At a Glance. Mechanical pressure measurement is the cheapest reliable reading in a plant, and it fails for predictable reasons. A pressure gauge such as WIKA A2G-mini or the 100.10 / 100.12 family is a Bourdon-tube instrument: the medium enters the tube directly. A diaphragm seal such as WIKA 981.10 puts a flexible diaphragm between the medium and the instrument and transmits the pressure through a filled capillary. The question is which of those two arrangements the process actually demands. Both sit inside our pressure measurement range, and the full range is on the WIKA page.
1. The decision is made by the medium
| Process condition | Plain gauge | Diaphragm seal |
|---|---|---|
| Clean, non-corrosive gas or water | Suitable | Not required |
| Corrosive medium (acids, chlorides, ammonia) | Short life, corrosion risk | Required |
| High viscosity, sludge or pulp | Blocked pressure port | Required |
| Crystallising or polymerising medium | Port plugs solid | Required |
| Media temperature above the instrument limit | Requires a siphon or cooler | Handled with a filled system |
| Hygienic process with CIP / SIP cleaning | Not acceptable | Required, flush design |
| Highly toxic or hazardous medium | Risks release at the instrument | Required, sealed barrier |
| Frequent zero re-calibration | Expected on aggressive service | Wetted parts can be cleaned or replaced |
2. What the seal changes in practice
A diaphragm seal is a system, not a single item, and each element affects the reading:
- Diaphragm material must be compatible with the medium. Stainless steel covers most cases; aggressive media may call for a special alloy or a PTFE-lined diaphragm.
- Diaphragm diameter sets the stiffness of the measuring system. A seal with a small diaphragm adds more temperature error than a large one, because the fill fluid expands against a stiffer element.
- Fill fluid transfers the pressure and is selected for the process temperature range. A standard fill is not appropriate for very low or very high temperatures.
- Capillary length allows the instrument to be mounted away from a hot or inaccessible point, but longer capillaries add temperature effects and slow the response.
- Process connection determines whether the seal mounts on a flange, a thread or a hygienic connection. Flange-mounted versions such as the 981.10 family are common where the medium must not be trapped in a threaded pocket.
3. Accuracy, temperature error and zero adjustment
A gauge with a seal is a filled system, and its total error budget is larger than the instrument accuracy alone. Two effects dominate:
- Temperature error of the fill fluid. As the process temperature changes, the fill expands or contracts and moves the indicated zero. Zero-adjustable instruments, or seals mounted with a compensating arrangement, keep this manageable.
- Elevation of the instrument. If the gauge sits above or below the seal, the fill column adds a constant offset. This is corrected at commissioning with the process at zero pressure — a step that is routinely skipped.
Specify the seal and the gauge as one system and state the process temperature at the seal, not at the instrument. Those are frequently different by a hundred degrees or more.
4. Where a plain gauge is the right answer
Most readings in a plant do not need a seal, and adding one costs money and introduces its own error sources. Choose a plain instrument where the medium is clean, non-aggressive and within the instrument temperature range, where the port cannot block, and where the cost of periodic replacement is acceptable. Compact instruments such as A2G-mini suit exactly this case: a simple, small gauge for a local indication where a full-size instrument will not fit.
5. Specification checklist
- Medium, concentration and temperature at the connection point
- Pressure range, and whether vacuum or a compound range is required
- Required accuracy class and whether a liquid-filled case is needed for vibration
- Diaphragm and wetted-part material, and whether a hygiene standard applies
- Process connection type, size and standard
- Mounting orientation and the availability of a support or capillary bracket
Send the existing gauge model, its scale range and a photo of the installed assembly, and we will confirm whether a seal is genuinely needed or whether a different instrument would solve the problem at lower cost.