| Choice | Options | When |
|---|---|---|
| Construction | Bar stock / fabricated | Bar stock for anything that matters |
| Connection | Threaded, flanged, weld-in, Van Stone, sanitary | Follow the piping specification |
| Stem profile | Straight, tapered, stepped | Tapered where velocity is high |
| Material | SS316, SS304, SS310, Inconel, Hastelloy, Monel | Match the process, not the pipe |
| Bore | 6.5 / 8.5 / 10.5 mm | Match the sensor diameter |
| Insertion length | 50 to 1000 mm | Tip in the middle third of the bore |
| Lagging extension | 0 to 150 mm | Equal to the insulation thickness |
| Tests | Hydrostatic, DP, radiography, PMI | As the project specification requires |
A bar stock well is machined from a solid billet, so there is no weld anywhere the process touches. A fabricated well has a tube welded to a flange, and that weld is the thing that fails. The price difference is small; the failure mode is not.
If the spec prohibits threaded connections, no argument about cost changes that. Flanged, socket weld and Van Stone all exist for exactly this reason.
A straight stem is fine in slow liquid. In fast gas or steam, vortices shed off the well and shake it; a tapered stem raises its natural frequency and moves it away from the shedding frequency. This is what the wake frequency calculation checks.
ASME PTC 19.3 TW is the standard. We run it on request, free, before you order. A well that fails this calculation will eventually break off inside the line — which is a process release, not just a lost instrument.
A 6 mm sensor needs a 6.5 mm bore. A larger bore leaves an air gap that slows the reading. A smaller one will not accept the sensor at all, and you will find out at site.
The head must clear the insulation so it can be opened. Measure the insulation thickness and state it — a well that ends inside the lagging cannot be serviced.
Hydrostatic test, dye penetrant, radiography, positive material identification, and material test certificates all take time and cost money. Adding them after the order is placed means remaking the item.
Tapered bar stock well with a wake frequency calculation. This is not optional on main steam.
Tri-clover well with crevice-free construction and Ra 0.8 µm polished wetted parts.
Hastelloy, Monel or tantalum wetted parts — or a PTFE sleeved well where nothing metallic survives.
A check under ASME PTC 19.3 TW that the vortices shed by the flowing fluid do not excite the thermowell at its natural frequency. If they do, the well vibrates and eventually fails at the root.
Bar stock is machined from solid, so there is no weld in the wetted section. Fabricated wells have a welded joint that is the usual point of failure.
Match the sensor: 6.5 mm bore for a 6 mm sensor, 8.5 mm for 8 mm, 10.5 mm for 10 mm. An oversized bore leaves an insulating air gap.
The tip should reach the middle third of the pipe bore, so it sits in the flowing stream rather than the boundary layer at the wall.
If the line is insulated, yes — the extension must at least equal the insulation thickness so the sensor head remains accessible.
Send the line size, velocity, pressure and material. We run the ASME PTC 19.3 TW wake frequency calculation free before you order.
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