Thermocouples Explained


Precision Team


Max 6 minutes read

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    Complete Thermocouple Probe and Adapter Guide

    Save yourself hassle and unnecessary breakage of your gear by learning how to operate it properly.

    Use this handy, step-by-step guide to use your probes and glass adapters like a pro!

    How To Insert a Thermocouple Probe

    1. Avoid breaking your thermocouple adapter! Proper insertion of a thermocouple probe into a fixed (or non-fixed) thermocouple adapter may seem like common sense, but these adapters can break if you’re not careful. This is especially true if your probe is bent prior to insertion.
    2. Straighten the probe The probe should be straightened before insertion into a thermocouple adapter. Inserting a bent probe wall cause undue stress on the probe, which can cause you problems down the road!
    3. Unscrew the cap on the adapter Once the probe has been straightened, you can now unscrew the Chemthread cap on the thermocouple adapter. Make sure the O-Ring is conditioned (grease may be used) and that it is intact!
    4. Place the Chemthread cap onto the probe Start by inserting the Chemthread cap onto the probe, followed by the O-Ring, and finally by the glass adapter. The thermocouple probe should slide smoothly and easily onto the thermocouple adapter.
    5. Tighten the Chemthread cap After inserting the probe, tighten it (in place) using the Chemthread cap. Do NOT over-tighten the cap! Once the thermocouple is sealed against the O-Ring, the probe will no longer move up and down. If the probe is loose, the O-Ring may be cold – if this is the case, warm it up with body heat or something similar.  The O-Ring’s flexibility may be tested by tightening the cap without a probe inside and visually observing how the size of the opening decreases when tightened.
    6. Bend the thermocouple as needed! The thermocouple probe may now be bent to suit your needs and situation.
    7. Insert the probe First, insert the stir bar by turning the boiling flask on an angle and gently inserting the stir bar into the lowest resting neck. Tilt the flask upright again slowly, such that the stir bar sits gently on the bottom. Next, carefully insert the probe and adapter into the round bottom flask.
    8. Need to remove the probe? When you want to remove a thermocouple probe from an adapter, you must remember to straighten the bend in the probe before you remove the adapter.
    9. Unscrew the Chemthread cap Once the thermocouple probe has been straightened, you can now unscrew the Chemthread cap completely, releasing the glass adapter and allowing the user to remove the probe from the adapter in a smooth and easy fashion.

    Links to products in this FAQ:

    Thermocouple FAQ

    What is a Thermocouple?

    A thermocouple is an electrical device consisting of two dissimilar electrical conductors, forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage – which is essentially a “reading” or measurement. This measurement can then be interpreted by an input device such as a temperature monitor (or regulator) in the form of actual degrees.

    Which Thermocouple Adapter Should I Choose?

    When choosing a thermocouple adapter, it is important to know what size thermocouple should be used (common sizes are 1/4″ and 1/8″) and what size ground glass joint will be used inside (common sizes are 10/18, 19/22, and 24/40). The next most important question to answer is: What kinds of materials/products will the adapter(s) be used around? For corrosive environments, PTFE or glass will do best. For high-temperature operations (internal temperatures over 260°C), PTFE and glass adapters have the potential to fail due to plastic (PPE) caps or parts. Providing this information to one of our sales representatives will allow us to give you multiple viable options. We manufacture and sell various types of thermocouple adapters for most common sizes and applications.

    Do thermocouple adapters leak?

    Some boiling flasks have 1/8″ thermocouple adapters located directly on the flask to reduce connection leaks. These adapters allow for the probe to slightly move around inside the flask. As long as the probe cannot easily move up and down inside the adapter (meaning the O-Ring is not properly seated), the connection will provide an adequate vacuum seal.

    What if there is still a leak?

    If you’ve followed the steps correctly at the top of this page, you should be all set. If, however, you cannot properly seat the thermocouple probe to seal inside the adapter (e.g. the probe moves up and down loosely), the following steps should be followed:

    1. Separate the components of the adapter and inspect the Viton O-Ring that compresses beneath the compression cap. If the Viton O-Ring is damaged, it may require replacement.

    2. If the O-Ring experiences excessive exposure to alcohol, it will degrade and may require replacement.

    3. If the O-Ring is extremely hard and not malleable, it may require some manipulation via slight heat (to improve elasticity) or even slight lubrication (you can use a small amount of vacuum grease for this).

    4. After ensuring that the O-Ring is exceptional and malleable, place it on top of the glass fitting, followed by the black (PPE) or tan (PEEK) cap, and begin screwing the cap into place.

    5. Once the cap stops turning, it has met the O-Ring. You should now peer through the opening in the cap to ensure that the O-Ring is indeed seated properly against the cap and glass.

    6. While looking through the cap’s aperture, begin tightening the cap, observing the O-Ring’s aperture shrinking down to smaller diameters.

    7. Lightly attempt to push the thermocouple probe inside the adapter with the cap tightened – it should not allow the probe through without excessive force. This means that the adapter is functioning properly, and it is ready to be assembled onto the system. Now you may loosen the cap until it is back to only just meeting the O-Ring (barely finger tight).

    8. Finally, insert the probe into the adapter (for fixed adapters ensure that the probe is STRAIGHT and NOT BENT until after inserting through the adapter). After the desired positioning is achieved, tighten the adapter’s cap until the thermocouple probe is snug and cannot move easily up and down unless force is applied. The adapter is now properly assembled and should provide a vacuum-tight seal for operation.

    Are temperature probes supposed to move?

    Once probes are placed, they should not move or be moved. This assumes they were placed in an optimal position at the start. If, in the course of operation, a thermocouple probe is moved to a sub-optimal position, then it is ok to move it back to the proper position for more accurate readings!

    If my PTFE melts off my thermocouple probe, can I still use it?

    Yes, you can still use your thermocouple probe. Underneath the PTFE is a stainless-steel probe which can still be used and will remain accurate.

    Can I bend my thermocouple probe?

    Yes, you can bend your thermocouple probe. Bending the thermocouple probe will not harm the probe unless bent to excess. Bending the thermocouple probe will allow for a more accurate reading in most cases, compared to when just the tip of the probe is sitting in the solution or vapor you are trying to monitor.

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