Clamp type thermocouples

[Jean-Sebastien Stoezel]

Hi:

I'm interested in those very low cost thermocouple clamps:


The ad says it's rated up to 400C which is interesting since K thermocouples are typically rated much higher. I would need to use these at a higher temperature rating.
I'm wondering whether the advertiser doesn't really know what he's selling or if the limiting factor is based on the construction or materials (say the way the bung is welded or brazed to the clamp). As is I believe the thermocouple itself could be used at a much higher temperature.
I asked the seller directly and he has no idea what limits the sensor to such a low rating.

Any opinion on the validity of the spec and use of this clamp setup over the advertised upper temperature limit?

Thanks,
Jean

[Steve Davis]

At that price, buy a few and see .

I would think that the accuracy would be quite suspect .. especially in "the wind". Would be tough to get SOLID contact for decently accurate readings. Seems it should have a 'bump' on the inside....

Steve, FV80

[Jean-Sebastien Stoezel]

Hi Steve:

Happy New Year. Who said this would be flapping in the wind ?

I'm not too concerned about absolute accuracy, just interested in relative readings. You're right that the contact wouldn't be very good close to the bung.
There's also the possibility to just use a basic thermocouple and sandwich it between the pipe and the clamp. I think this would work as long as the junction is electrically isolated from both the pipe and the clamp?

Jean

At that price, buy a few and see .

I would think that the accuracy would be quite suspect .. especially in "the wind". Would be tough to get SOLID contact for decently accurate readings. Seems it should have a 'bump' on the inside....

Steve, FV80

[Steve Davis]

... I think this would work as long as the junction is electrically isolated from both the pipe and the clamp?

Jean

Would be REALLY tough to electrically isolate it and still get good heat transfer. I'm thinking EXHAUST pipe and you might be thinking of something else. Depends on just how much higher than 400C you need to go. I'm confident that the sensor would not STOP at 400.. just might start to get off the curve.

Got down to FIVE degrees (F) here early part of the week (and stayed there for a number of hours).. I thought about you, wondering what your temp was...
sd80

[Jean-Sebastien Stoezel]

Yes I am thinking exhaust pipe... Though I don't need an absolute reading, as long as the temperatures can be accurately compared from one of 4 headers, with an accuracy within +/10C, possibly even more than that.

How awful for you! Last week we had an instance of -37C and -51C with the wind. Today we have -8C and I drove to work with the panoramic roof open. Amazing how your body adapts and things get relative.

Sometimes it's not about absolute temperature values

Would be REALLY tough to electrically isolate it and still get good heat transfer. I'm thinking EXHAUST pipe and you might be thinking of something else. Depends on just how much higher than 400C you need to go. I'm confident that the sensor would not STOP at 400.. just might start to get off the curve.

Got down to FIVE degrees (F) here early part of the week (and stayed there for a number of hours).. I thought about you, wondering what your temp was...
sd80

[Steve Davis]

Well.. now if you're gonna talk wind chill...
Sorry.. off topic.
I found this for you
http://www.hoskin.ca/catalog/index.p...toqreiu101e9t7

I'll bet you can find someone there that can tell you about the temp limit. They don't actually mention the limits on that page, but do say you can get any 'type' of sensor.
sd80

[BLS]

Jean,

*From memory here - k type is known to have variation from sensor to sensor due to the materials used. As I recall, the max range is much higher than the useable/linear range. You are correct a K type can go much higher. I'm guessing the 400C spec is where the signal degrades considerably. I have used these in lower temperature applications, maximum 200-250C without issue. I'm sure you can look up the k type characteristics...


*not all that good

Regards,
Barry

[sc94]

Why do you think you need isolated T/Cs? What are you measuring with. 400 °C is somewhere around the Curie temp for one of the metals in type K if I recall correctly. That might be one of the reasons they limit you to that range.

[Jean-Sebastien Stoezel]

Wouldn't you short circuit the connection if you were to use a regular exposed bead thermocouple?

Why do you think you need isolated T/Cs? What are you measuring with. 400 °C is somewhere around the Curie temp for one of the metals in type K if I recall correctly. That might be one of the reasons they limit you to that range.

