Thermocouples

[jik22]

I've already got a K-type thermocouple in the car from the previous EGT gauge. However, I've now upgraded to the EMS2 (http://www.madman.co.za/digital_car_gauges.htm) management gauge and am wondering whether all K type sensors are the same, or whether I need one specifically to suit?

Anyone any ideas?

[foz.in.oz]

The "type" of a thermocouple is a standard and therefore any K type thermocouple if connected the correct way should give the same reading as any other K type (note:red thermocouple cables aren't always the +ve connection). "Should" being the operative word. Even thermocouples of the same brand/manufacturer can give slightly different readings hence calibration is often required. If in doubt you can connnect your guage to an "injector". This device outputs a given voltage and the value corresponds to a chart for each thermocouple type. If the guage is a true K type guage at the known voltage the readout should correspond to the predicted value from the chart. I don't know who may be able to help you with this as I used to play with this sort of stuff in Blighty but since coming to Oz have got out of touch. Maybe there is another geek/nerd out there who may be able to help.

[jik22]

The "type" of a thermocouple is a standard and therefore any K type thermocouple if connected the correct way should give the same reading as any other K type (note:red thermocouple cables aren't always the +ve connection). "Should" being the operative word. Even thermocouples of the same brand/manufacturer can give slightly different readings hence calibration is often required.

Thanks. The gauge can be calibrated for different senders on any of it's inputs, but of course, that's only helpful if you know what you're calibrating it too!

I noticed it had polarity on it, but am interested in why? I figured it was just a heat-sensitive resistor??

[81stubee]

From my limited understanding

A Thermocouple is two metals disimilar that when joined together generate voltage, and heat (increases????) this. However these voltages are incredibly small.

One mistake I made (told by an engineer) was I extended the probe leads to standard Figure 8. Apparently this created a secondary voltage because of the differnt metals and threw my reading out.

Some engineer will have a better explanation, maybe even google

Stu

[BigTim]

Alrighty, a question in my area of expertise :cool:

Short answer - Yes, the K type thermocouple you have will be fine provided it can be mounted securely. Suggest you do a quick search on this site as there are a number of threads discussing best placement and potential mounting issues etc.

For a quick check once connected and the EMS2 is set for a K-type, simply see if your sensor is reading air temperature and blow on it or hold in your hand - reading should increase to approx body temperature. Or you could get fancy and immerse the thermocouple in an ice-water bath to check zero degrees C and boiling water to check 100 degrees C. Remember that it will only be accurate to +/- 1 or 2 degrees, but this is generally not an issue when reading things like exhaust temperatures in the high hundreds of degrees.

Please note that the color coding of the cores is not necessarily what you would think - the most common K-type coloring is a yellow sheath over a yellow wire (positive) and a red wire (negative), yes - the red wire is the negative in this case. Other color codes are available if this is not what you have - check this link for other possibilities Colour Code Chart (http://www.temperature.com.au/Support/ColourCodeChart.aspx)

81stubee - you're quite correct, a thermocouple is simply two dissimilar metals joined together which create a very small (milli or micro (1/1000 or 1/1000000 of a volt)) voltage output in proportion to the applied heat. Chromel and Alumel (both Nickel alloys) are used to make K-type thermocouples. The reason your reading was out when standard figure-8 wire was used to extend it was that two additional thermocouples were created at the joins, Chromel-Copper and Alumel-Copper. These would each also generate voltages depending on their temperature. The problem can be avoided by only extending thermocouple cables using the same type of compensation cable. More details can be found here Thermocouple application note from Pico Technology (http://www.picotech.com/applications/thermocouple.html) if you are interested in the technicalities :eek: For the stated range of use e.g. a K-type thermocouple is typically -200 to +1200 deg C the temperature response curve is predictable, fairly linear, and can be further linearized by software in the instrument as in the case of the EMS2 instrument (according to the user manual) for better accuracy still.

[jik22]

Short answer - Yes, the K type thermocouple you have will be fine provided it can be mounted securely. Suggest you do a quick search on this site as there are a number of threads discussing best placement and potential mounting issues etc.

OK, thanks. As I said, it's already installed - plumbed in via the EGR blaking plate on the 300TDi and was fine with the old EGT gauge. Seems I should be fine with this one too from what you and others have said. I also know roughly what the EGT was at idle and under load on a highway run, so I'll know if there's something major wrong anyway.


