Pressure Gauge Thread

I'm in the process of putting together a hydraulic test rig and acquired a (new) Budenburg gauge from E-bay at a very reasonable price. It came with a red plastic protector on the brass connector that is marked 1/8th BSP. However, this is split (forced on?) and doesn't seem to be quite the same size as the actual brass fitting.

I've calipered the fitting and it is about 11.1mm (7/16th?) diameter over thread. My tables say a 1/8 BSP should be about 13/32nd (10.3mm) and a 1/4 should be 17/32nd (13.49mm) approx. diameter. As it seemed to be a bit large for 1/8 and too small for 1/4 - I checked the TPI. As far as I can tell (using a Whitworth thread gauge) it's near enough 19tpi, although the fit isn't perfect. I wondered if it could be metric - in which case it could be 1.25mm pitch?)

So as 1/8th BSP is 28tpi and 1/4 is 19tpi, I don't seem to have a thread that looks like it is BSP. So I'm wondering if there is another (metric) standard for these pipe/gauge fittings that would be about 11mm over thread and 1.25 pitch??

Or have I missed something with regards to BSP threads? Is there a 7/16th, 19tpi thread standard used for gauges? Any advice would be welcomed.

Regards,

IanT

Are you trying to fit a BSP tapered thread to a BSP parallel thread?

Is it perhaps NPT(National Pipe Thread, USA), 1/8 is 27tpi, and the small end of the taper is .3625", 1/4 is 18tpi, and .4774" at the small end of taper. Just another thread to confuse things a bit more, OH dear.

Ian S C

They have a comprehensive web site, have you looked up your particulat gauge to see what standard thread options it has?

J

I think you'll find there are three options of thread, BSP, NPT, and metric, not Knowing the part number of your gauge, I could not see which thread, but you should be able to find it quite easily. Ian S C

Hi Ian
To confuse things more M10 x1 is close the 1/8" bsp
Nobby

Edited By Nobby on 05/11/2013 13:02:59

Thanks for the replies.

As far as I can tell - it's not a tapered thread (although I'm no expert in this area). I have looked on the Budenburg site and I cannot find the actual gauge there but have looked at several similar ones. They generally state that gauges are available in 1/4, 3/8 & 1/2 BSP or 1/4, 3/8 or 1/2 NPT - or "other as required".

Of these the 1/4 versions seemed the most likely - with 19tpi for BSP and 18tpi for NPT. However, it's the major diameter that has flummoxed me. A 1/4 BSP should be about 13.716mm major diameter and a 1/4 NPT about 13.15mm (0.518" ). The tapping drill for a 1/4 NPT is 7/16 which is about the outside thread diameter I am measuring.

I have thought about it being a metric thread and my tables for M10 fine show both 1mm & 1.25mm pitches. This could almost be 1.25mm (at just over 20 tpi) but I would expect the outside diameter to be much nearer 10mm (?).

Thank you for help.

Regards,

IanT

Edited By IanT on 05/11/2013 13:32:43

Unlikely but what about 7/16UNF or CEI20 both of which are 7/16OD x 20tpi

Edited By JasonB on 05/11/2013 13:55:31

Sometimes pressure gauges have special threads to stop them being swapped onto the wrong fitting on complex or critical equipment. Doesn't help, but an odd thread may be why it is on ebay.

Neil

UNF threads are common on high pressure coned fittings (just as an example).

The sealing method may give some clue - if it is a parallel thread then generally there should be provision for a seal of some kind (O ring, copper washer, Dowty seal, coned pipe end or ferrule).

Is the gauge a fairly 'ordinary' pressure range, or something a bit more extreme?

Thank you for your suggestions guys.

The 100mm gauge is scaled to 15 bar (217psi) which is fine for my needs but also has another scale marked for degrees C - so I'm not really sure of the original intended use but it seems to be pressure/temp related.

The thread doesn't seem to have any taper but it does have a 8.5mm smooth lead-in to it that is about 4mm deep. There is no seal fitted but the threaded part of the connector runs into a large flat (hex) piece that is 21.4mm across flats and this could be used as one face of any seal. I will see if I have a 7/16th UNF tap to compare it with.

If it's a standard sized tap that I can identify, then I should be able to make a usable fitting to mate with (and seal) it. I wasn't sure if I was missing something a bit more obvious, as there doesn't seem to be too much correlation (in pipe threads) between the actual major thread diameter and the nominal size.

Regards,

IanT.

hi ian,

i would suggest you take the gauge apart and re-machine the gauge thread to a standard size. shove some bar up the hole and add loctite or shellac and then shove it in the 3 - jaw. most commercial pressure gauges have very odd threads as mentioned and quite a lot of meat on the body to enable easy reducing and re threading. alternatively buy another gauge with a standard parallel thread. they arent difficult to come by. if you are using same as a test rig it will need checking against a proper calibrated gauge anyway.

chers,

julian

Hi Julian,

If I don't have a tap to match this thread (or cannot purchase one) then I think that is what I will have to do - assuming I can disassemble the gauge. As you say there is enough meat on the connector to do this and I'm sure I can devise a custom sealing method (O-ring or similar) for my 'adaptor'.

I'm not too concerned about calibration as it's purely for my own use at this time and I simply want to be able to monitor the ability of boilers/fittings to 'hold' a pressure for a short while - and I'm sure the gauge will be good enough for this purpose. Of course, if I get the chance I will check my gauge against one of the (Gauge 3) Society's calibrated test rigs.

Thanks for everyone's input.

Regards,

IanT