A Form 3 pressure gauge is a back-connected, panel-mounted Bourdon-tube pressure gauge with a front flange ring that secures the case from behind a control panel. Eugène Bourdon patented the underlying curved-tube principle in France in 1849, and the Form 3 case style was later codified under EN 837-1 as the standard back-flange panel layout. The Bourdon tube uncoils as pressure rises, driving a sector-and-pinion movement that rotates a pointer across a calibrated dial. The result is a clean, flush-fitting indicator visible across a control room — used on hydraulic power packs, steam skids, and water-treatment manifolds rated up to 1,000 bar.
Pressure Gauge Form 3 Interactive Calculator
Vary gauge pressure, full scale, Bourdon tip travel, sweep, and accuracy class to see the pointer angle, tube motion, and allowable error.
Equation Used
The calculator treats the Bourdon tube and sector-pinion movement as a linear calibration: pressure fraction of full scale gives the same fraction of full-scale tip travel and pointer sweep. The accuracy band is the EN-style class percentage multiplied by full-scale pressure.
- Bourdon tube tip motion is linear with pressure over the calibrated span.
- Sector-and-pinion movement converts full-scale tip travel to the specified pointer sweep.
- Accuracy class is percent of full-scale pressure per EN 837-1 style rating.
- Overrange indication is shown numerically but the model does not include permanent tube set.
The Pressure Gauge (form 3) in Action
The Form 3 designation refers to the case geometry, not the sensing element. The pressure connection enters through the back of the case, and a triangular or round front flange (the "front ring") clamps the gauge into a panel cut-out from the operator side. Inside, a C-shaped Bourdon tube — typically phosphor bronze for ranges below 60 bar, 316 stainless for higher ranges or aggressive media — does the actual sensing. As pressure pushes the inside of the flattened tube outward, the tube tries to straighten. That tip motion is tiny, often less than 5 mm at full scale, so a sector-and-pinion movement multiplies it into 270° of pointer travel.
The geometry of that linkage is where accuracy lives or dies. The link length between the tube tip and the sector arm must be set so the pointer hits zero and full scale exactly — adjustment is done by sliding the link pivot in a slot during calibration. If the pivot drifts, you get linearity errors that show up as a gauge reading correctly at zero and full scale but bowing 2-3% high in the middle of the range. EN 837-1 accuracy class 1.0 means ±1.0% of full scale — so a 0-100 bar gauge is allowed to read ±1 bar anywhere on the dial.
Things go wrong in predictable ways. Pulsation from a piston pump shakes the movement and you'll see the pointer hash-mark its way to an early death — that's why a snubber or glycerine-filled case is standard on pump-discharge installations. Overpressure beyond 130% of full scale takes a permanent set in the Bourdon tube, and the gauge will read low forever after. Hydrogen embrittlement on stainless tubes in refinery service causes slow zero drift. And if you panel-mount a back-entry gauge without supporting the process pipe, the entire pipe weight hangs off the 1/4 NPT fitting on the back of the case — that cracks brass sockets within months.
Key Components
- Bourdon tube: C-shaped flattened tube, typically 50-65 mm radius for a 100 mm dial. Phosphor bronze for ranges 0-60 bar, 316L stainless for higher pressures or corrosive media. The tube cross-section flattens to roughly 0.8 × 4 mm so internal pressure tries to round it out, generating tip motion.
- Front flange (mounting ring): Triangular or circular ring fixed to the front of the case, with 3 clearance holes for M4 or #8 panel screws. The flange sits proud of the panel face and clamps the case from the operator side, so the gauge can be removed without breaking the process connection if installed correctly.
- Sector and pinion movement: Brass or stainless gear train that converts 3-5 mm of Bourdon tube tip motion into 270° of pointer rotation. Backlash must be under 0.2° or you get hysteresis on the dial — most movements use a hairspring on the pinion shaft to load the gears one-sided and eliminate backlash.
- Back-entry pressure socket: 1/4 NPT, 1/2 NPT, or G1/4 male thread silver-brazed to the Bourdon tube. Located on the rear axial centreline of the case. Torque to 25-40 Nm with PTFE tape — never use the case as a wrench point, the socket-to-tube braze joint will fracture.
- Dial and pointer: Aluminium dial printed with the calibrated scale, accuracy class (1.0, 1.6, or 2.5), and units. Pointer is balanced aluminium or steel, friction-fit on the pinion shaft so it can be reset to zero with a pointer puller during recalibration.
- Case (often glycerine-filled): Stainless or painted-steel housing, 63 mm, 100 mm, or 160 mm dial size. Glycerine or silicone fill damps pointer flutter from pulsation and lubricates the movement — fill level should sit at 75-80% to leave room for thermal expansion.
Industries That Rely on the Pressure Gauge (form 3)
Form 3 gauges go wherever an operator needs to read pressure from the front of a panel without the gauge body sticking out into walkway space. They're the default on hydraulic power packs, pneumatic control panels, steam skids, water-treatment manifolds, and test benches. The back-flange layout means the process plumbing lives in the cabinet behind the panel, the operator sees only a clean dial face, and a failed gauge swaps out from the front in under 5 minutes if the panel cut-out is sized correctly to EN 837-1.
