A clearance issue water gauge

[runner42]

I have had a hazardous few days due to two water gauge glass blowouts in a the shed. The first resulted in the gauge glass developing a small crack after some considerable time and usage which was alarming but resulted it a non catastrophic stream of steam lasting several minutes until the boiler pressure dropped to almost zero. So I replaced the gauge glass which previously was straightforward, but this occasion I had diffulculty in placing the gauge glass between the top and bottom positions and when doing up the securing nuts the glass fractured. This was due to the fact that using a 4mm dia gauge glass in a 4mm dia securing nut caused the nuts to bear only slightly on the gauge glass and cause the fracture. I should have been alerted to this lack of clearance between the gauge glass and securing nuts and correct it before attempting another glass replacement. The third attempt was done with some extra care and I decided not to do the securing nuts too tight. However under pressure the gauge glass leaked and and required a little more tightening, but a soon as I turned the bottom nut a quarter turn the glass blew out and the results didn't need further description. You don't realise how much energy is stored in an 80 psi boiler until its provided with an immediate open pathway. So the 4mm gauge glass needs a slight clearance between it and the hole in the securing nut, what is recommended. 

I have fitted a 3/16" dia gauge glass in lieu and this works perfectly, but the red striped gauge glass that I prefer is only available in 4mm dia.

Brian

[andyhigham]

I am struggling to make sense of this. Is there a typo?
If the 4mm glass is too tight in the nuts, 3/16" will be even tighter! 3/16" = 4.76mm

[jordanleeds]

Any slight misalignment will crack the glass make up a alignment bar and check this too

[runner42]

Sept 23, 2023 19:43:41 GMT jordanleeds said:

Any slight misalignment will crack the glass make up a alignment bar and check this too

[blr]

I made up an alignment bar and it looked OK, alignment isn't the issue it is putting a 4mm glass tube into a 4mm hole, without clearance tightening the nuts is putting pressure on a tightly held glass tube. Result cracking. DY didn't specify the size of the hole it was drawn as captivating the glass tube. With water gauge construction two screw in items cannot achieve alignment perfectly that's why some clearance is required to enable the glass tube to float slightly. What this clearance should be is the question.

I used a smaller sized glass tube which works fine, I incorrectly assumed it was 3/16" dia without measuring it. It is smaller than 4mm.

Brian

[chrisb]

5/32” is the closest imperial to 4mm - 3.97mm

[millman]

I make all my own water gauges and give 20 thou clearance in the fittings and 30 thou in the nuts. I use 60 thou section silicon o rings to seal the glass and have never had any problems. I always check alignment with a steel bar, usually a drill shank, if I cannot get a good alignment I try another fitting, I usually make three or four sets of fittings at a time, while you are set up for doing one it doesn’t take much longer to do a few more, that’s my philosophy anyway, others will (probably) disagree.

[SteveW]

Runner42,

Not long awake but a thought occurred ... is your preferred 4mm gauge glass with red line actually a bit oval?

[jma1009]

Hi Brian,

Schellbach gauge glass is rarely perfectly round. It is also more brittle than ordinary pyrex gauge glass. Neither can be guaranteed to be of a precise diameter (an old Reeves catalogue gives a "nominal" diameter with quite a range possible for the "nominal" size). The union nuts and register for the bottom of the glass, and the hole through the top of the glass should be made/altered to suit the glass to hand, or choose a piece of glass to suit the water gauge. Say 5 thou around the glass should be fine.

Cheers,
Julian

[runner42]

Sept 24, 2023 9:39:40 GMT jma1009 said:

Hi Brian,

Schellbach gauge glass is rarely perfectly round. It is also more brittle than ordinary pyrex gauge glass. Neither can be guaranteed to be of a precise diameter (an old Reeves catalogue gives a "nominal" diameter with quite a range possible for the "nominal" size). The union nuts and register for the bottom of the glass, and the hole through the top of the glass should be made/altered to suit the glass to hand, or choose a piece of glass to suit the water gauge. Say 5 thou around the glass should be fine.

Cheers,
Julian

Thanks Julian,

your vast experience gained over the years puts you in a position to be aware of all the potential pitfalls in supplied products and this is a case in point. Your emphasis on reworking the top hole to provide the required clearance is logical, so when I receive the new supply I shall measure its diameter and provide 5 thou clearance between the top hole and the glass.

