Mystery Pressure Gauge

Might find that the person who engraved the dial did not have, or bother to load, the lower case characters? If used for Life Jacket testing it could be a locally produced item by RAF techies in accordance with a SEM?

Fair enough. Have you ever seen it written M/M in this application before – perhaps before metric units were more familiar?

I’ve got a mk9a which would normally indicate 240mph in one & three quarters needle rotations, but gives around 100 mph on the first rotation …… it’s only marked in the range required and maybe not done by Munro

Reason for new face in mm- Because the spec to which the life jackets need the periodic recertification calls for “pressurise to 80mm of water +/- 5, and this reading must not drop more 10mm in 30minutes”. Doing this with a water Mano can be just painful, i.e. spills, just which part of the miniscus do you take the measurements from……, dead easy with an old ASI calibrated in mm of water.

I agree, I’m not really convinced that M/M is millimeters either!
Andy

To answer my own question. 100mm H20 / 0.14psi pressure differential is only about 90 mph. So it’s either not millimetres, or it’s not a Mk9 ASI.

Why did whoever replaced the face write mm as M/M? It is a basic error for someone involved in instrument making – and it is repeated on the back. Still not convinced it’s not something per something.

What is the range of an ASI in terms of differential pressure?

It’s a Mk9 pattern Air Speed Indicator (ASI) which has its MPH or Knots face replaced by one in mm of water. The mk9 ASI was produced between the early 1930’s and mid 1950 in large quantities by several companies including Munro. When Munro made this they were making aviation instruments, there current activities has nothing to do with its original intended use.

100mm of water = 0.14psi which is way too low to be connected with aircraft cabin pressurisation.

I’ve seen former ASI’s used in a safety equipment section for leak checking life jackets.

I think Munro historically made pneumatic instruments for aircraft. Concentrating on meteorological equipment will be a modern phenomenon as aircraft use fewer ‘suck and blow’ instruments.
Andy

That being the case an instrument from a Met Hastings?

According to their website Munro manufacture meteorological equipment. So surely that is what this unit was used for. See below…

http://munroinstruments.com
We have over 150 years’ experience in the manufacture and supply of meteorological equipment. Our clients include government authorities, agriculturalists, defence ministries, airports, seaports and highway authorities. To view our meteorological & environmental product range, please click on the icons below or download our brochure here.

Agreed, another possibility.

Here is a thought:

This could have been an early instrument to measure cabin pressure or cabin air conditioning system.
Basically an aircraft grade environmental system.
If not WW2 then early post war air liners.

Then there is the definite use of upper-case ‘M’s, both on the face and on the note on the reverse. It is unlikely that both the instrument maker and whoever wrote the inscription would be unaware of the correct convention for writing ‘mm’, even if it was a foreign system to them.

The final reason I am dubious about M/M meaning millimetres is the lack of fine calibration on the gauge. If you needed to change from inches to mm to get finer definition, (and even go to the trouble of changing the stated reference medium to get a pleasing range) you’d gradate the scale more finely to make use of these smaller units. As it is, each step is 5 ‘whatever-they-ares’. The the normally chosen inches Hg is naturally finer-gradated than this, in easy-to-read single-unit steps:

[ATTACH=CONFIG]257503[/ATTACH]

What if we took the gauge literally and took it to mean Metres (or Mass or, if the gauge really was scientifically correct, Molar mass) per Metre (or Mass or Molar mass) of Water, ranging from 5 to 100, ie the measured thing might be up to a hundred times that of water (300 times is overload) After all, no-where does it say ‘percentage’, and there’s a very clear ‘/’ on both sides.

