Anything between angled brackets has probably been interpreted as some form of HTML and disappeared into a forum black hole.

Test – putting the words “black hole” between angled brackets here <black hole>

Rob

Edit – seems ok in the body of a post

Ahh an inclined manometer. Used to measure low pressure differentials. Many used special low density fluids often kerosine based.

Robert.

I think it’s a DP gauge normally used to check filter banks in building heating and venting systems.

High DP means time to change the filter. Some ‘roll’ type filters used a HP to trigger a motor to wind the filter onto a new section.

Michael

A modern version
https://www.poddymeter.co.uk/products/precision-inclined-manometers/

EDIT This is interesting but too old for your manometer

https://dn790006.ca.archive.org/0/items/illustrateddescr00caserich/illustrateddescr00caserich.pdf

On 25 February 2026 at 20:35 Michael Gilligan Said:

I soaked the label off the bottle, and have the text [with only the single most important word being illegible ! ]

.

MichaelG.

That says “FLUID”

Rob

Hi Michael,

It’s an incline manometer. I used to use one to measure the pressure drop across a pitot tube when measuring the gas velocities during stack testing. It’s inclined because the pressures are small and the incline expands the scale so it’s easier to read.

The liquid had a density of I think 0.86 which also lengthened the scale. The density needs to be correct for the scale reading to be accurate. When the first digital manometers first came out they were very expensive but I was keen to get one. I guess that tells you what I thought of the incline manometers.

Regards,

Alan C.

The Manometer is an accurate differential pressure gauge but accuracy depends on the density of the fluid. We used to change the fluid in the ones in an aircraft instrument shop every year. Density can change due to evaporation of volatile components. Inclined manometers normally use low density fluids. Why increase sensitivity by inclining and then reduce it with a high density fluid? High or low is relative to water (1).

The manometer can be calibrated directly in pressur (inches of water, PSI, millibar, whatever)  or if intended for velocity measurement or similar it may have custom calibration for a specific transducer like a pitot-static probe, venturi etc.

Robert.

It is unlikey that the unit was supplied with a bottle of “fluid” if it was intended for use with water.

A manual almost https://ehive.com/collections/6274/objects/633426/instruction-leaflet-of-draught-guage

Info but paywalled Journal of Scientific Instruments, Volume 29, Number 2

Robert.