Brian

[ettingtonliam]

I'm sure the late (and latterly much derided) LBSC will have had words of wisdom on the subject of clearances in water gauges, I just can't recall them, other than not to make the clearance hole in the nut so large that the rubber seal squeezed out.

[chris vine]

One other thought is on the ends of the tube.
I wonder if the bottom end is touching on the step in the lower fitting. When you tighten the nut it could press the cut end into the step and crack it then.

I always flame polish the ends in the blowtorch, so they are rounded and smooth. This makes the ends much less susceptible to damage and only takes a few seconds to do. Just turn the tube in your fingers while heating the end to red heat. You can see it melt slightly and heal up any rough parts. Just don't touch it too soon (or plunge into water to cool it)...

All best
Chris.

[uuu]

Sept 25, 2023 10:17:29 GMT ettingtonliam said:

I'm sure the late (and latterly much derided) LBSC will have had words of wisdom on the subject of clearances in water gauges, I just can't recall them, other than not to make the clearance hole in the nut so large that the rubber seal squeezed out.

I've looked up in "Shop Shed and Road". LBSC's fittings are drilled No.12 for 3/16" glass. So that's a nominal 0.1890" hole for a 0.1875" tube. So hardly any clearance at all. Hmmm. That's for the top fitting. The bottom is described as having a 3/16" hole drilled part way in - so no clearance at all. The clamping nuts "need no special description", so the holes in them are not dimensioned.

Wilf

[jma1009]

Sept 25, 2023 13:48:06 GMT uuu said:

Sept 25, 2023 10:17:29 GMT ettingtonliam said:

I'm sure the late (and latterly much derided) LBSC will have had words of wisdom on the subject of clearances in water gauges, I just can't recall them, other than not to make the clearance hole in the nut so large that the rubber seal squeezed out.

I've looked up in "Shop Shed and Road". LBSC's fittings are drilled No.12 for 3/16" glass. So that's a nominal 0.1890" hole for a 0.1875" tube. So hardly any clearance at all. Hmmm. That's for the top fitting. The bottom is described as having a 3/16" hole drilled part way in - so no clearance at all. The clamping nuts "need no special description", so the holes in them are not dimensioned.

Wilf

Except there is no such thing as 3/16” dia gauge glass! Approximately, yes! But no more than that as is clear from an old Reeves catalogue because they could not guarantee the precise diameter.

I’ve got part of Arthur Grimmett’s left over stock of gauge glass from when he made water gauges for Reeves and Kennions (the other part went to the IWMES). For Stepney I wanted some just under 7/32” dia to fit the clearance size for the 1/4” x ME 40 TPI for the cap in the top fitting. I ‘miked up’ everything that looked like 7/32” dia and found only one piece of glass that fitted the bill. Everything else was either just a bit too big or a bit too small.

I would not be surprised if LBSC had quite a stock of ‘gauge glass’ himself. It isn’t gauge glass as such unlike full-size. It is laboratory Pyrex glass usually. Where exact tolerances didn’t matter. From memory Arthur obtained his stuff from dealers in Southampton off St Mary street. I don’t know where he got the Schellbach glass from.

[ettingtonliam]

This is dancing around handbags! Yes, its generally agreed that gauge glass tube varies in diameter, but apart from Millman nobody else is saying what clearances they use! If the tube is so variable, then presumably its wise to err on the large size when drilling, to suit whatever tube is to hand in a few years time when the current piece has been used up.
I have made one or two water gauges but so long ago that I can't remember what I did, other than having been told to make the fittings a loose fit on the glass and rely on the rubber seal to prevent leakage.

[Jim]

Couldn't agree more.

Jim

[andyhigham]

Something else to think about.
The bottom fitting will usually have a shoulder/counterbore to stop the glass dropping down to block the hole. Before tightening the bottom nut, lift the glass slightly so the nut doesn't force the glass into the shoulder/counterbore

[JonL]

I hope its not detracting too much from the important parts of the question here to ask, does anyone have a good supplier for the types of gauge glass with the white or white and red line on the back? I took Julians comment about Schellbach glass but could only find a manufacturer of laboratory equipment. I assume they are the makers, but I can't seem to find a supplier.

Thank you

[nick952]

Any good?

www.livesteammodels.co.uk/shop/index.php?route=product/category&path=61

[dhamblin]

Also available from Polly Model Engineering (both red and blue, although the latter had lower availability).

Regards,

Dan