[ReynardF1000]

The junction voltage output of a type K TC is established and recognized by NIST for up to 1,260C. At no point is the output "linear". The voltage output is sensed by the circuitry/'chip in your display/control device and compared against the standard K-type curve, providing a temperature.
If this sensor is a "tube and wire" product (looks like it), the junction is either welded ("grounded") to the sheath, or, adhered to the sheath with an adhesive ("ungrounded"). Inexpensive adhesives are limited to around 400 C; commonly silicone rubber and its family. The fiberglass insulated leadwire on this product is likely limited to ~400C by the inexpensive adhesive used to bind the fiberglass strands together, as well. If/when the adhesive(s) fails, the indicated temperature will drop off and be of little value.
"Ungrounded" TC's are used to isolate "stray" currents that are VERY COMMON on metalic surfaces of machines. The output signal of the K-TC is from ~0 - 50mV: that's 50/1,000 of a volt. Even a very small currant leakage will cause the displayed signal to be useless.
"Point contact" temperature sensing, like used with this sensor (tip of sheath at 90 degrees to clamp), is notoriously inaccurate. Air WILL pass over the sheath, either from natural or forced convection and WILL effect the sensed temperature: Sensed temperature will be lower than pipe temperature.
Given that you're wanting a relatively accurate temperature of the outside of your exhaust pipe (+-10C @ 400+C), this will not work. If you're happy with ~+-25-50C, you'll be fine, until the adhesive fails and the sensed temperature drops.
The best approach is to braze a nut to your exhaust pipe tube and install an exhaust sensing TC. They are built for the purpose and will sustain the temperature and vibration of your exhaust, and provide the accuracy you want (+-10C).
TMI?
Greg Lindahl - Watlow Electric Mfg., BSEE.

[Jean-Sebastien Stoezel]

Hi Greg:

Thanks for the reply. This is the level of details I was expecting to get.
I have ordered bung type thermocouples for EGT readings but I feel reluctant to install them on my headers.

First the headers are coated and welding or brazing will damage the coating. Then there is the issue of the effect of the bung and the probe to the gas flows. The headers I have a rather small section 1.5" I believe, will I be loosing performance by instrumenting my headers?

I'd probably be fine with a +/-50C difference actually.

Jean

The junction voltage output of a type K TC is established and recognized by NIST for up to 1,260C. At no point is the output "linear". The voltage output is sensed by the circuitry/'chip in your display/control device and compared against the standard K-type curve, providing a temperature.
If this sensor is a "tube and wire" product (looks like it), the junction is either welded ("grounded") to the sheath, or, adhered to the sheath with an adhesive ("ungrounded"). Inexpensive adhesives are limited to around 400 C; commonly silicone rubber and its family. The fiberglass insulated leadwire on this product is likely limited to ~400C by the inexpensive adhesive used to bind the fiberglass strands together, as well. If/when the adhesive(s) fails, the indicated temperature will drop off and be of little value.
"Ungrounded" TC's are used to isolate "stray" currents that are VERY COMMON on metalic surfaces of machines. The output signal of the K-TC is from ~0 - 50mV: that's 50/1,000 of a volt. Even a very small currant leakage will cause the displayed signal to be useless.
"Point contact" temperature sensing, like used with this sensor (tip of sheath at 90 degrees to clamp), is notoriously inaccurate. Air WILL pass over the sheath, either from natural or forced convection and WILL effect the sensed temperature: Sensed temperature will be lower than pipe temperature.
Given that you're wanting a relatively accurate temperature of the outside of your exhaust pipe (+-10C @ 400+C), this will not work. If you're happy with ~+-25-50C, you'll be fine, until the adhesive fails and the sensed temperature drops.
The best approach is to braze a nut to your exhaust pipe tube and install an exhaust sensing TC. They are built for the purpose and will sustain the temperature and vibration of your exhaust, and provide the accuracy you want (+-10C).
TMI?
Greg Lindahl - Watlow Electric Mfg., BSEE.

[ReynardF1000]

You're very welcome Jean-Sebastien.
Just be aware that this sensor will probably not live very long.
I've been using intrusive exhaust sensors for a long time. It's just the last ~5-10mm of sensor tip that's sensing the heat. If you have a large sensor, you don't need the entire length to extend into the pipe. I use very small OD sensors made with mineral insulation packed in the tube, ungrounded. The OD is 3mm and overall length is 20mm. I keep the end where the leadwires come out as cool as possible and use high temperture potting in the transition area. This is about a $75 USD sensor, and I've never had one fail.
The problem with sensors is that people don't treat them carefully. They either purchase a low quality product and it fails, or purchase a high quality product and break or burn the wires. Or, if you take care of them, they will last for many years. There are no moving parts to fail, except leads..

