Bourdon Tubes under suction

[JonL]

I've fitted my Steam Chest pressure gauge by Installing a second smaller union in the Snifting valve (which in the LBSC Britannia is fitted at the 6 o'clock position inside the smokebox just inside the door). From this union I have then run 1/16 pipe to a 0-100 psi gauge in the cab (working pressure 90psi). 

My only concern is that about once every 6 or 7 laps of our track I shut the regulator and the snifting valve doesn't open, generating a suction in the header. Usually I notice straight away as the wheels try to lock and crack the regulator again and it works the next time, or I don't shut it completely. But sometimes there is a vacuum there and I am now wondering how likely that is to damage the bourdon tube? 

Or am I doing my usual trick and worrying about nothing?

[chrisb]

Given that 30" hg is a vacuum of 14.7 psi. I would doubt it will affect the 100 psi Bourdon tube. In industrial use we have bourbon tube gauges that read -1 to 3 bar.

[jackrae]

It really depends on the cross-section of your bourdon tube. If the tube has been rolled quite flat then it's cetrainly possible it will collapse under vacuum and hence lose it's pressure calibration. If you cannot resolve your snifting valve problem and you have concerns about your pressure guage then I'd suggest performing a vacuum test on it followed by a pressure calibration to verify, either way, whether it can survive how you're treating it.

[chris vine]

Hi Jon,

Luckily, the steam chest pressure gauge is not even slightly safety oriented, so if it does get damaged it doesn't matter more than being annoying.
To test whether it has been damaged, you only have to look at the reading compared to the boiler pressure when the loco is stopped, in mid-gear, brakes on and regulator wide open. It should read about the same.

Enjoy,
Chris.

[JonL]

Thank you all