I will start off by telling you- Your answers are going to go WAY over my head, and I'm hoping someone will have to time to break things down for me, bitesize.

I am looking for a Cylinder Head Temperature Sensor and Gauge solution for my VW Bus. There are 2 solutions that exists, One is a Thermocouple Temp Sensor that simply connects to a gauge,, but the issue is
"...Because the thermocouple is calibrated for 72F, the readings will only be accurate at that temperature in the engine compartment. If it's colder, the CHT temperature will appear higher giving a false impression that the engine has fully warmed up. If the temperature is higher, say 100F or more, then the reading will be lower by 30F (the difference between the engine compartment temp and the calibration temp).
The reason this happens is that the VDO CHT wiring harness contains a 1 ft. length of thermocouple wire and the rest of it is regular wire so the cold junction will be in the engine compartment. Even if VDO ran thermocouple wire all the way to the dash, then the cabin temperature at the cold junction would still affect the the readings.
-The cure is use a system that has a cold-junction compensating thermistor. What this does is measure the temperature at the cold junction so the gauge can adjust the reading. Very simple but for some reason you can't buy this type of system from the low budget gauge vendors."

-Source: http://www.ratwell.com/technical/VDOGauges.html

My question is- Can I make a cold-junction compensating thermistor solution simply and affordably?

I Guess this thought is where my knowledge runs out.. This is something I found on a quick search..
https://www.ebay.com/itm/MAX31855-K...192331747576?_trksid=p2385738.m4383.l4275.c10

This is pretty much what I want to make..
http://www.dakotadigital.com/index....ct_id=758/category_id=-1/mode=prod/prd758.htm

 

There are IC solutions that combine cold junction compensation with amplification, making it much easier to interface with microcontrollers and ADCs. I'm not sure if these would help you though - if the gauge expects a non-amplified signal, then these chips might be counterproductive.

Here's an example of one on a breakout board that makes it easier to work with, avoiding the need for surface mount soldering:
https://www.adafruit.com/product/17...E6SdiOlhJXwbek9nns5bkUqv6Uhu0GXRoCiGIQAvD_BwE

Here's the same chip from the board above, by itself:
https://www.mouser.com/productdetai...=sGAEpiMZZMsLZKpoLkeUsb1V1VenCV8PF1t0h2xCOEg=

Having said that, I think a far better solution would be to use a display module that includes proper, built-in cold junction compensation, and run thermocouple wire (or the slightly cheaper thermocouple extension wire) all the way from the thermocouple to the display. You'd have to do some shopping to find just the right one, but there are countless options available on eBay and Amazon, starting under $10.

I've been using a cheap PID controller in my home-made espresso machine for 6+ years now, and it's been wonderful. It was $30-ish on Amazon, and this is the 12VDC equivalent:
https://www.amazon.com/Lightobject-ETC-JLD612-DC-Fahrenheit-Temperature-Controller/dp/B00C9M6ZJC

The ones I've looked at so far have all been single channel, but I imagine you can find dual channel displays with suitable inputs too. Unless you have to make that particular display work, I'd just go for something that's built from the ground up to read a thermocouple properly (in other words, with cold junction compensation.)

 

Just in the interest of keeping things simple there are thermocouples and there are thermistors, they are each totally different temperature sensors. Thermocouples require CJC where thermistors do not. Now looking at your link. The simple option using the display you referenced would be to run some thermocouple wire from the actual thermocouple to the gauge. As ebeowulf mentions thermocouple extension wire can be had relatively inexpensive. Twenty five feet runs about $30. Then you also need to consider the insulation on the extension wire for the environment. I would think about PFA Insulation as can be seen here in this link. Depending on you configuration choice of the actual thermocouple you may want to get a few Type K connectors also.

