A Pressure Gauge is a mechanical instrument that measures fluid pressure and displays it on a dial. Its core component is the Bourdon tube — a curved, flattened metal tube that straightens slightly as internal pressure rises, driving a pinion-and-sector linkage to swing the pointer. We use it to give operators a direct, power-free reading of system pressure so they can spot blockages, leaks, or overpressure before damage occurs. A 100 mm 0–10 bar gauge with class 1.6 accuracy is standard kit on boilers, hydraulic rigs, and gas regulators worldwide.
Pressure Gauge Interactive Calculator
Vary pressure, gauge span, accuracy class, and pointer sweep to see Bourdon tube travel, pointer angle, and allowable reading error.
Equation Used
The gauge converts pressure fraction P/FS into pointer rotation over the selected sweep. For the standard 100 mm Bourdon gauge described, full-scale tube-tip travel is about 4 to 8 mm. Accuracy class is applied to the full-scale span, so a class 1.6, 0-10 bar gauge has an allowable error of 0.16 bar.
- Bourdon tube response is treated as linear over the dial span.
- Standard 100 mm dial gauge tip travel is 4 to 8 mm at full scale.
- Accuracy class is percent of full-scale span.
- Pointer angle is limited to the selected sweep for display.
Inside the Pressure Gauge (form 1)
The form-1 Pressure Gauge — what most people picture as the round dial gauge — works on elastic deformation. Process fluid enters the socket, fills the C-shaped Bourdon tube, and tries to round out the tube's flattened cross-section. Because one end is anchored at the socket and the other is free, that rounding effect makes the free tip travel through a small arc — typically 4 to 8 mm at full scale on a 100 mm dial. A link transfers that motion to a sector gear, which drives a pinion on the pointer shaft, multiplying the tip travel into a 270° pointer sweep across the dial.
The geometry has to be right or the gauge lies to you. The Bourdon tube wall thickness sets the range — a 0–1 bar gauge uses a thin-wall tube, a 0–600 bar gauge uses a much heavier section. Get the linkage geometry wrong and you see non-linearity: the pointer reads correct at zero and full scale but droops 2-3% in the middle. If you notice the pointer not returning to zero after pressure release, you have either a permanently deformed tube (over-pressured past the elastic limit), a sticking pinion, or a broken hairspring on the pointer shaft. Pulsation is the other big killer — a pump that cycles 50 times a second will work-harden the tube and crack it within months unless you fit a snubber or a glycerine-filled case to damp the movement.
Accuracy class matters more than dial size. A class 1.6 gauge is accurate to ±1.6% of full scale across the middle 75% of the range — so a 0–10 bar class 1.6 gauge can read ±0.16 bar anywhere on the dial. For trade-custody or safety-critical work you want class 0.6 or better, and you size the range so the normal operating point sits at roughly 2/3 of full scale. Run a gauge pinned at the top of the dial and you will work-harden the tube and lose accuracy inside a year.
Key Components
- Bourdon Tube: The flattened, C-shaped (or helical, or spiral) elastic element that deflects under pressure. Material is typically phosphor bronze for ranges up to 60 bar and 316 stainless for higher ranges or corrosive media. Tip travel at full scale is 4-8 mm on a standard 100 mm dial gauge.
- Movement (Sector and Pinion): The brass or stainless gear train that converts the Bourdon tip's small linear travel into 270° of pointer rotation. The sector gear meshes with a pinion on the pointer shaft, with a hairspring keeping the gears in single-flank contact to eliminate backlash. Backlash above 0.5° shows up as pointer hysteresis between rising and falling pressure.
- Link and Adjustable Connection: The short rigid link between the Bourdon tip and the sector gear, with a slotted screw connection that lets the technician adjust span. Moving the connection point closer to the sector pivot increases span; moving it away decreases span. Calibration is iterative — you adjust span at full scale, recheck zero, repeat.
- Pointer and Hairspring: Aluminium pointer pressed onto the pinion shaft, with a flat coil hairspring loading the gear train against backlash. A weak or broken hairspring gives erratic reading and slow return to zero.
- Socket and Pressure Connection: The threaded process connection — typically 1/4 NPT, 1/2 NPT, or G1/2 — brazed or welded to the Bourdon root. The socket carries the full process pressure and bears the gauge weight, so it has to be torqued to spec (usually 30-50 Nm for G1/2) without using the case as a wrench point.
