill advised regarding this temperature business.
1. Measure the gas temperature not the head temps
2. OK to set at 350F to start off with but it is critical IMHO to monitor the compressor in real time and see what number you are getting. Then reduce the 350F setting down to say a few degrees above the maximum you are normally seeing. That way and I assume you have incorporated a high temperature cut out switch when the compresor cuts out high temp you can asses the reasons Hot day, extra high pressure, Dog sleeping in front of the fan shrold etc and make the judgement call to wake the dog.

Also get a good quality digital room temperature and humidity reading monitor as well I will try and look up some suitable models. Iain

@iain/hsm temp switches are replacing the plugs on the 2nd and 3rd stage heads which will be measuring gas temperature inside of the head itself. Where Rix suggested to put them, but also the most convenient place to put them since the plugs are already there. The logic I was using with fixed temp switches was to have the temp cutoff at the point that the teflon is going to start degrading. Will likely leave that for now and when I start designing the full PLC, then go to thermocouples so I can watch them in real time

I have lab grade temp/humidity sensors that I can use.

You mentioned adjusting the actual balance of the crank for high pressure use. Do you have any documentation on that being done? Like I said above, this is likely going to spend 98% of its life pumping from 2500-4500psi, so if there is something I can do to make the compressor happier running at high pressure, I'm very curious

@RayfromTX perfectly fine with me to split out.

So that says to me that head temp is not really what I am looking for. Unfortunately, looking at pressure rated 316 sensors that might fit the (2nd and 3rd) stage head plugs or NPT for inline at the head exit-port in addition to the re-plumbing to accomplish it, the cost/hassle for my limited use seems impractical.

While this has been somewhat RIX-specific ...


Stewing over it a bit, I began to wonder if attaching a plain thermocouple sensor on the outlet tubing, immediately after the NPT-Tube fitting might give me the closest possible approximation of gas temperature without opening the system up. My expectation is that the tubing temp should stabilize pretty close to the gas temp.


My thinking would be to bed the thermocouple tip to the tubing with some "grease", wrap with some high-temperature tape for mechanical isolation and thermal insulation and then clamp it in place with small SS worm-drive clamps.

For thermocouples, I was thinking something like these: 5pcs 3M K Type Mini-Connector Thermocouple Temperature Probe Sensor Measure Range -50~700°C

For "grease" I have not found a truly high-temperature thermal conductive paste, they seem to top out near 250c. Since I can use some around the shop anyway, one thought was to consider a high-temperature copper based anti-seize like: Permatex 31163 Copper Anti-Seize Lubricant

For the tape I was looking at special flue tapes, kapton, and even specialty fiberglass/PTFE tape for heat-sealers. But, now I'm thinking plain-old ptfe/teflon thread tape should do the trick as it will likely conform to seal without crushing.

As for monitoring there are some cheap 4-channel monitor/loggers to get me started like: PerfectPrime TC41, 4-Channel K-Type Digital Thermometer Thermocouple Sensor -200~1372°C/2501°F, 20 x 4 Data Log Storage Function

With 4 channels, I can measure the 1st, 2nd, and 3rd stage outlets and the probably do the same at the filter stack inlet for

• Conceptual critiques?
• Do I need to externally wrap the thermocouple/tube contact areas with thermal insulating material drive the internal/external temp differential down?
• Specific concerns about one of the components?
• What am I not thinking about?

 

[broncobowsher]

Working in the automotive R&D world for years. Surface temp of the tubing will be really close to the gas inside of it. I would actually get a little distance from the head (a heat sink). But still away from cooling fins.

We have done some taps into slightly higher pressure hydraulic stuff, but never true high pressure gas. Unless you need to know exact temps inside, the exterior is close enough. I've heard of soldering K-type thermocouples onto the sides of transmission cooler lines. No extra insulation used. I am a little more OCD and I do put a hint of insulation on. For a compressor like this, high temp silicone probably. Takes the heat, not very thermally conductive.

The stuff I do is generally short term. A few weeks and I am done with it. Although I have seen some of my handwork many years later on rare occasion and the stuff still works.

 

[-JD-]

Working in the automotive R&D world for years. Surface temp of the tubing will be really close to the gas inside of it. I would actually get a little distance from the head (a heat sink). But still away from cooling fins.

We have done some taps into slightly higher pressure hydraulic stuff, but never true high pressure gas. Unless you need to know exact temps inside, the exterior is close enough. I've heard of soldering K-type thermocouples onto the sides of transmission cooler lines. No extra insulation used. I am a little more OCD and I do put a hint of insulation on. For a compressor like this, high temp silicone probably. Takes the heat, not very thermally conductive.

The stuff I do is generally short term. A few weeks and I am done with it. Although I have seen some of my handwork many years later on rare occasion and the stuff still works.

High temp silicone is perfect! Brilliant!, thank you. I probably don't even need the clamps, just tape it in place tightly and I'm good to go!

Good to 500F so as same as the Teflon rings/seals which need to be protected. Better yet, it is just $5 at the local Home Depot: https://www.homedepot.com/p/Super-Glue-1-in-x-10-ft-Black-E-Z-Fuse-Silicone-Tape-15408-6/204237144

Need to figure out optimal placement ...

 

[-JD-]

I pulled the trigger on the Thermocouple stuff and the silicone tape (since it was available immediately and at the same price as HD). I think I'm into it for around $60 to experiment. If nothing else, I'll probably find other uses for the monitoring unit around the shop.

