What a pressure gauge is
A pressure gauge indicates the pressure at the point where it is mounted. It has a dial, a pointer and a connection, it needs no supply, and it sends nothing to a control system. It answers the question in front of whoever is standing at the tapping point rather than the one in the control room, and for that job it remains the simplest and most robust instrument there is.
It is worth being precise about the boundaries, because a gauge is often specified where a different instrument belongs. A transmitter produces a continuous signal and is chosen when the value has to travel, be logged or be acted on. A pressure switch produces a contact at a set point and nothing else. A gauge produces a local indication - and where that indication is the whole requirement, everything an electronic instrument adds is cost, supply and another way to fail.
What decides a gauge is not its measuring principle, which is settled, but three figures that move together: the dial size, the accuracy class available at that size, and the range list. Choosing any one of them without the others is how a gauge ends up reading a small fraction of its own scale, which is the most common reason a reading is worth less than the buyer expected.
How a pressure gauge measures
Most industrial gauges work on the bourdon principle. A curved tube with an oval cross section straightens slightly as pressure is admitted, and a linkage and a sector gear turn that small movement into the rotation of the pointer across the dial. The element is a solid metal part, with the case itself as the only barrier between the pressure and the atmosphere, which is why a gauge tolerates heat, vibration and overpressure that would damage an electronic instrument, and equally why it cannot be corrected, compensated or adjusted in service.
The accuracy class describes the whole assembly as a proportion of full scale, not the tube alone. The gear train, the bearings, the mounting of the pointer and the printing of the dial all contribute, and the same principle can be built to a finer class on a larger dial because a given angular error covers more length of scale. That is why dial size and class are quoted together on a specification rather than offered as independent choices.
Two properties follow from being mechanical, and both are worth stating plainly. The first is that the reading exists only at the dial: nothing leaves the instrument, so a gauge cannot feed a recorder, a controller or a historian. The second is that a gauge has no reference of its own. It reads against the atmosphere at the dial unless it is built as a sealed or an absolute instrument, so its zero moves with the weather and with altitude, by an amount that is irrelevant for most duties and decisive for some.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Dial size and accuracy class | A small dial, a standard process dial, or a large dial | The two are not independent: a finer class is normally available only at the larger sizes, so choosing a size chooses the accuracy band. It also decides how far away the reading can be taken and how much room the tapping point needs. |
| Range and pressure reference | A positive-pressure span, a vacuum span, or a compound span crossing zero | What the dial can show at all. A positive-pressure gauge reads a line below atmosphere as an off-scale condition, and a vacuum gauge has no scale above zero, so the reference is fixed by the duty before the maximum figure is discussed. |
| Range placement against the working pressure | Where the normal working pressure sits on the dial | What the class is worth in practice. The class applies to full scale, so a gauge reading near the bottom of a wide span reports a larger absolute error at the point that matters than the same gauge reading near the top of a narrower one. |
| Process connection | Thread family, size and standard, and the material of the connection | Whether the gauge fits the port and whether it seals. Parallel and tapered threads carry the same nominal sizes and do not interchange, and the connection is usually the only wetted part the buyer thinks to specify. |
| Case, window and filling | Case material and finish, window material, and whether the case is liquid filled | Whether the gauge survives its surroundings rather than its medium. A filled case damps needle movement in a vibrating installation, and a case material chosen for a wet or corrosive environment has nothing to do with chemical compatibility of the wetted parts. |
How to choose a pressure gauge
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| Normal working pressure | It sets both the range and the portion of the dial that is actually going to be read, and therefore what the class delivers at the point of interest. | State the pressure the line usually holds, not only its maximum, and the spike or test pressure it may see. |
| Whether the reading crosses zero | A positive-pressure gauge and a vacuum gauge are different instruments, and only a compound gauge covers both in one dial. | Say whether the process ever sits below atmospheric in normal operation, including start-up and shutdown. |
| Thread already at the tapping point | It decides what will physically fit, and the connection is a part of the wetted boundary. | Give the thread type and size at the port, or send a photograph of it. Note the family as well as the size, since G and R are not interchangeable. |
| Ambient conditions at the gauge | Heat, vibration, direct sun, washdown and a corrosive atmosphere each change the case specification rather than the measurement. | Describe where the gauge will be mounted, including whether it faces outdoors or is hosed down. |
| What happens to the reading afterwards | A local indication and a signal that has to travel are different requirements, and confusing them leads to a gauge doing a transmitter job. | Say whether the value has to be logged, transmitted or acted on, or only read at the point. |
The nearest alternatives
These are the types an enquiry for a pressure gauge usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a pressure gauge |
| Digital pressure gauges | Shows the same local reading on a display rather than on a dial, and runs from a battery. Chosen where a numeric value is wanted instead of a position of a needle, or where the reading has to be resolved more finely than a dial allows. It carries the same limitation: nothing leaves the instrument. |
| Pressure transmitters | Produces a continuous electrical signal. Where the value has to travel to a controller, a recorder or a plant system, this is the instrument, and a gauge alongside it becomes a local check rather than the measurement. |
| Differential pressure gauges | Reads the difference between two ports rather than one pressure against atmosphere. Used across filters, strainers and flow elements, where the useful figure is the drop rather than the line pressure - and where a single-port gauge gives no useful answer at all. |
| Vacuum gauges | Scales from atmosphere downwards. Where the whole working range is below atmospheric, a dedicated vacuum instrument gives usable resolution where a compound dial has half its scale dedicated to a range the process never uses. |
| Pressure switches | Produces a contact at a set point and nothing else. Correct where the requirement is to act on a pressure rather than to read it, and where a visible reading is also wanted, the two instruments sit beside each other rather than one replacing the other. |
Where a gauge and a switch or a transmitter are both wanted at one tapping point, a manifold or a twin-connection block is usually tidier than two separate tappings.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- Normal working pressure, maximum pressure and any test or spike pressure
- Whether the reading crosses zero, or is entirely above or below atmospheric
- Dial size required, or the accuracy class the duty has to meet
- Thread type, size and standard at the tapping point
- Medium, and whether it is compatible with the wetted parts offered
- Ambient conditions: heat, vibration, weather and washdown
- Whether the process medium is one the instrument must not be used on
- Whether customisation of the dial face or the range is required
- Quantity, destination port and required delivery date