[Dan McMahan]

We had to use exhaust sensors in the 2 strokes, I don't know if they affected the performance. Welding wil degrade your coating around the weld area, how much depends on how much heat you build up in that area. A good welder should be able to do it without hurting it too much.

[Roux]

using this for exhaust temps. My experience with FF application was to insert TC's into the exhaust gas flow (AIM probe K types) and used them to tune the two throats of the weber and the dump tubes to get even exhaust temps. We did this on a chassis dyno and got pretty close to matched temperatures. Then a few hours later on track I logged the same two TC's in the same two pipes and found changes of over ~200F from what the engine did on the dyno. The pipe nearest the front was coolest and all I can attribute this to was the cooling airflow going through the engine bay at high speed was carrying away heat so well that the front pipe was 'cold' and the next pipe was hot, but not as hot as on the dyno where there was hardly any cooling air flow. So even with the TC junction nominally in the middle of the primary (away from the tube wall) the effective of running the car on track changed the reading by a lot and I gave up the idea of getting a meaningful piece of data except on the dyno.

The ports were about 6 inches from the exhaust valve.

One idea would be to wrap the header and the temp sensor to avoid ambient effects

One other option is to make your own type K TC's by ordering the wire from McMaster and then welding a ball on the wire using a Tig trick. this way you can choose to encapsulate with epoxy or shrink wrap for low temp stuff, and with small ceramic beads for the really high temp applications.

To put a ball on the pair of small wires, lay them side by side on a piece of copper. About 1/8 back from the free ends put another piece of metal like copper on top of the wires to act as a quench. Now strike a very low tig arc about an inch away from the ends on the copper 'base' and slowly move the arc nearer the ends until you see them melt, ball up and join into one puddle. As soon as you see some kind of action, get the arc away, so that you get the smallest ball possible. This gives faster response and the smallest lump to deal with. I have taken the alumel and chromel wires and with sandpaper (400?) thinned the wire to less than half the original thickness for the last inch or so, to get a ridiculously small ball.

[ReynardF1000]

Gasflow in exhaust pipes varies greatly, especially in or near elbows. If you're not exactly in the same flow region of each pipe, temperatures will vary greatly. Sounds like you're careful to be the same distance from the exhaust valve. I've found a great deal of temperature difference at different distances from the exhaust valve.
If the junction is inside the exhaust pipe, outside flow will not affect sensed temperature. This is because the junction is where most of the temperature is sensed. To measure exact temperature sensing, you must have 10-20 diameters of the wire immersed in a uniform temperature. This is another very important reason a TC won't measure accurate temperature when pressed against the outside of a pipe, for example (the other is air flow around the sensor body will create a low sensed temperature).
Greg

[Jean-Sebastien Stoezel]

Greg:

Thanks for the extra information. Hmmmm... $75 per sensor is much more than the $5 I wanted to spend! Do you have a part number for those sensors?

Jean

You're very welcome Jean-Sebastien.
Just be aware that this sensor will probably not live very long.
I've been using intrusive exhaust sensors for a long time. It's just the last ~5-10mm of sensor tip that's sensing the heat. If you have a large sensor, you don't need the entire length to extend into the pipe. I use very small OD sensors made with mineral insulation packed in the tube, ungrounded. The OD is 3mm and overall length is 20mm. I keep the end where the leadwires come out as cool as possible and use high temperture potting in the transition area. This is about a $75 USD sensor, and I've never had one fail.
The problem with sensors is that people don't treat them carefully. They either purchase a low quality product and it fails, or purchase a high quality product and break or burn the wires. Or, if you take care of them, they will last for many years. There are no moving parts to fail, except leads..

[ReynardF1000]

Hi Jean.
I suggest you speak with your local Watlow distributor and say you want an AF style TC in 1/8" OD x 2.5" OAL, Alloy 600 sheath with 1/8" NPT SS fitting. They should be able to help you.
Greg