A mechanical gauge is right when a person has to read pressure on the spot and no power is available. A transmitter is right when a controller has to read it. Everything else — dial size, accuracy class, case filling, diaphragm seal — follows from that first answer.
Who reads the number decides the instrument
A Bourdon tube gauge needs no supply, no wiring and no configuration. It reads at the pipe, keeps reading when the panel is dead, and ignores noise from a variable-speed drive.
A transmitter does the opposite: it turns pressure into a signal a control system can act on. On the WIKA range that is usually 4–20 mA, or 4–20 mA with HART data on the same pair; process transmitters are listed as 4–20 mA, 4–20 mA HART or bus signals. Where the sensor belongs to a machine rather than a plant, ask for IO-Link: the A-1200 carries it, with a 360° status LED, ranges from 0 … 0.4 to 0 … 1,000 bar and a switching time of 5 ms or less.
A third case is often missed: a local reading plus a switching action, with no controller between them. The PGS23.1x0 switchGAUGE covers it — 100 and 160 mm, class 1.0, ranges from 0 … 0.6 to 0 … 1,600 bar, up to four contacts, magnetic snap-action, reed, inductive for hazardous areas or electronic for direct PLC input.
| Mechanical gauge (232.50 / 233.50) | Transmitter (A-10 / S-20) | |
|---|---|---|
| Supply | None | 8 … 30 V DC, A-10 current output |
| Output | Pointer on a dial | 4 … 20 mA, 0 … 10 V, HART or IO-Link |
| Read at the pipe | Yes | Only with an add-on indicator |
| Read at the panel | No | Yes, through the PLC or DCS |
| Typical accuracy | Class 1.0 or 1.6 of span | 0.25 % or 0.5 % non-linearity (A-10) |
| Fits | Local indication, utilities, field checks | Trends, interlocks, recorded values |
Dial size and mounting
WIKA builds the 232.50 and 233.50 in three sizes, and the size is not cosmetic: it sets the accuracy class, the range ceiling and how far away the dial can be read.
| Size | Accuracy class | Range ceiling | Window | Read from |
|---|---|---|---|---|
| 63 mm (2½”) | 1.6 | 1,000 bar | Polycarbonate | Around 1 m |
| 100 mm (4”) | 1.0 | 1,000 bar | Laminated safety glass | 2 to 3 m |
| 160 mm (6”) | 1.0 | 1,600 bar | Laminated safety glass | 4 to 5 m |
Those distances are a practical guide, not a standard. What matters is graduation spacing: if the operator has to lean in to separate two adjacent marks, the dial is too small.
Mounting follows the pipework. Lower mount, or radial, puts the connection at the bottom of the case so the gauge sits above the tapping. Lower back and centre back mounts sit on the back of the case for panel work or a bracket; on the 232.50 the centre back mount comes on the 63 mm size only. A front flange clamps through a panel cut-out and a U-bolt holds the case to a stanchion.
A filled gauge needs one extra rule: mount it upright and vent the case afterwards. The 233.50 has a fill plug, and on ranges up to 300 psi a compensating valve, so trapped air can escape and the filling can expand without loading the window.
Accuracy class and range, matched to the job
Gauges are graded under EN 837-1 in classes given as a percentage of span. The classes met in process work are 0.5, 1.0, 1.6 and 2.5, and WIKA applies 1.6 to the 63 mm 232.50 and 1.0 to the larger versions.
Two mistakes are common. The first is buying accuracy the application cannot use: a class 1.0 gauge read from three metres, at 40 °C, on a line that surges when a pump starts, delivers nothing like 1 % of span, because temperature error alone is ±0.4 % of span per 10 K. The second is sizing the range at the working pressure. Keep the normal operating point between half and three-quarters of span, so there is room above it and the reading is not squeezed into the first marks.
The data sheet also caps how hard the instrument may be worked, and the limit depends on size:
| Size | Steady | Fluctuating | Short time |
|---|---|---|---|
| 63 mm | ¾ of span | ⅔ of span | Full span |
| 100 and 160 mm | Full span | 0.9 × span | 1.3 × span |
A transmitter needs the same care. The A-10 quotes non-linearity of 0.25 % or 0.5 % of span, which is the linearity term alone — temperature effect, long-term drift and supply variation sit outside it. The S-20 quotes long-term stability better than 0.1 %, comes in three accuracy classes, and offers adjustment temperatures of +4, +40, +60 or +80 °C, which is the version to specify when the panel runs hot.
Liquid filling for pulsation and vibration
The 233.50 is the 232.50 with a filled case, glycerine as standard and silicone oil where temperature or application asks for it. The filling does one job well: it damps the pointer. On a pump discharge the needle of a dry gauge blurs, and the movement takes that oscillation for years until sector and pinion wear.