- Dial and Window: Aluminium dial with printed scale, and a polycarbonate or glass window. Process pressure gauges over 16 bar should use safety glass or laminated polycarbonate so a Bourdon rupture doesn't fire shards at the operator's face.
- Case (Dry or Liquid-Filled): Steel, stainless, or ABS case. Glycerine or silicone fill damps pointer flutter on pulsating systems and lubricates the movement — a liquid-filled gauge on a piston pump outlet lasts 5-10× longer than a dry gauge in the same service.
Where the Pressure Gauge (form 1) Is Used
You find Pressure Gauges anywhere a fluid is contained under pressure and somebody needs to know the value at a glance. The form-1 dial gauge dominates because it needs no power, no electronics, and no calibration cable — you screw it in and read it. Where it falls short is data logging, remote indication, and very low pressure (below ~25 mbar) where a diaphragm or capsule element does the job better. The applications below show the working range from craft food production right up to high-pressure hydraulic rigs.
- Brewing: Steam-jacket pressure indication on a 10 hL mash tun at Wicked Weed Brewing — a 0–4 bar liquid-filled gauge with a pigtail siphon protecting the Bourdon tube from live steam.
- Hydraulics: Outlet pressure monitoring on Enerpac P-392 hand pumps — a 0–700 bar glycerine-filled class 1.6 gauge that absorbs the pump's pulsation without flutter.
- Compressed Air: Tank gauge on Ingersoll Rand SS3 reciprocating compressors — a 0–10 bar dry gauge sized so the 8 bar cut-out point sits at 80% of full scale.
- Fire Protection: Wet-pipe sprinkler riser gauge per NFPA 13 — a 0–20 bar gauge with a 1/4 NPT connection, dual-marked in psi and bar, retrofitted across thousands of commercial buildings.
- Gas Distribution: Outlet gauge on Harris 25GX oxy-acetylene regulators — a 0–10 bar gauge for the cutting-torch line, with brass internals certified for oxygen service.
- HVAC: Chilled-water differential pressure on Armstrong 4380 base-mounted pumps — paired suction and discharge gauges reading 0–6 bar to verify pump head against the curve.
- Pharmaceutical: Sterile process pressure on a WFI loop, using a Wika 990.10 diaphragm-seal gauge with a flush 1.5" Tri-Clamp connection so there's no dead leg for bacteria to colonise.
The Formula Behind the Pressure Gauge (form 1)
The deflection of a Bourdon tube tip under pressure is governed by Wuest's relation, which links tip travel to pressure, tube geometry, and material modulus. You rarely solve it from scratch — gauge manufacturers do that — but understanding it tells you why a low-range gauge has a thin-wall tube and why over-pressuring kills accuracy. At the low end of the typical range (around 1/4 of full scale) the tip travel is small, the linkage works in its most non-linear region, and reading errors of 2-3% of FS are common. At nominal (around 2/3 of full scale) the tip operates in the linear sweet spot and full-scale accuracy class is achievable. At the high end (above 90% FS) you're approaching the elastic limit of the tube — repeated cycling here permanently sets the tube and shifts zero.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| y | Bourdon tip deflection at the free end | m | in |
| P | Applied gauge pressure | Pa | psi |
| R | Mean radius of curvature of the C-tube | m | in |
| a | Major semi-axis of the flattened tube cross-section | m | in |
| b | Minor semi-axis of the flattened tube cross-section | m | in |
| t | Tube wall thickness | m | in |
| h | Tube height (axial dimension of the cross-section) | m | in |
| E | Young's modulus of the tube material | Pa | psi |
| K | Geometry constant capturing arc angle and end conditions | dimensionless | dimensionless |
Worked Example: Pressure Gauge (form 1) in a craft chocolate factory's tempering loop
A craft chocolate maker in Brooklyn is specifying a Pressure Gauge for the hot-water heating loop on a Selmi Plus EX tempering machine. The loop runs at 1.5 bar nominal, with cold-fill at 0.8 bar and a relief valve set at 3.0 bar. The gauge sits on a 1/4 NPT tee at the manifold inlet. We need to confirm whether a 0–4 bar 63 mm class 1.6 dial gauge will read the operating point with usable resolution at the low, nominal, and high end of the duty range, and what tip deflection the Bourdon sees at each point.