Please note that the color coding of the cores is not necessarily what you would think - the most common K-type coloring is a yellow sheath over a yellow wire (positive) and a red wire (negative), yes - the red wire is the negative in this case.

That's exactly what I have.

However, it brings up another interesting point. It's currently terminated with two "U" shaped crimp terminal, which connected it to the terminal block on the old gauge. However, the new gauge has a loom, from which the two wires for connection to the thermocouple are unterminated, so I need to join them somehow. What's going to be the best method? (It's interesting to note that even the thermocouple (http://landyonline.co.za/shop/popup_image.php/pID/51) sold with the EMS gauge would need to be joined/re-terminated to fit though!)

[rick130]

WARNING, THREAD HI JACK

Tim, most of the industrial sensors I work with (usually NTC) can be extended to whatever length you need just using twin core copper cable, what type of sensors are these ?

[Dougal]

I've fitted my EGT thermocouples with mini K type plugs and sockets.

It makes for easy disconnect if I'm working in the engine bay and also fits my handheld digital reader (many multimeters have sockets for them too). This also avoids the "cold temp junctions" mentioned above.

This is a mini K Type connector:
https://www.aulro.com/afvb/images/imported/2008/04/542.jpg

[jik22]

I've fitted my EGT thermocouples with mini K type plugs and sockets.



That looks ideal, as I'm wiring the whole new Mud console up with connectors so I can just unplug and remove it.

However, while I can find these on the end of thermocouples, I can't find anywhere that actually sells the connector (Or at least, it's not coming up on a web search of the obvious sources). Any ideas where you get them?

[EchiDna]

Thermocouple Connectors - Miniature Thermocouple Connectors, Standard Thermocouple Connectors (http://www.tcaus.com.au/gen/connectors.htm)

RS Australia | Welcome to RS Online (http://australia.rs-online.com/web/search/searchBrowseAction.html'method=retrieveTfg&Ne=4294934505&Ntt=thermocouple+connector&Nr=AND%28avl%3aau%2csearchDiscon_au%3aN%29&Ntk=I18NAll&Ntx=mode%2bmatchallpartial&N=4294856265+4294835434&Nty=1&binCount=108#breadCrumb)

nice long URL for you!!

[jik22]

Thermocouple Connectors - Miniature Thermocouple Connectors, Standard Thermocouple Connectors (http://www.tcaus.com.au/gen/connectors.htm)


Thanks. Found them in the end at Farnells - cost more than the thermocouple! :eek:

[Dougal]

Thanks. Found them in the end at Farnells - cost more than the thermocouple! :eek:

RS are usually a lot cheaper than Farnell.

[jik22]

RS are usually a lot cheaper than Farnell.

Both extortionate, but it's a one-off and I'm keen to have everything connected via plugs so I can easily lift the whole dash out should the need arise.

Converted the stereo/speakers to ISO plugs, did all the power connections with a similar plug to the one on the ARB switch/locker harness, and used a smaller multi-way for the sender connections on the EMS gauge.

Finally, the iCom came with one for it's power lead, and decent thick power cables too, which was a nice surprise.

Just need this now and it's all done. :)

[slug_burner]

a heat sensitive resistor would be a thermistor not a thermocouple

[BigTim]

jik22 - EchiDna's got just the thing there with those mini K-type connectors. They make life as easy as possible.


WARNING, THREAD HI JACK

Tim, most of the industrial sensors I work with (usually NTC) can be extended to whatever length you need just using twin core copper cable, what type of sensors are these ?

Rick130 - NTC generally stands for Negative Temperature Coefficient and is typically associated with thermistors (contraction of "thermal resistors"), i.e resistors that change their resistance based on the temperature applied - NTC type resistance decreases as temperature increases - PTC type resistance increases as temperature increases. Thermistors can be extended using twin core copper cable without issues in most cases, as opposed to thermocouples which generate a small voltage which typically increases with the sensed temperature - extending a thermocouple cable with copper wires can create additional thermocouple effects known as cold junctions which can cause inaccurate measurements as noted in previous posts.
Both can be used to measure temperature, but they have different ranges and accuracies, and different measuring techniques and technologies are used.