Just throwing that ‘out there’ as another option

Edit: Keeping it literal, the only use of upper-case M as a standard unit seems to be ‘Molar’ as in Moles per Litre. Water’s is roughly 55, at 25 degrees

Well, we are and we aren’t. Ultimately we are still measuring pressure (and the VC-10 trials were focused on pressure, not loading) whether in lbs per square inch or how much water or mercury that can push. It is conventional to measure manifold pressure with a gauge, but the mystery gauge is not conventional in the application as it is marked in m/mH2O, where mmHg is the closest ‘conventional’ measure used in this context, while inches Hg seems far more common (in the US, UK and subservient dominions 😉 . Not saying you’re wrong, I am only a dabbler!

Regarding bearings, the specs for non-engine bearings for late-1930’s British aircraft might include a decimal of an inch, especially where imported bearings were initially specified (and most of them were, initially) but I haven’t seen mm. Thinking about it now it might be interesting to convert these inch decimals (I am thinking specifically of some ‘mystery’ SKF and Hoffman bearings in the Whirlwind parts catalogue) into mm, see if I get whole numbers.. OK, maybe not worth-a-TV-documentary-interesting, but..

I imagine the mix is down to the issue of shaft diameter – surely there wouldn’t be a knock-on effect of forced metrication of other components just because the bearings are foreign spec? Would it in fact be likely that it’s the balls, the tricky bit done by cunning Swedes, and the subsequent O/D that might be metric?

Beermat, we are talking different applications of the term pressure, ie PSI (wing loading) as distinct from inches (or mm) of mercury or water displacement in a tube, applied to manifold pressure in an engine. The mystery gauge is quite conventional in that application. Of the 23 roller bearings in the 1930Â’s RR Kestrel, and probably Peregrine, half the dimensions are in croissant, typically half imperial, half metric in the one bearing. All the bearings are Specials, made expressly by Hoffmanns for application in that engine, so they are RR specified. Why? Maybe to force you to buy OEM replacement bearings only…maybe they figured that these wear components were so performance critical, they could prevent both Arthur Daly and Dodgy Pierre supplying aftermarket bearings, then dealing with warranty claims that eventually reveal a basis in cheap, substandard bearings.

I did not know that about RR engine bearings being metric. Really very surprising!

I have found a report on Super VC-10 flight tests from 1972 – http://naca.central.cranfield.ac.uk/reports/arc/rm/3707.pdf – that has all the pressures over the wings in pounds per square inch, and which includes the line “The majority of gauges had a range of + 1 p.s.i, to – 5 p.s.i, with one revolution of the pointer being equivalent to 1 p.s.i. However, for the leading edge and the 0.001 c pressure holes at each station the pressures were read on + 3 p.s.i, to – 2 p.s.i, gauges having 5 p.s.i, per revolution” and the footnote “*Since the experimental work was carried out using precise measurements in inches, it has been thought better to retain these rather than convert to SI units”.

AndyY, the reason I surmised that the dial had been overpainted is that there is paint in the countersink & the screw slots, which I would not expect to see on a pre manufactured dial.
As for static use, say in a wind tunnel, then surely U tubes would be a better device, not very practical for use in flight testing.

I would believe that the British would not condescend to use the metric system of the perfidious continentals except half the dimensions in RR engine bearings seem to be metric. At the end of the day it has more practical flexibility than 64ths…I am familiar with inches or mm on the ‘water gaugeÂ’ from the design of street sweeper ‘vacuumÂ’ systems, which are ‘old schoolÂ’ British engineering in the antipodes. Mr Dyson probably has a water gauge next to his coffee pod machine.

Trouble is, if the British using the imperial system wanted to more finely calibrate something more usually measured in inches, they would normally resort to fractions of an inch.

Having said that, there are some 1950’s technical reports from Farnborough that use mm of Mercury. Just can’t find anything test-related using mm H2O. Pressure differentials are most usually expressed as a figure devoid of units.

Isn’t this just a standard manifold pressure gauge, except precise for precise calibration? Ie 25mm gives you 25 points of measurement, rather than 1 inch giving you 1 point of measurement. So the gauge would be logical in a calibration device for standard supercharger gauges, or fitted to a new engine installation requiring precise feedback…