- Hydraulic power units: Bosch Rexroth ABPAC standard power packs use 100 mm Form 3 glycerine-filled gauges on the panel face to read pump-discharge pressure up to 350 bar.
- Steam and process plant: Spirax Sarco BCV31 boiler-blowdown skids carry 160 mm Form 3 gauges with siphon loops to keep the Bourdon tube below 60°C while reading saturated steam at 10 bar.
- Water treatment: Grundfos Hydro MPC booster sets fit Form 3 panel gauges on the manifold cover to show suction and discharge pressure on 0-16 bar ranges.
- Compressed air systems: Atlas Copco GA-series rotary screw compressor cabinets use 63 mm Form 3 gauges on the control door for line pressure indication at 0-13 bar.
- Hydraulic test rigs: Parker PVS series hydraulic test stands mount 100 mm Form 3 class 1.0 gauges on the operator console for proof-pressure testing of valves and cylinders.
- Fire protection: Viking deluge valve trim panels use 100 mm Form 3 gauges to display upstream and downstream water pressure on sprinkler riser assemblies.
The Formula Behind the Pressure Gauge (form 3)
The fundamental relationship for sizing a Bourdon-tube gauge — Form 3 or otherwise — is the working-pressure rule from EN 837-1. You don't pick a gauge whose full scale matches your operating pressure; you pick one with headroom, because steady operation at full scale fatigues the tube and pulsation events spike well above nominal. At the low end of the typical range, sitting at 25% of full scale, the gauge resolves poorly and small changes are hard to read. At the high end, above 75% of full scale, fatigue life of the Bourdon tube falls off a cliff. The sweet spot — and what EN 837-1 effectively codifies — is steady operation around 50% of full scale with peaks staying below 75% for static loads or 65% for pulsating loads.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| PFS | Required full-scale range of the gauge | bar | psi |
| Pop | Steady operating pressure of the system | bar | psi |
| kutil | Utilisation factor — 0.75 for static service, 0.65 for pulsating service per EN 837-1 | dimensionless | dimensionless |
Worked Example: Pressure Gauge (form 3) in a CNC machining centre coolant skid
A machine-tool builder in Stuttgart is fitting a Form 3 panel gauge to the high-pressure coolant skid of a DMG MORI NHX 5000 horizontal machining centre. The skid runs a Chen Ying 3-piston coolant pump at a steady 70 bar discharge with pulsation peaks measured at 84 bar on a piezo transducer. The 100 mm gauge sits in the operator door, EN 837-1 class 1.6, glycerine-filled. Pick the right full-scale range.
Given
- Pop = 70 bar
- Service type = pulsating (3-piston pump) —
- kutil = 0.65 —
- Dial size = 100 mm
Solution
Step 1 — apply the EN 837-1 utilisation rule for pulsating service. The pump is a positive-displacement 3-piston unit, so peaks of ~20% above mean are expected. We want the steady reading at or below 65% of full scale:
Step 2 — round to the next standard EN 837 range. Standard ranges are 0-60, 0-100, 0-160, 0-250 bar. 0-100 bar is below the calculated minimum, so we step up to 0-160 bar:
Step 3 — verify the operating point sits in the sweet spot. At 70 bar steady, the pointer rests at 70/160 = 44% of full scale. The 84 bar peak sits at 53%:
Compare this against the wrong choices. If you'd picked 0-100 bar (low end of plausible options), the steady reading sits at 70% and pulsation peaks hit 84% — you'd be running into the fatigue zone and the Bourdon tube would take a permanent set within 6-12 months on a machine that runs 16 hours a day. If you'd jumped to 0-250 bar (high end of plausible options), the steady reading sits at 28% — the pointer barely moves off the bottom third of the dial and the operator can't resolve a 5 bar drop signalling a clogged coolant filter. The 0-160 bar choice puts the pointer in the most readable middle band of the dial and keeps tube stress in the elastic range with margin to spare.
Result
Specify a 0-160 bar Form 3 panel gauge, 100 mm dial, class 1. 6, glycerine-filled, with G1/4 back entry. The pointer rests at the 7 o'clock position around 70 bar — the most readable zone of a 270° dial — and pulsation peaks at 84 bar (53% of full scale) sit comfortably below the 65% pulsating-service limit. Across the realistic range, a 0-100 bar gauge would push the pointer into the fatigue zone, while 0-250 bar would crush resolution. If your installed gauge reads consistently 3-5% low after a few weeks, the most likely causes are: (1) a snubber not fitted on a pulsating service letting peak spikes overpressure the tube and take a permanent set, (2) ambient temperature above 60°C at the gauge case shifting the Bourdon tube's elastic modulus, or (3) glycerine fill leaking past the bezel seal so the movement no longer damps and the pointer hash-marks itself out of calibration.
