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Tuesday, 25 October 2011

Make your own Thermocouple sensor

If you're unfamiliar with thermocouples, OmegaEngineering's Introductionto Thermocouples is a good primer that contains several other usefulinformation resources about the selection and use of thermocouples.
One of the common tools that researchers use to measuretemperatures of objects or organisms is the electronic thermocouple. Fora few hundred dollars you can have a handheld electronic thermocouplereader and flexible thermocouple leads. The thermocouple readers arequite durable and last for years. On the other hand, the thermocouplewires eventually break after lots of usage. Most people purchasepre-made thermocouples from a source such as Omega Engineering primarily for theconvenience/cost tradeoff. However, it is fairly easy, if somewhat timeconsuming, to make and repair your own thermocouple leads using suppliesfrom Omega, which can save money in the long run compared to purchasingnew thermocouple leads every time you break one. This tutorial is basedaround the assumption that you have one of the typical Omega handheldthermometers (pictured above) and use T-type thermocouples with“miniature” connectors.


If you or your lab have standardized on some other type of thermocouple,maybe K or J type, perhaps using “standard” round-pin connectors, youcan still use the same principles described here, but the part numberswill differ.

When I aim to measure temperatures of inverts and algae, rocktemperature, or the air temperature in the field, I use T-typethermocouples with wire of a few sizes. For now we’ll considerfine-gauge wire as it is the most versatile (but least robust) form formeasuring temperatures of small things. It’s always desirable to be ableto say that the thermal mass of your measurement instrument doesn’tinfluence the temperature of the object you’re measuring (i.e. placing alarge stainless steel temperature probe at 20°C on a small snail at 40°Cmight cool the snail during the measurement process), so using smalldiameter thermocouples is good.

It can make sense financially to purchase thermocouple wire in bulk sothat you can make and repair thermocouples for years to come. Forinstance, you can purchase 40-gauge T-type insulated wire from thispage on Omega’s web site. The particular wire I would order is partnumber TT-T-40-SLE-100. This part number would get you 100 feetof 40-gauge T-type thermocouple wire with a "Neoflon" insulation. Notemy recommendation for getting the “Special Limits of Error” wire ratherthan regular wire. It costs a bit more, but Omega specs this wire to ahigher standard, so that it should be within 0.5C of the actualtemperature, rather than the 1C of the regular cheaper wire, even beforeyou calibrate.

To go with the wire, you’ll need to purchase the connectors that allowyou to plug the wire into your handheld thermometer, and in this case Irecommend connectors from thispage, using this part number combination in particular: HMPW-T-M.For starters you only need the male plugs to plug into your handheldthermometer, but you can use the female plugs to make "extension cords"by placing a female plug at one end and male plug at the other end of alength of thermocouple wire. If you have a pile of old non-functionalthermocouple leads sitting around, you can reuse the connectors.

With the wire and connectors in hand, you can now start making your ownthermocouple leads. To put things together, you’ll need the followingequipment:
Soldering iron and regular solder for electronics
Dissecting scope or magnifying glass so you can see
Razor blade or scalpel
Small screw driver, such as a jeweler’s screwdriver
Fine forceps
Some tape

Assembling the thermocouple:
Start by cutting off a length of thermocouple wire to the desiredlength. I usually make my leads a few feet long (3-4’). We’ll start byattaching the leads to the plastic connector.


Unscrew the cover on the connector. Underneath you should see two screwterminals.


On a T-type connector, one terminal will be copper colored and the otherwill be silver colored. They each have a screw in them, which is how youwill attach the thermocouple wires. Unscrew the terminal screws.Sometimes your connectors will have little Teflon washers under theterminal screws, or maybe a metal flange to help hold the wire downagainst the terminal. You can use these during reassembly, or discardthem.

Take your thermocouple wire and place one end under the dissectingscope. I like to use a bit of scotch tape to hold the wire in place sothat I can work on it with both hands. While looking through the scope,take a sharp razor blade and scrape off some of the insulation from thewire.

With the thermocouple wire I recommended above, there is a clear outerjacket, and then separate red and blue insulation around the copper andconstantan wires. Place the razor blade at a shallow angle to the wire(i.e. nearly horizontal) about 1” from the end, and scrape towards theend of the wire. Do this gently to avoid cutting through the metal wireitself. You just want to take off the clear insulation and the coloredinsulation, leaving the bare wires intact. The 1” length gives youplenty of bare wire to wrap around the terminals in the next step. Cutaway the excess insulation so that things are neat and tidy.


Once you have your wires bared, bring in the connector. Again, this isprobably best done under the scope. With your wire still taped down,position the connector so that the bared portion of the wire will sitcompletely inside the connector when closed.