Given
- Range = 0–4 bar
- Dial size = 63 mm
- Accuracy class = 1.6 % FS
- Full-scale tip travel yFS = 5 mm
- Plow (cold-fill) = 0.8 bar
- Pnom (operating) = 1.5 bar
- Phigh (relief setpoint) = 3.0 bar
Solution
Step 1 — for a properly designed Bourdon tube operating in its linear range, tip deflection scales linearly with pressure, so we can use the ratio form. Compute the absolute accuracy band first:
That ±0.064 bar applies anywhere on the dial. Now check if the operating point gives usable resolution.
Step 2 — at nominal 1.5 bar, the tip deflection from zero is:
The pointer sits at 37.5% of full scale. That's below the 2/3 sweet spot we'd ideally want, but acceptable. Resolution-wise, ±0.064 bar at 1.5 bar is ±4.3% of reading — fine for process indication but not for trade custody.
Step 3 — at the low end, cold-fill at 0.8 bar:
The pointer sits at 20% of full scale. The ±0.064 bar absolute error becomes ±8% of reading — the operator can confirm the system is pressurised but can't dial-in a precise cold-fill pressure from this gauge alone. If precise low-end reading mattered, you'd drop to a 0–2.5 bar range instead.
Step 4 — at the relief setpoint 3.0 bar:
The pointer sits at 75% of full scale, which is exactly where you want a high-alarm point — visually obvious and inside the linear region of the tube. If the relief opens, the gauge will read clean and recover to zero without permanent set, because we're well below the 90% FS overload threshold.
Result
The 0–4 bar 63 mm class 1. 6 gauge gives ±0.064 bar absolute accuracy with the nominal 1.5 bar operating point landing at 37.5% FS and a Bourdon tip deflection of 1.88 mm. That tip travel is enough for the linkage to read linearly, and you'll see the pointer respond visibly to a 0.1 bar pressure change. Across the duty range, low-end resolution at 0.8 bar is marginal (±8% of reading), nominal is comfortable, and the relief setpoint at 75% FS is the sweet spot — readable at a glance and safe for the tube. If your installed gauge reads consistently low or sticks during pressure rise, suspect (1) a clogged 1/4 NPT socket from chocolate or scale ingress restricting fluid into the Bourdon, (2) a sluggish glycerine fill that's gone cloudy and stiff after years of UV exposure on the case window, or (3) the gauge mounted with the dial face horizontal when the movement was calibrated for vertical mounting — a 90° orientation change can shift zero by 1-2% FS on a small-dial gauge.
Pressure Gauge (form 1) vs Alternatives
The form-1 Bourdon-tube Pressure Gauge isn't the only way to read pressure on a process line. For very low pressures, corrosive media, or anywhere you need a 4-20 mA signal back to a control room, you'll be choosing between a Bourdon gauge, a diaphragm capsule gauge, and an electronic pressure transmitter. Here's how they compare on the dimensions that actually matter for selection.
| Property | Bourdon Pressure Gauge | Diaphragm Capsule Gauge | Electronic Pressure Transmitter |
|---|---|---|---|
| Practical pressure range | 0.6 to 6,000 bar | 2.5 mbar to 25 bar | Vacuum to 10,000 bar |
| Typical accuracy class | 0.6 to 2.5% FS | 1.0 to 2.5% FS | 0.05 to 0.5% FS |
| Power required | None | None | 12-36 VDC, 4-20 mA loop |
| Remote / data-logging capability | None — local read only | None — local read only | Native 4-20 mA, HART, or fieldbus |
| Cost (100 mm class 1.6 example) | $25-80 | $60-150 | $200-1,200 |
| Lifespan on pulsating service | 6-24 months dry, 5-10 yr liquid-filled | 2-5 yr | 10+ yr |
| Best fit application | Local indication on steady or damped pressures | Low-pressure gas, sanitary, sub-1-bar systems | Closed-loop control, alarms, data logging |
| Failure mode if overpressured | Tube ruptures, pointer pegs at FS | Capsule deforms permanently | Sensor zero shifts, may survive 2-4× overpressure |
Frequently Asked Questions About Pressure Gauge (form 1)
Mounting orientation. Small-dial gauges (63 mm and below) are calibrated in a specific orientation — usually dial-vertical — and the weight of the pointer plus sector gear shifts the zero when you rotate the gauge into a different plane. A 90° change can move zero by 1-2% FS on a 63 mm gauge.