Ron

 

Just in the interest of keeping things simple there are thermocouples and there are thermistors, they are each totally different temperature sensors. Thermocouples require CJC where thermistors do not. Now looking at your link. The simple option using the display you referenced would be to run some thermocouple wire from the actual thermocouple to the gauge. As ebeowulf mentions thermocouple extension wire can be had relatively inexpensive. Twenty five feet runs about $30. Then you also need to consider the insulation on the extension wire for the environment. I would think about PFA Insulation as can be seen here in this link. Depending on you configuration choice of the actual thermocouple you may want to get a few Type K connectors also.

Ron

Hey again,
This as a solution was also mentioned in the vw forums, and I guess I forgot to include the findings, that being that even if the entire extension is thermocouple wire, the last foot, being in the cab of the vehicle, will still be influenced drastically by the outside temperatures. If it's 100° f for example, a difference of 60° in accuracy was recorded..

 

What is the benefit, if any, of using a thermocouple sensor, when I could use say a thermistor and not have to worry about CJC?
Could I make this sensor work without worrying about all of this?
https://www.ebay.com/i/112652923979...3D711-117182-37290-0%26rvr_id%3D1395001254899

 

The use of thermocouple wire for the entire run will not be influenced by the last foot or anything else as it is TC wire from the hot junction all the way to your display which should have CJC.
Using a thermistor is fine as long as the expected temperatures are within the range of the sensor. We choose sensors for their range and environment. Type K thermocouples are popular because of a wide range and depending on sheath a good range of environments.

I have to run but should be back later. We can also look at other sensors and displays.

Ron

 

OK, life is good as we now have a Christmas Tree.
As to sensors in general. When we want to measure for example cylinder head temperature we want a "Surface Mount" sensor. The use of thermocouples is popular because they can be rugged and work in harsh environments. Type K thermocouples offer a wide range of temperature making them popular. All families of temperature sensors have their good and bad points so we try to choose a sensor for its intended application. This web page has quite a bit of information and covers CJC in some detail. CJC only comes into play where the thermocouple alloys meet, for example copper terminals or other wire alloys. Most thermocouple displays take CJC into consideration where the TC alloys connect to the device. They create what is commonly called an "Ice Point Reference" junction electronically.

All you need to do is find suitable thermocouples for your application and a suitable display which should include any CJC. One big factor is the cost. This is a matter of what you plan to spend to get the results you want.

Ron

 

Hey again,
This as a solution was also mentioned in the vw forums, and I guess I forgot to include the findings, that being that even if the entire extension is thermocouple wire, the last foot, being in the cab of the vehicle, will still be influenced drastically by the outside temperatures. If it's 100° f for example, a difference of 60° in accuracy was recorded..

That's why I suggested using a different display entirely. I don't know the details on the VDO unit they're discussing in the vw forums, but if what they claim is true, it means that gauge doesn't include cold junction compensation, which is just silly. There are countless display options that do include cold junction compensation, and they're quite inexpensive, so why would you struggle to make that thing work?

 

Ron beat me to it. Well put.

 

What is the benefit, if any, of using a thermocouple sensor, when I could use say a thermistor and not have to worry about CJC?
Could I make this sensor work without worrying about all of this?
https://www.ebay.com/i/112652923979?chn=ps&ul_ref=http%3A%2F%2Frover.ebay.com%2Frover%2F1%2F711-117182-37290-0%2F2%3Fmpre%3Dhttps%253A%252F%252Fwww.ebay.com%252Fi%252F112652923979%253Fchn%253Dps%26itemid%3D112652923979%26targetid%3D379756163652%26device%3Dm%26adtype%3Dpla%26googleloc%3D9032306%26poi%3D%26campaignid%3D962079499%26adgroupid%3D51784542430%26rlsatarget%3Daud-384261922707%3Apla-379756163652%26abcId%3D1129816%26merchantid%3D101826266%26gclid%3DCj0KCQiAyNjRBRCpARIsAPDBnn102nlwx47tgsPgQNWBAnePdIYjCZJlCOMjXH8jz__DvBgCJ4MXgVEaAkMOEALw_wcB%26srcrot%3D711-117182-37290-0%26rvr_id%3D1395001254899