Then use your forceps to wind the bare wires around their respectiveterminal screws.


The wire color matters here: the copper colored wire needs to be screwedonto the copper-colored terminal, while the silver constantan wire needsto be screwed onto the silver terminal (again, this assumes you’re usingT-type thermocouple wire). Follow this up by carefully screwing down theterminal screws. It is fairly easy to accidentally break the wire atthis step, especially if the wire catches on the terminal screw andstarts tugging as you screw down. Use your forceps to keep everything inplace. The goal here is to get the bare wire of the thermocouple lead indirect contact with the terminal, using the screw to push the wire downonto the terminal. Also note that your two bare wires should not toucheach other inside the connector housing, so keep them separate!


With the wires screwed down, it’s time to add in a strain relief beforereplacing the plastic cover on the connector. The connectors from Omegasometimes come with square brass or copper pieces that sit in adepression at the distal end of the connector housing to act as a strainrelief. Typically these pieces simply sever the fragile wire as youscrew the connector cover back on. Alternatively, the connector may comewith a round brown silicone rubber piece with a hole through its middle.This hole is too large to grip the very fine 40 ga wire, but works fineon larger diameter wire. You can make use of the silicone rubber pieceby slipping the wire underneath the rubber so that it is pressed betweenthe rubber and the plastic of the connector case. In either case, Ialways just replace the square metal pieces or round rubber piece with asimilarly sized square piece of rubber that sits in the depression.

The rubber grips the thermocouple lead without cutting it, providing ameasure of strain relief for the terminal connections inside thehousing. I cut out little pieces of neoprene rubber purchased from McMaster-Carr, such as the rubber in thisassortment: part number 9455K666 found on this page.


At this point the thermocouple is half finished. You now need tocomplete the junction at the far end of the thermocouple lead, whereyou’ll be measuring your temperature. We typically use solder to makethis connection, as it is easy and fairly durable. The pre-madeconnectors you buy from Omega use a welded junction at the sensing end,which is more durable than solder, and will not melt at hightemperatures. Soldered connections are fine for the full range ofbiologically-relevant temperatures you’ll find in the field, but youcouldn’t use a soldered joint to measure the temperature in your mufflefurnace for example, as the solder would melt.

As before, you need to bare the thermocouple wires to make theconnection. Tape your wire down to the dissecting scope platform, anduse your razor blade once again. I recommend only baring ~1/4” or so. Inspecial cases you might want a long bare wire lead, but for mostpurposes a short bare section at the sensing end is desirable from adurability standpoint.


With the two wires bared, use your forceps to twist the two leadstogether.


This provides a bit of mechanical strength to the joint, and makes thesoldering easier. For the soldering step, fire up your iron, clean thetip (wipe it on a wet sponge), and melt a bit of solder onto the tip.Again, this next step is best done under the scope. Take your solderingiron and touch it to the twisted wire pair. If you have enough solder onthe tip of the iron, the wire pair should be “submerged” in solder.


I then draw the iron tip out towards the end of the twisted wire pair,dragging the solder pool along the twisted wires. Solder doesn’t stickwell to the wire pair, so ideally the twisted joint will retain a bit ofsolder on the two wires via surface tension. Do this quickly (touch irontip to wires, draw iron tip towards end of wires) until a bit of solderremains.


It doesn’t take much to hold the wires together. Once you have somesolder on the joint, you can test the thermocouple by plugging it intoyour handheld thermometer. You should get a temperature reading, and itshould quickly approach air temperature if the tip of the thermocoupleis sitting free in the air. Assuming the thermocouple gives you areading, you can now trim off any excess bare wire from the tip that youdon’t think you’ll need, using the razor blade or wire clippers.Occasionally you’ll end up with a big blob of solder on the tip, and Ilike to cut that off to keep the tip low-profile.
You should now have a working thermocouple.


It would be prudent to calibrate your thermocouple lead. At the veryleast, dip your thermocouple tip in a stirred ice water bath to get a 0degree C reading, and go find a water bath or some other warm water of aknown temperature (a calibrated alcohol thermometer would be usefulhere) and check your thermocouple against the water temperature. Eachthermocouple lead that you make may have a slightly differentcalibration, so it’s worth checking them and writing the calibration ona piece of tape that you stick on the thermocouple lead. Most of thetime your thermocouples will all read within 0.1°C of each other, butsome may be further off.

Now that you can make your own thermocouples, you can also repair brokenthermocouples using the same methods. You might occasionally rip thewire out of the connector, or the sensing tip might get flexed too manytimes and break, or you might wind the wire too tightly and break itsomewhere within the insulation. In every case you can just cut off theoffending end of the wire and make a new, shorter thermocouple. Be sureto recalibrate it!

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