Polyimide Capillary Tube for Thermocouples and RTDs: ID, Wall and Cut Length
Engineer's Guide · 2026-09-16 · 5 min read
A thermocouple or RTD sheath has two jobs that pull against each other. It has to insulate a millivolt signal from a hot, often conductive environment, and it has to let heat reach the sensing element fast enough that the reading means something. Wall thickness sits between those two requirements, and polyimide is the material that lets you go thinner than most alternatives without losing the insulation. What follows is the specification side of that trade.
The four numbers on a capillary drawing
| Dimension | What it controls | What goes wrong when it is wrong |
|---|---|---|
| Inner diameter | Whether the thermocouple or RTD element fits, and how much air gap sits around it | Too tight and the element will not slide through after a bend; too loose and the element moves and reads a lagging average |
| Wall thickness | Thermal response time and dielectric strength, in opposite directions | Thick wall slows the reading; thin wall creases during assembly and loses insulation margin |
| Concentricity | Even wall around the bore | An off-centre bore puts a thin side in one direction, so the part passes inspection and fails in service |
| Cut length and end squareness | How the sheath seats in a fitting or a weld boss | A skewed or burred end leaks past the ferrule and the sensor reads the ambient, not the process |
Polyimide film carries a very wide temperature span, quoted on our sensor page as minus 269 C to plus 400 C, with a thin wall held to tight tolerance. That span is what makes the material useful for cryogenic rigs and high-temperature probes with the same drawing. It is not a licence to ignore the service conditions, because the wall is thin and the polymer is hygroscopic unless it is conditioned or coated.
Thermal response is a wall thickness problem
Response time is dominated by the thermal mass between the process and the sensing junction. Thin the wall and you shorten the time constant. That is the whole reason polyimide is used here rather than a metal sheath or a thick polymer one.
- For a bare junction inside a capillary, halving the wall moves the time constant in the right direction but does not halve it, because the air gap and the element's own mass still dominate.
- Filling the capillary with a thermal compound or a powder removes the air gap and cuts the lag far more than thinning the wall further.
- On a probe where the element sits against the bore, wall thickness matters more because conduction, not air, is doing the work.
- For cryogenic work, the same logic holds at the other end of the range, and the material stays flexible enough that the sheath does not crack on cooldown.
Moisture is the hidden variable
Polyimide takes up moisture. That changes dimensions slightly and, more importantly, changes dielectric performance. A sheath that measured correctly in the warehouse can read differently after a humid week on the shop floor.
Specifying the sheath
- State the element first: thermocouple wire gauge or RTD body diameter, and the length of the sensing zone.
- Set the bore from the element with a stated clearance. Say whether the element has to slide through after bending, which is the requirement that usually sets the number.
- Set the wall from the response time you need, then check the dielectric margin at your working voltage and temperature.
- State concentricity rather than assuming it. On a thin wall it is the difference between a part that insulates evenly and one that does not.
- State the cut length, end squareness and whether the ends get deburred or flared, and specify the finish if the sensor sees a cleanroom.
- State the environment: continuous temperature, peaks, chemical contact, humidity and whether the sheath is inside a metal well or directly exposed.
Where polyimide is the wrong sheath
Three cases. Where the job is low friction or mould release, because polyimide is not a release surface and a fluoropolymer is. Where continuous temperature sits comfortably below the polyimide range and the extra cost buys nothing, since PET film tube covers moderate motor and transformer duty for far less. And where the sensor is scrapped rather than repaired and the sheath cost dominates the assembly, in which case a softer, cheaper polymer is the right engineering answer.
The dimensional side of the specification, including how to read ID and OD tolerance and concentricity on a polyimide datasheet, is on the polyimide tolerance guide. Where the material is the only one that survives, that is a different discussion and we keep it on the radiation and semiconductor page.
Seen in context: the polyimide capillary tubing route brings together every part that goes into this job.
Products for this job
These are the polyimide tubing routes we run, from sub-millimetre bore to sensor-sheath wall. Send the drawing and the duty, and we will confirm the grade and the wall against both.
Related reading
- Polyimide Tube in Radiation and Semiconductor
- Polyimide Tube Wall Thickness and ID Tolerance
- PEEK vs PI (Polyimide) vs PTFE
- Heat Shrink Temperature Ratings
Common questions
What is a polyimide capillary tube used for?
It sheathes thermocouples and RTDs, insulates fine sensor wires, and lines high-temperature probe bodies. The usual reason it is chosen over a metal well or a fluoropolymer is that it holds a very thin wall across a wide temperature span, so the signal is insulated without slowing the reading.
What temperature can a polyimide capillary take?
Our sensor capillary is quoted from minus 269 C to plus 400 C. That spans cryogenic rigs and high-temperature probes. Derate for continuous duty at the top of the range and confirm the grade against your actual service temperature.
How do I choose the wall thickness?
Start from the response time you need and the dielectric margin at your working voltage, then confirm what the supplier can hold at your bore. Thin wall shortens the time constant, but the air gap around the element usually matters more, so a filled capillary often beats a thinner wall.
Does the bore tolerance matter as much as the wall?
Yes. The bore decides whether the element slides through after bending and how much it can move once installed. A loose bore gives a lagging reading because the element is not where the sheath is measuring.
Why does moisture come up for polyimide parts?
Polyimide absorbs moisture, which shifts dimensions slightly and changes dielectric behaviour. On tight-tolerance parts that means a conditioning step before final measurement, and your drawing should state the humidity and temperature exposure the part will see.
Can polyimide be substituted for PTFE in a sensor sheath?
Only if you do not need low friction. PTFE is the release and low-friction surface. Polyimide holds thinner walls and tighter dimensions and takes higher continuous temperature, but it is not a release material.
Can the tube be supplied to our drawing?
Yes. Bore, wall, cut length and end finish are made to your drawing. WELLELE makes polyimide capillary tubes for temperature sensors with thin, tight-tolerance walls and references IEC 60672 and ASTM D5487 on the material side. Send the print with the element dimensions and the quantity.
Is there a certificate with the batch?
Each production batch ships with a certificate of analysis, and first-article inspection and dielectric testing can be added against your specification. Ask for the dimensional report to cover bore, wall and concentricity, not only the outside diameter.
Specifying this part?
Send the drawing for a manufacturability review.