Here's a good summary of the pros and cons of each sensor type. Personally, I'm a big fan of thermocouples for most sensing applications I encounter because they're very fast, very rugged and simple, and very durable. Also, their consistency from one unit to the next, while not quite as good as high end RTDs, is better than all but the most expensive thermistors (which often have very wide tolerances.)

http://www.tempco.com/Sensors/RTD.htm

 

The important thing to recognize here, as Ron also mentioned, is that each temperature sensor type has a completely different output type, in the form of a voltage or resistance curve relative to the temperature they're measuring. Each of the three technologies is completely incompatible with the others. You can't mix one sensor with a gauge meant for a different sensor. You must pick your gauge and sensor to work with one another.

If I were in your shoes, I'd probably stick with thermocouples, both because I trust them in a harsh automotive environment, and because there seem to be a lot of existing hardware options to help you install them into your motor. However, I would not waste money on any gauge that doesn't handle the compensation - run thermocouple wire all the way to the gauge/display electronics, and use a display with cold junction compensation built in.

 

Thanks everyone for the comments- I am learning a lot here!

I think the reason the vw community is going through all the trouble with the VDO gauge- is because it's a analog, vintage looking gauge. If there was some external CJC that would work well for the VDO gauge, the vw guys would go crazy..
For me, I don't really care what my gauge looks like as long as it's pretty small and accurate. I will try to do my research and select a gauge with built in CJC.

 

Thanks everyone for the comments- I am learning a lot here!

I think the reason the vw community is going through all the trouble with the VDO gauge- is because it's a analog, vintage looking gauge. If there was some external CJC that would work well for the VDO gauge, the vw guys would go crazy..
For me, I don't really care what my gauge looks like as long as it's pretty small and accurate. I will try to do my research and select a gauge with built in CJC.

I don't know what the VDO gauge costs, but it looks like there are others making analog (or semi-analog) thermocouple gauge options for anyone who really prefers that.

... Although $100 extra is a lot to pay for an analog look in my opinion!

https://www.speedhut.com/gauge/GL25...DcalmhsAFpTO7tznWWrrGFdI7hi8J0mYaAk40EALw_wcB

https://www.glowshiftdirect.com/whi...-2aCFRZFc3IR0jS0Y83hvG2ATNl8pAVkaAqULEALw_wcB

 

I don't know what the VDO gauge costs, but it looks like there are others making analog (or semi-analog) thermocouple gauge options for anyone who really prefers that.

... Although $100 extra is a lot to pay for an analog look in my opinion!


https://www.speedhut.com/gauge/GL25...DcalmhsAFpTO7tznWWrrGFdI7hi8J0mYaAk40EALw_wcB

https://www.glowshiftdirect.com/whi...-2aCFRZFc3IR0jS0Y83hvG2ATNl8pAVkaAqULEALw_wcB

The VDO gauge costs about $40 typically. It also only reads to 600° f. I agree not really worth it

I don't know what the VDO gauge costs, but it looks like there are others making analog (or semi-analog) thermocouple gauge options for anyone who really prefers that.

... Although $100 extra is a lot to pay for an analog look in my opinion!

https://www.speedhut.com/gauge/GL25...DcalmhsAFpTO7tznWWrrGFdI7hi8J0mYaAk40EALw_wcB

https://www.glowshiftdirect.com/whi...-2aCFRZFc3IR0jS0Y83hvG2ATNl8pAVkaAqULEALw_wcB

 

Here is one I have found, very simple and without a backlight.. And still quite expensive. http://www.testequipmentdepot.com/l...Fv7DpLrz8oPPLKAaV9r_0J-pi8y_31icaAnicEALw_wcB
Do you have a good place or technique to look for a CJC thermocouple display?

 

Well, for most electronics I buy exclusively from big, trusted suppliers like Digikey, Mouser, Allied, etc.

However, when it comes to things like this, they usually only carry high-end, expensive, fail-safe, mission-critical level stuff.