Two fixes: either re-zero the pointer in the installed orientation by lifting it off the pinion and re-pressing it at the correct position, or specify a gauge with an external zero-adjust screw next time. For 100 mm and larger gauges this effect is usually below the accuracy class and not worth chasing.
Two reasons working in opposite directions. You want the operating point high enough on the dial that the pointer lands in the linear, accurate middle band of the tube's deflection curve — below 25% FS the linkage geometry is non-linear and the accuracy class doesn't really apply. But you also need headroom above the operating point for transients, surges, and pump start-up spikes without slamming the pointer into the stop pin and over-deflecting the tube.
2/3 FS gives you both: linear operation at the duty point, plus 50% headroom for spikes. If you pin a gauge above 90% FS regularly, the tube work-hardens and zero drifts within months.
Liquid-filled wins for pulsation that's continuous and rapid (above ~5 Hz), and where the operator needs to actually read the pointer mid-cycle — glycerine fill damps flutter to zero and lubricates the movement, giving 5-10× the service life. The downside is glycerine clouds with UV exposure and the fill plug can vent in cold climates.
A snubber (porous metal disc or capillary restrictor) is cheaper and works for slower or intermittent pulsation, but it adds response lag — you won't see fast pressure spikes, just a smoothed average. Rule of thumb: for a piston pump under 100 bar, snubber is fine; above 100 bar or above 5 Hz pulsation, spec the liquid-filled gauge.
Probably not the tube itself — a slow return is almost always a sticking movement, not a deformed Bourdon. The most common cause is contamination in the sector-and-pinion gear train: dust, varnished glycerine, or fine debris from a corroding case. The hairspring isn't strong enough to overcome the friction so the pointer creeps back instead of snapping back.
Diagnose by tapping the case lightly during return — if the pointer jumps down in steps, it's mechanical stiction in the movement. If it returns smoothly but slowly, suspect a partially blocked socket or pigtail siphon trapping fluid behind the Bourdon. A genuinely deformed tube doesn't return slowly; it returns to a non-zero offset and stays there.
Check the test conditions first. Accuracy class is specified at a reference temperature (usually 20°C) and in the calibrated mounting orientation. A 20°C swing in ambient temperature shifts the Bourdon material's modulus enough to throw a class 0.6 gauge by 0.4-0.8% FS. Add wrong orientation and you can easily see 1.5% error on a gauge that's perfectly within spec.
The other thing people miss is the head correction — if your deadweight tester piston sits 200 mm above or below the gauge connection, the static head of the test fluid (oil at ~0.08 bar/m) shifts the reading. For high-accuracy gauges you have to test at the same elevation, in the same orientation, at the same temperature as the spec sheet calls out.
No, and this is a safety issue, not a performance one. Oxygen-service gauges are degreased to a specific standard (typically per ASTM G93 Level C or better) and use only oxygen-compatible internal materials — no hydrocarbon lubricants in the movement, no glycerine fill, no soft-soldered joints. A standard gauge has machining oil residues in the Bourdon tube and brass sector gear that will ignite in a high-pressure oxygen environment.
Buy a gauge marked "Oxygen — Use No Oil" or "O₂ cleaned" with a green dial label. The cost difference is 30-50% and not worth saving against a flash fire risk.
The diaphragm seal adds a sealed fill fluid (silicone oil, glycerine, or NaK depending on temperature range) between the process and the Bourdon tube. That fill fluid has thermal expansion and viscous drag, both of which slow the response and add a temperature-dependent zero shift. A capillary line longer than 1 m makes it worse — every metre of capillary adds roughly 0.3-1.0% FS of zero shift per 10°C ambient swing.
If response speed matters, keep the capillary as short as possible (under 500 mm), use a low-viscosity fill, and insulate the capillary if the ambient swings hard between day and night. If you can direct-mount a Bourdon gauge with a pigtail siphon instead, you'll get faster response and better accuracy at lower cost.
References & Further Reading
- Wikipedia contributors. Pressure measurement. Wikipedia
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