First steps will be testing in ice and boiling RO water to see if things are "close enough" I'm not expecting precision lab equipment ....

 

[ACHiPo]

(Some of you may have seen that while I'm SLOWLY going over and cleaning up my 20+ year old (seemingly, almost-pristine) RIX SA-6AG and preparing to fire it up, I'm planning for the future. While I will probably create a specific unit/build thread, I have some more generic questions that are not "type", much less unit-specific, so I think deserving of their own threads ...)

Based on suggestions made in response to my pressure monitoring point thread (Question - Hydraulic Test Ports for local or remote gauge-points and possibly controls?), I am considering how to implement practical (for me) continuous temperature monitoring. Hopefully it will be helpful to others if it results in a relatively easy, low-cost, and reasonably accurate way to monitor compressor temperatures.

At first I was considering mounting ring-lugged thermocouple probes under the head bolts. In doing a deep-dive into this, I ran across a thread where this was being discussed for long-duration, high-oxygen % use. Some relevant quotes for reference w/ highlighting by me:






So that says to me that head temp is not really what I am looking for. Unfortunately, looking at pressure rated 316 sensors that might fit the (2nd and 3rd) stage head plugs or NPT for inline at the head exit-port in addition to the re-plumbing to accomplish it, the cost/hassle for my limited use seems impractical.

While this has been somewhat RIX-specific ...


Stewing over it a bit, I began to wonder if attaching a plain thermocouple sensor on the outlet tubing, immediately after the NPT-Tube fitting might give me the closest possible approximation of gas temperature without opening the system up. My expectation is that the tubing temp should stabilize pretty close to the gas temp.


My thinking would be to bed the thermocouple tip to the tubing with some "grease", wrap with some high-temperature tape for mechanical isolation and thermal insulation and then clamp it in place with small SS worm-drive clamps.

For thermocouples, I was thinking something like these: 5pcs 3M K Type Mini-Connector Thermocouple Temperature Probe Sensor Measure Range -50~700°C

For "grease" I have not found a truly high-temperature thermal conductive paste, they seem to top out near 250c. Since I can use some around the shop anyway, one thought was to consider a high-temperature copper based anti-seize like: Permatex 31163 Copper Anti-Seize Lubricant

For the tape I was looking at special flue tapes, kapton, and even specialty fiberglass/PTFE tape for heat-sealers. But, now I'm thinking plain-old ptfe/teflon thread tape should do the trick as it will likely conform to seal without crushing.

As for monitoring there are some cheap 4-channel monitor/loggers to get me started like: PerfectPrime TC41, 4-Channel K-Type Digital Thermometer Thermocouple Sensor -200~1372°C/2501°F, 20 x 4 Data Log Storage Function

With 4 channels, I can measure the 1st, 2nd, and 3rd stage outlets and the probably do the same at the filter stack inlet for

• Conceptual critiques?
• Do I need to externally wrap the thermocouple/tube contact areas with thermal insulating material drive the internal/external temp differential down?
• Specific concerns about one of the components?
• What am I not thinking about?

I like your approach--thermal grease with the TC clamped securely to the lines of interest. Another way to skin the cat (albeit more complicated) would be to use thermally conductive epoxy, perhaps potted into machined aluminum clamps around the lines.

The myriad of bluetooth transducers that connect to a smart phone is pretty amazing. I haven't looked for this application, but I wouldn't be surprised if you could find reasonable TCs combined with bluetooth transceivers that you could monitor with your phone or a PC.

 

[ACHiPo]

I pulled the trigger on the Thermocouple stuff and the silicone tape (since it was available immediately and at the same priceprice as HD). I think I'm into it for around $60 to experiment. If nothing else, I'll probably find other uses for the monitoring unit around the shop.

First steps will be testing in ice and boiling RO water to see if things are "close enough" I'm not expecting precision lab equipment ....

Looks like I was a little late! Looking forward to the results.

 

[-JD-]

I like your approach--thermal grease with the TC clamped securely to the lines of interest. Another way to skin the cat (albeit more complicated) would be to use thermally conductive epoxy, perhaps potted into machined aluminum clamps around the lines.

The myriad of bluetooth transducers that connect to a smart phone is pretty amazing. I haven't looked for this application, but I wouldn't be surprised if you could find reasonable TCs combined with bluetooth transceivers that you could monitor with your phone or a PC.

If you have any specific suggestions at hand for thermal grease or epoxy available easily, affordably, and in small quantities, I'm all ears. A quick search for specific use compounds did not yield any stuff rated for my 500f minimum target. I needed the Copper anti-seize anyway, it is rated to well over 1000f, and with the copper, should be reasonably thermally conductive since it is relatively quite electrically conductive. I only need a dab at the thermocouple junction.

The thermocouples could easily find their way into an RPi / Arduino /ESP32 type control system integration eventually or even just a BT/Wifi server for remote monitoring. For now a $32 4-channel display seems pretty attractive to get me started.

 

[Tracy]

For no work at all, you can glean the same information with a thermal camera. That is what I use when I am diagnosing compressors and boosters. Instant information about exactly what you want.

 

[rhwestfall]

I have just been using a laser thermometer....

 

[ACHiPo]

For no work at all, you can glean the same information with a thermal camera. That is what I use when I am diagnosing compressors and boosters. Instant information about exactly what you want.
View attachment 772124

IR cameras are great for comparing temps, not so great for absolute temp measurement—they’re sensitive to the emissivity of what they’re measuring.