Please help me with the thermowell size

Question

I started to build ebiab kettle and I got lost in NPT sizes, I have 7mm thermometer hole in my kettle and I don’t want to drill bigger, what size to choose?Planning to buy it from AliExpress or similar site.

Thank you for your help.

Answer 1

Please help me with the thermowell size ... I got lost in NPT sizes, I have a 7 mm thermometer hole in my kettle and I don’t want to drill bigger, what size to choose?

When searching I found that imperial (inches) is more common than metric, and such small sizes don't turn up in search results anywhere near as often as larger sizes, I understand that you don't want to drill.

• Choosing the thermowell length:

  A properly installed element will project into a liquid a distance equal to its sensitive length plus at least one inch. Above all, be sure that dead length, i.e., that required to pass through walls, pipe fittings, etc., is taken into account when choosing the necessary well insertion length.

  Thermocouples and thermistors have short sensitive lengths. They can be used with shorter insertion length thermowells. Bimetal thermometers, resistance thermometers and liquid-in-glass thermometers have sensitive portions between 25 to 51 mm (1 to 2") long. Therefore, the minimum standard insertion length of 64 mm (21 ⁄2") must be entirely immersed in liquid for proper accuracy. Filled system thermometer bulbs may have sensitive portions from one to several inches in length. Determine the sensitive length of the bulb before choosing an insertion length.

While you might think you can get away with a 1/4" thread what you should be searching for is a "M7 NPT Stainless Steel Thermowell" - but your list of choices at that size seems limited, here is one for 1/4" (a little small and not interchangeable). Searching AliExpress for "1/4" NPT Thermowell" turns up nothing, except a lot of 1/2" ones. Searching for "M7 NPT Thermowell" turns up nothing at all.

You will have a more difficult time finding a 7 mm thermowell compared to any other size, but they are available. Generally a thermowell is used to protect the thermocouple from corrosion or abrasive particles or where the vessel is under pressure, otherwise a simple temperature probe could be used instead. At 7 mm it won't take much wear, be particularly strong, nor permit as wide a selection of internal probes, thus it's the least common size.

A thermowell certainly has an advantage in boil over situations, or where the cat is likely to tip it over.

A 7 mm temperature probe is much more readily and less expensive than a thermowell (which still requires a probe).

This is a thermowell:

This is a temperature sensor (probe):

That is from Prairie Brew Supply, called the "Temperature Sensor – Waterproof (DS18B20)", the Probe is 7 mm in diameter and roughly 26 mm long. Overall length (including wire) is 6 feet. AliExpress seems to have a limited selection, I'd advise you to expand the list of places you are willing to purchase from.

If you do decide to drill for tapping a 1/2" NPT hole you'll want to drill a 45/64ths inch hole. This chart suggests you can get away with a 23/32nds inch hole which will be an easier drill to find.

Metric to Imperial conversions:

0.250" = 6.35 mm
0.276" = 7 mm

0.375" = 3/8" = 9.525 mm
0.394" = 10 mm
0.4375" = 7/16" = 11.112 mm

0.472" = 12 mm
0.5" = 12.7 mm