A corrugated tube pressure gauge measures pressure by sensing the elastic deflection of a thin metal capsule or bellows whose walls are formed into concentric corrugations. Unlike a Bourdon gauge — which uses a curved tube uncoiling under pressure — the corrugated tube relies on stacked diaphragm flexure, giving it far better sensitivity at low pressures. The deflection drives a lever-and-pinion linkage to a pointer. You see this mechanism in HVAC draft gauges, aneroid barometers, and altimeters where pressures are well under 1 bar.
Corrugated Tube Pressure Gauge Interactive Calculator
Vary pressure, full-scale travel, and lever ratio to see capsule deflection amplified into gauge linkage motion.
Equation Used
The worked example shows a 1.0 in WC gauge producing about 2 mm of capsule motion and using a 10:1 pivot lever. This calculator scales capsule displacement linearly with pressure, then multiplies that displacement by the lever ratio to estimate linkage travel.
- Capsule deflection is linear over the calibrated pressure span.
- Lever ratio is an ideal geometric amplification with negligible backlash.
- Overrange is reported when pressure exceeds the selected full-scale pressure.
How the Corrugated Tube Pressure Gauge Actually Works
The active element is a thin-walled metal capsule — typically beryllium copper, phosphor bronze, or stainless — formed into concentric ring-shaped corrugations. When pressure enters the capsule, the corrugations let the end faces move axially without the wall material having to stretch. That converts pressure into a clean linear displacement, usually 0.5 to 3 mm of full-scale travel for a 50 mm capsule. A lever multiplies that travel, a sector gear converts it to rotation, and a pinion drives the pointer across the dial.
The corrugations exist for one reason: to give the capsule a low, repeatable spring rate. A flat diaphragm of the same thickness would resist pressure mostly through membrane tension, which is non-linear and stiff. Corrugations let the wall flex like a series of small hinges, so the spring rate becomes nearly linear and the gauge reads accurately across the full scale. If the corrugation depth is too shallow — below roughly 0.4 mm on a thin-wall capsule — you lose linearity at the low end. Too deep and the capsule loses pressure capacity and starts to buckle inward at overpressure.
Failures usually come from three places. Fatigue cracks appear at the inner crests of the corrugations after enough pressure cycles — that is where bending strain peaks. Solder or weld joints around the capsule rim leak after thermal cycling, which shows up as a slow zero drift you can't trim out. And if you overpressure the capsule past its elastic limit, you get a permanent set: the pointer no longer returns to zero when pressure is removed. That last one is the single most common cause of a scrap gauge in HVAC service.
Key Components
- Corrugated Capsule: Two thin metal diaphragms with concentric corrugations welded or soldered at their rims to form a sealed chamber. Wall thickness is typically 0.05 to 0.15 mm, with 3 to 6 corrugation rings per face. Material choice (beryllium copper for sensitivity, 316 stainless for chemical resistance) sets both the spring rate and the fatigue life.
- Capsule Stack (optional): Two or more capsules joined in series to multiply axial travel. A 4-capsule aneroid stack in a precision altimeter delivers 4× the displacement of a single capsule for the same pressure change, which is why aircraft altimeters can resolve 20 ft of altitude.
- Drive Lever and Hairspring: A pivot lever picks up capsule travel and amplifies it geometrically — typical lever ratios run 8:1 to 20:1. A small hairspring preloads the linkage to remove backlash; without it the pointer hunts by 1-2% of full scale on small pressure changes.
- Sector Gear and Pinion: Converts the lever's arc motion into pointer rotation. The sector tooth pitch and pinion bore must mesh with no perceptible backlash — pinion bore tolerance is typically held to ±0.01 mm, because every 0.02 mm of slop reads as roughly 0.5° of pointer wander.
- Zero and Span Adjusters: Zero adjustment shifts the linkage takeoff point; span adjustment moves the lever's pivot along its slot to change mechanical advantage. Calibration is done at zero and 90% of full scale, iterating until both points sit within tolerance — usually ±1% or ±2% of span.
- Case and Process Connection: Sealed case with a static port for gauge or differential reference. The process connection (often 1/4 NPT or 1/8 NPT) ports pressure into the capsule. For absolute-pressure variants the capsule is evacuated and the case sees the measured pressure.
Industries That Rely on the Corrugated Tube Pressure Gauge
Corrugated capsule and bellows gauges live in the low-pressure end of the instrumentation world — anywhere a Bourdon tube would barely twitch. You find them measuring inches of water column, millibars, or absolute pressures at altitude. The capsule's high sensitivity and low hysteresis make it the default choice when the full-scale range is under about 1 bar and accuracy needs to be better than ±2%.
- HVAC: Dwyer Magnehelic-style differential pressure gauges across air handler filter banks, reading 0-1 inch water column to flag a clogged filter.
- Aviation: Kollsman aneroid altimeters using a stacked 3- or 4-capsule aneroid as the primary altitude reference in light aircraft and gliders.
