BSP Screwplug Sizes - A Large PITA!
It should be simple.
You measure something, and that is the size.
Not if you are measuring a BSP (Bristish Standard Pipe) screwplug, sadly.
The size was originally based on the inner diameter measured in inches of a steel tube for which the thread was intended. This often causes confusion, as "normal" people think the size refers to the outside diameter of the male thread.
For example, the most common BSP Thread Size for UK Immersion Heaters is 2¼” BSP. If you measure roughly across the threads to check the diameter, it looks closer to 2½”, 2.58″ or 65.7mm, to be precise. Best be using a vernier gauge to be that accurate!
If you ask for a 2½” BSP immersion heater to be made, it will arrive with a thread size of almost 2.96″ or 75.18mm, which will be really annoying as there is no way it will fit, it will be the thread size you asked for but not the size you need. It is the same for all other BSP threads so caution is required.
Here at IHUK we believe prevention is better than cure, so here you go -
- Cut a strip of paper that you can wrap around the thread you want to measure.
- Draw a Line across it with a pen or pencil.
- Wrap the paper strip once around the thread and make a second mark where the paper overlaps the first mark.
- Unwrap the paper and measure the distance between the marks. This should correspond to one of the measurements on C=Circumference (C) mm above.
Why is shipping so expensive?
Long parcels and exotic locations, such as the Ireland, Highlands & Islands, can often prove to be prohibitively expensive with courier companies. We have to cover all bases for web orders, when folks want the convenience of plonking it in a basket & chucking in their credit card details. The website is covered with invitations to "Ring Jamie for a deal" and a conversation really is best. It also removes the issues of "Bits" like stats & compression glands, which we will be sure to ask if you need, thus preventing confusion.
What is the temperature of the sheathe of my element?
It is almost impossible to accurately measure the sheathe temperature of the elements, as the measuring device senses temperature of the medium the element is in, ie, the air, water or oil, rather than the temperature of the stainless steel tube.
The following is as close as we can realistically get, humblest apologies.
| Watt Density | Approx Sheath Temp |
|---|---|
| 5 W/in² | ~150–200 °C |
| 10 W/in² | ~180–230 °C |
| 15 W/in² | ~210–260 °C |
| 20 W/in² | ~230–320 °C |
| 25 W/in² | ~260–430 °C |
Bloody hot, simply put!
Or if you prefer ChatGPT's equally vague version -
At 12 W/in² watt density, the sheath temperature is moderate and well within the safe range for most immersion heating applications—this is why it’s commonly used in process tanks and plating/phosphate baths.
Typical sheath temperature at 12 W/in²
In water or aqueous solutions (with some circulation)
Sheath temperature: ~180–250 °C
In still liquids (no agitation)
Sheath temperature: ~220–300 °C
Why this range occurs
The temperature rise depends on heat transfer:
ΔT=qh\Delta T = \frac{q}{h}
Lower watt density → lower heat flux → lower sheath temperature
Better circulation → higher hh → cooler sheath
At 12 W/in² (~18,600 W/m²):
Heat flux is relatively low
So the temperature rise above the liquid is modest
Practical interpretation
If your tank is running at:
90 °C (typical manganese phosphate)
Then sheath temperature is roughly:
~200–260 °C (with circulation)
~230–300 °C (still)
Why 12 W/in² is widely used
This is considered a “safe industrial design range” because:
Minimises scale/sludge overheating
Reduces element burnout risk
Extends heater life significantly
For manganese phosphate (Gardobond-type baths)
Typical best practice:
10–15 W/in² → ideal
Keeps sheath temperature generally <300 °C
So 12 W/in² is right in the sweet spot.
✅ Final answer:
At 12 W/in², expect a sheath temperature of roughly:
➡️ 180–300 °C, depending on circulation and fouling
💡 If you want, I can calculate the exact expected sheath temperature for your tank using your flow conditions and chemistry—that’s how process engineers predict heater life before installation.