Since you need a temperature display, but not a control system I'd be more willing to try other sources, like Amazon and eBay. Personally, I trust Amazon more than eBay, but that's as much feeling as fact-based.

https://www.amazon.com/gp/aw/s/ref=is_s?k=12v+thermocouple+gauge+

https://m.ebay.com/sch/i.html?_from...0.X12v+panel.TRS0&_nkw=12v+panel+thermocouple

With parts coming from unknown sources, you could do a few tests of your own before installing it if you want to play it safe.

 

Well, for most electronics I buy exclusively from big, trusted suppliers like Digikey, Mouser, Allied, etc.

However, when it comes to things like this, they usually only carry high-end, expensive, fail-safe, mission-critical level stuff.

Since you need a temperature display, but not a control system I'd be more willing to try other sources, like Amazon and eBay. Personally, I trust Amazon more than eBay, but that's as much feeling as fact-based.

https://www.amazon.com/gp/aw/s/ref=is_s?k=12v+thermocouple+gauge+

https://m.ebay.com/sch/i.html?_from...0.X12v+panel.TRS0&_nkw=12v+panel+thermocouple

With parts coming from unknown sources, you could do a few tests of your own before installing it if you want to play it safe.

Yes but these displays don't have CJC built in..

 

Yes but these displays don't have CJC built in..

What makes you so sure of that? The vast majority of of devices that have a thermocouple input also have cold junction compensation. In fact, your aftermarket vw part is the first I've heard of that seems not to. I've always known it was theoretically possible, but I'd never run into one before.

As an example, I know for sure that the JLD612, listed below, has working CJC, based on extensive experience with it. It doesn't mention it anywhere in the sales listing, because most people would consider it to be a necessity, not an optional feature. This unit is overkill for what you need - it has PID logic, several output options, alarms, dual display, and can read nearly every type of temperature sensor out there... and it's still only about $30.

https://www.amazon.com/gp/aw/d/B00C...rds=jld612&dpPl=1&dpID=41bregD4ztL&ref=plSrch

Based on that, I wouldn't be surprised to find $10 units that just accurately display temperature for a type K thermocouple, including the requisite CJC.

I can't personally vouch for all of the cheaper display-only units, but I strongly suspect that they'd be fine. It's easy enough to check for CJC functionality when you get a unit, just to be sure.

 

I have this https://www.dynoscooter.com/product...-gauge-with-14mm-spark-plug-connector-722-et3

for a leaf blower. The idea is to remember to put the oil in the gas and to not overwork the thing. My leaves were not applicable for a manufacturer's leaf blower because they grind up as dust and can make the head overheat.

==

All of the information concerning the link with the K thermocouple stuff is pure garbage and inaccurate.

About $50.00 should get you an extension wire and connectors that are 25 feet long. See: https://www.omega.com/pptst/EXGG_KX_WIRE.html
p/n FF-K-20-SLE-25

Connector type are your choice, K type thermocouple uses a yellow jacket. There are regular and miniature connectors. You have to use the connector and the special extension wire. The cables would easily be able to be shortened. There are screw terminal connections in the connector body.

The key is that the wire junctions be isothermal or the same temperature, but you don;t want multiple thermocouples. Extension wire has some of the same characteristics of a thermocouple and as long as the connection at the junction of the real K thermocouple wire and the extension wire is the same temperature, everyone is happy.

If you just short the temperature of the measuring instrument, it should read the temperature of the terminals at the meter.

It takes a lot of effort to make the connections at the meter isothermal. The negative side of the meter goes tot he red wire of the thermocouple..

There is a range where cold junction compensation operates and that should be spelled out in the meter. You need another kind of sensor that is able to know the temperature directly of the terminal. The meter has to be able to "look up" the mV value of the temperature of the terminals of the measuring instrument and be able to measure the mV value of the leads attached to the instrument.

Lots of stuff here https://www.omega.com/techref/ to read.