Bourdon Tube Pressure Gauge

A bourdon tube gauge is the instrument most people picture when they hear the word gauge: a round dial, a needle, and a threaded connection underneath. It measures by mechanical movement rather than by electronics, it needs no supply, and it is read at the tapping point. This range covers a 60 mm dial in 2.5 class and a 100 mm dial in 1.6 class, with brass connections on an iron case, spanning vacuum to 40 MPa on the smaller dial and offered across a wider range list on the larger one.

The two sizes are not the same instrument at different scales. They carry different accuracy classes and different connection threads, so which one suits a job is decided by the reading that has to be trusted and by the port that already exists, in that order.

How a bourdon tube gauge works

The sensing element is a curved tube with an oval cross section, closed at one end and connected to the pressure at the other. Pressure inside the tube tries to straighten the curve, and the free end moves. That movement is small, so a linkage and a sector gear turn it into the rotation of the pointer across the dial.

Two consequences follow. The first is that the gauge is entirely mechanical, so it holds its reading with no power and survives conditions that would upset electronics, but it also cannot be logged or transmitted without a separate transmitter. The second is that the accuracy class is a statement about the whole assembly, not just the tube: the linkage, the gears and the way the pointer is mounted all contribute, which is why a larger dial can be built to a finer class than a smaller one using the same principle.

The dial is graduated in MPa. Range lists step in roughly the 1, 1.6, 2.5 progression that gauge scales use worldwide, which means a working pressure can usually be placed in the upper part of a standard dial rather than needing a special range.

How to select the right gauge

Fix the range from the normal working pressure, not the peak. The accuracy class is a proportion of full scale, so a gauge reading a small fraction of its scale reports a larger absolute error than the same gauge reading near the top of it. Choosing a range that puts the working pressure in the upper half of the dial is the single decision that most improves what the reading is worth, and the step sizes in the standard list make that possible without going to a special range.

Then decide between pressure, vacuum and compound, which is fixed by what the process does rather than by its magnitude. A line that only ever sits above atmosphere takes a positive-pressure dial; one that only reads below atmosphere takes the vacuum form; one that crosses zero in normal operation takes a compound dial, and a positive-pressure gauge will not do in its place.

Then settle the dial size and the connection together, because they move as a pair: 60 mm comes with M14×1.5 and 100 mm with M20×1.5. Where an existing port has to be matched, the thread decides the size, and where the reading has to be trusted to a stated proportion of full scale, the class decides it, and the two usually agree. G1/2 and ZG1/2 are available to order where the port calls for them.

Finally, decide the case. The standard case is iron with a sprayed finish, and stainless steel case versions are named as Y-60BF and Y-100BF for installations where the surroundings rather than the medium are the problem. The material does not give those versions their own class, thread or range list, so a stainless case requirement should be confirmed at the enquiry rather than assumed from the standard data.

Prohibited services

These gauges are not suitable for oxygen or for other oxidising and combustible gases. The movement and the bourdon tube carry lubricating grease and have not been degreased, and the supplied material states expressly that the gauges must not be used on oxygen, liquid oxygen, hydrogen, acetylene, propane or high-purity gases of that kind. It cites GB/T 1226-2017, under which an oxygen system must use a gauge intended for oxygen service, and notes that grease meeting pressurised oxygen can ignite.

An oxygen duty therefore needs a gauge marked for it. The same applies in the other direction to any medium that attacks brass, since the connection is brass and the material does not list the other wetted parts. For those duties the usual answer is a separating diaphragm or a chemical seal ahead of the gauge, which puts a compatible diaphragm between the medium and the instrument and leaves the gauge measuring a clean fill fluid.

Measurement limitations and selection considerations

The media stated for this range are dry compressed air, mains water and ordinary neutral non-corrosive fluids, within an ambient temperature of -40 to 70°C. Nothing beyond that is documented, so process temperature, vibration and the sealing of the case are all questions to raise rather than figures to assume.

A mechanical gauge is also the wrong instrument where the value has to travel. If the pressure has to reach a control room, a recorder or a PLC, a transmitter produces the signal and a gauge alongside it becomes a local check rather than the measurement. Where the requirement is a contact that changes state rather than a reading, a pressure switch does that job without a dial.

The thread warning in the material is worth repeating at the point of installation: parallel threads such as G seal against a gasket or tape on a flat face, tapered threads such as R and NPT seal on the thread form itself, and the two families carry the same nominal sizes without interchanging. Fitting one where the other belongs either leaks or damages the port, and the sizes are close enough for the mistake to be made by eye.

Ordering

The range list, the dial size and the thread together define the gauge, and the material also lists customisation of the dial plate, the range and the case material. A quotation needs the working pressure, whether the reading crosses zero, the port thread already present, the dial size if the class matters, and whether the location calls for a stainless case.