Filling is not a substitute for a bad installation. Where pulsation is severe the data sheet calls for a restrictor as well, and the gauge belongs on a bracket rather than a vibrating pipe run.
It also costs something: the medium ceiling falls from +200 °C dry to +100 °C filled and ambient narrows to -20 … +60 °C with glycerine. For a clean, steady line at ambient temperature, dry is the better instrument.
Diaphragm seals: when the medium is the problem
A seal separates the instrument from the medium with a diaphragm and transmits pressure through a fill fluid. WIKA seals are rated from -90 to +400 °C and intended for aggressive, corrosive, heterogeneous, abrasive, high-viscosity or toxic media. The threaded version, model 990.10, takes up to 25 MPa and its lower body can be changed without dismantling the seal system, which helps when the connection has to change but the instrument stays.
Reach for a seal when:
- The medium crystallises, polymerises or sets as it cools, so a narrow bore would block.
- The medium is viscous, carries solids or fibres, or would sit in the socket and read nothing.
- 316L is not enough, so the wetted surface must be something else.
- The process runs hotter than the instrument allows and a cooling element or capillary is needed.
- The instrument must come off for cleaning or calibration without breaking into the process.
It is a measurement decision as well as a barrier: the seal adds a diaphragm, a fill fluid and often a capillary, so response slows, temperature error grows with fill volume, and the assembly has to be filled and zeroed as a system, with an elevated or suppressed zero where the installation calls for it. Bought as loose parts and assembled on site, it is a source of error that never quite goes away.
Hygienic connections and where they end
Where the wetted surface must be cleanable and sterilisable, the connection changes shape. Clamp ferrules to DIN 32676, sterile screwed connections and flush diaphragms replace the threaded socket, so no crevice holds product and cleaning in place (CIP) and sterilisation in place (SIP) reach the surface. Pharma, food and beverage buyers ask for seals whose design meets EHEDG or 3-A expectations, and that requirement belongs in the enquiry because it fixes geometry, surface finish and diaphragm. The same logic drives the flush transmitter: the S-11 is WIKA’s flush sensor for viscous and solids-containing media, where a recessed port would clog.
The boundary is easy to state: hygiene geometry is for surfaces cleaned or sterilised in place. A clean, dry compressed-air header does not need a clamp ferrule. Match the connection to the cleaning regime, not the industry label.
The four questions that fix the specification
Work through these in order and the part number falls out. Out of sequence they produce a gauge reading 1.5 bar on a 25 bar scale.
- Range. What is normal operating pressure, what is the highest it can reach, and what happens on a blocked outlet or a dead-headed pump? Set normal operation at half to three-quarters of span, then check it against the steady and fluctuating limits above.
- Process connection. Thread type and size, and where it sits. G ½ B and G ¼ A, ¼ and ½ NPT are the common ones; the 232.50 and PGS23.1x0 use G ½ B with 22 mm flats, the A-10 uses G ¼ A to DIN EN ISO 1179-2 or ¼ NPT. Then lower, lower back, centre back, flange or clamp.
- Material and seal. What the medium does to 316L, and whether a seal, another diaphragm material or a coating is needed. Check medium temperature against the dry and filled limits before choosing the filling.
- Output and accuracy. Does a person read it, or a controller? If a controller, which signal — 4–20 mA, HART, IO-Link — and what accuracy the loop needs, remembering that non-linearity, temperature effect and long-term drift add up.
Putting the enquiry together
Send the four answers, plus the medium, its temperature, the ambient conditions and the range in the units your maintenance team uses. That is enough to specify a 232.50 or 233.50, an A-10, S-20 or S-11, a PGS23.1x0 or a seal to suit. Our WIKA brand page lists the families we supply, the instrument category covers the rest of the pressure and temperature range, and the full product index covers the other categories. To set a check interval for an existing transmitter, the notes on calibrating a pressure transmitter in the field work from its drift figure. Send the details through the enquiry form and we will come back with the model, connection and options that fit.
Figures here come from WIKA data sheets PM 02.02 (models 232.50 and 233.50), PV 22.02 (PGS23.1x0), PE 81.60 (A-10) and PE 81.90 (A-1200), and from the WIKA product pages for the S-20, S-11 and the diaphragm seal range. Specifications change without notice and options vary by size, so confirm range, connection, materials and certification against the current data sheet before ordering. This article is technical guidance, not a substitute for the data sheet and not a statement of availability for any individual part number. FINACO supplies WIKA instrumentation through the manufacturer and established channels; we are an independent supplier, not the manufacturer.