Choosing the Pressure Gauge (form 3): Pros and Cons
The Form 3 isn't the only panel-gauge layout in EN 837. The two competitors you'll see on real panels are the Form 1 (lower back-entry, no flange — held in by the process pipe alone) and the Form 7 (back-entry with a U-clamp bracket instead of a front flange). Each has a place, and the wrong pick costs you either time at install or money at maintenance.
| Property | Form 3 (back entry, front flange) | Form 1 (lower entry, stem mount) | Form 7 (back entry, U-clamp bracket) |
|---|---|---|---|
| Panel cut-out time | Single round hole + 3 small flange holes | No cut-out — gauge hangs off pipe | Round hole + bracket fasteners behind panel |
| Front-of-panel access for replacement | Yes — remove 3 screws, swap from front | No — must access process pipe | Partial — bracket sits behind panel |
| Accuracy class typically available | 1.0 or 1.6 | 1.6 or 2.5 | 1.0 or 1.6 |
| Pressure range typically available | 0-0.6 bar to 0-1,000 bar | 0-0.6 bar to 0-600 bar | 0-0.6 bar to 0-1,000 bar |
| Cost (100 mm glycerine-filled) | €35-€90 | €20-€60 | €45-€110 |
| Vibration tolerance | Good — flange-mounted to panel | Poor — cantilevered on pipe | Good — bracket-mounted to panel |
| Best application | Control panels, operator consoles | Local pipe-mounted indication | Retrofit where panel cut-out exists but no flange holes |
Frequently Asked Questions About Pressure Gauge (form 3)
That's a linearity error in the sector-and-pinion movement, almost always caused by the link between the Bourdon tube tip and the sector arm being set at the wrong angle. EN 837-1 class 1.0 movements need the link perpendicular to the sector at mid-scale — if the link sits at, say, 75° instead of 90°, the angular conversion isn't linear and the dial bows.
You can't fix this from the front. It needs the gauge open on a comparator with a deadweight tester, the link pivot loosened in its slot, and the position walked in until zero, mid, and full scale all read within tolerance. On a class 1.6 panel gauge it's usually cheaper to replace the gauge than to send it out for recalibration.
Read it from the operator station and pick the smallest dial that's legible. The rule of thumb is the dial diameter in mm should be roughly equal to the viewing distance in cm — so a 63 mm gauge reads cleanly at 60 cm, a 100 mm gauge at 1 m, and a 160 mm gauge at 1.6 m.
The other driver is accuracy class. 63 mm gauges max out at class 1.6, while 100 mm and 160 mm are available in class 1.0. If you need to spot a 5% pressure deviation reliably on a 0-100 bar dial, you want the 100 mm.
Glycerine expands roughly 0.05% per °C. If the case was filled at 20°C with no air gap and the workshop now sits at 35°C, the trapped fluid pressurises the inside of the case and pushes the Bourdon tube from the outside — which makes it read high.
Properly filled gauges leave a 20-25% air bubble at the top of the case for exactly this reason. Either the bubble has dissolved over time, or the gauge was overfilled at the factory. Pop the fill plug on top of the case briefly to vent — if the reading drops back to expected, that confirms it. Refit the plug when half the air gap has returned.
Not without a snubber or a glycerine fill, and ideally both. Piston pumps generate sharp pressure spikes at every piston stroke — on a 3-piston pump at 1,500 RPM that's 75 spikes per second. The Bourdon tube can take it, but the sector-and-pinion movement cannot. You'll see the pointer flutter ±5-10 bar and within 200 hours the gear teeth will be visibly worn.
Fit a porous-disc snubber or a needle-valve snubber upstream of the gauge, sized to damp pulsation without slowing legitimate pressure changes below 1 second. A glycerine-filled case alone helps, but on a 3-piston pump it isn't enough on its own.
Pipe weight hung off the gauge. Back-entry gauges are designed for the process connection to be self-supporting — usually a short pigtail or siphon between a pipe-mounted bulkhead and the gauge. If the installer ran rigid pipe straight to the back socket and didn't clamp the pipe to the cabinet wall, the gauge fitting is the only thing holding the pipe up.
Brass sockets fatigue at the silver-braze joint between socket and Bourdon tube. Stainless lasts longer but still fails eventually. The fix is a P-clip on the pipe within 100 mm of the gauge, taking the weight before it reaches the case.
Permanent set in the Bourdon tube. The gauge has been overpressured at some point — either a hydraulic spike above 130% of full scale, or someone deadheaded the pump against a closed valve. The tube took a tiny plastic deformation and no longer returns fully to its zero shape.
You can pull the bezel and reset the pointer to zero with a pointer puller, but that's masking the problem — the linearity will be off across the whole range. The honest answer is replace the gauge and fit overpressure protection (a pressure-limiting valve or a gauge with an internal blow-out disc) so it doesn't happen again.
References & Further Reading
- Wikipedia contributors. Pressure measurement. Wikipedia
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