- Meteorology: Aneroid barometers (the classic Fischer Precision Aneroid is one example) measuring atmospheric pressure between 950 and 1050 mbar.
- Process Industry: Low-pressure tank vent and blanket gauges on chemical storage where line pressure sits between 1 and 50 mbar gauge.
- Cleanrooms and Labs: Room-to-room differential pressure indicators in pharmaceutical cleanrooms, holding ±5 Pa setpoints to maintain cascade pressurisation.
- Boilers and Combustion: Furnace draft gauges on industrial gas-fired boilers, measuring negative draft of −0.05 to −0.2 inch water column at the flue.
The Formula Behind the Corrugated Tube Pressure Gauge
The capsule behaves like a soft axial spring — pressure produces a deflection that is proportional to the area exposed and inversely proportional to the capsule's spring rate. Down at the low end of the typical range, sensitivity is what you want, so you choose a thinner wall and more corrugations and accept a lower overpressure margin. At the high end of the elastic range, you get faster response and better overpressure tolerance but lose resolution because the pointer barely moves for small pressure changes. The sweet spot sits where full-scale pressure produces 1.5 to 2.5 mm of capsule travel — enough for the linkage to read cleanly, not so much that the corrugations work-harden over time.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Δx | Axial deflection of the capsule end face | m | in |
| P | Applied differential pressure across the capsule | Pa | psi |
| Aeff | Effective area of the corrugated diaphragm (typically 0.6-0.8× the geometric area) | m2 | in2 |
| kc | Axial spring rate of the capsule | N/m | lbf/in |
| θp | Pointer rotation (after lever × gear amplification G) | rad | deg |
Worked Example: Corrugated Tube Pressure Gauge in an HVAC filter differential pressure gauge
A facilities team is sizing a corrugated capsule gauge to monitor pressure drop across a MERV 13 filter bank in a hospital air handler. The filter is rated to be replaced when ΔP across it reaches 1.0 inch water column (≈ 249 Pa). The capsule is 50 mm OD beryllium copper with effective area 1.2 × 10−3 m2, axial spring rate 120 N/m, and the linkage has total amplification G = 600 rad/m so that 270° of pointer sweep corresponds to full scale.
Given
- Aeff = 1.2 × 10−3 m²
- kc = 120 N/m
- G = 600 rad/m
- Pnom = 249 Pa (1.0 in WC)
- Plow = 62 Pa (0.25 in WC, clean filter)
- Phigh = 498 Pa (2.0 in WC, fully loaded)
Solution
Step 1 — compute capsule deflection at the nominal change-out pressure of 249 Pa:
Step 2 — convert that travel to pointer rotation using the linkage amplification:
That's clean placement — about a third of the way across a 270° dial, where the pointer reads with no parallax confusion. Now check the low end of the operating range. A new clean filter draws roughly 0.25 in WC (62 Pa):
That tiny pointer sweep is the whole reason you don't reach for a Bourdon gauge here — a Bourdon would barely register 62 Pa at all. At the high end, 2.0 in WC (498 Pa) corresponds to a fully loaded filter that should already have been changed:
4.98 mm is past the linear range of most 50 mm capsules — you are now stretching the corrugations into the work-hardening region, and after a few hundred cycles to that pressure you'll see permanent zero shift. So the capsule is correctly sized for an alarm at 1.0 in WC but should not be used as a continuous indicator out at 2.0 in WC.
Result
Nominal capsule deflection at the 1. 0 in WC change-out point is 2.49 mm, giving roughly 85° of pointer rotation on a 270° dial. At the low end (clean filter, 62 Pa) the pointer barely swings 21°, which is exactly what makes the corrugated capsule the right tool for this job — a Bourdon gauge would show essentially nothing. At the high end (498 Pa) the capsule is at 4.98 mm of travel, beyond its linear band, and repeated excursions there will cause permanent set. If the field gauge reads 20% low compared to a calibrated reference, check three things in this order: (1) a pinhole leak at the capsule rim solder joint, which lets internal pressure equalise to case pressure and bleeds off deflection, (2) a hairspring that has lost preload and let backlash creep into the sector-pinion mesh, and (3) condensation inside the case wetting the corrugations and adding effective mass that damps response.
When to Use a Corrugated Tube Pressure Gauge and When Not To
Pick a pressure sensing element to match the pressure range and accuracy you actually need. The corrugated capsule wins below 1 bar where Bourdon tubes have no useful sensitivity. Above that, the Bourdon is cheaper, more rugged, and easier to overpressure-protect. A flat diaphragm sits between them and beats both for fast response.
| Property | Corrugated Tube/Capsule Gauge | Bourdon Tube Gauge | Flat Diaphragm Gauge |
|---|---|---|---|
| Useful pressure range | 0-1 bar (best below 100 mbar) | 0.5 bar to 1000+ bar | 0-10 bar |
| Typical accuracy (% of span) | ±1% to ±2% | ±1% to ±3% | ±0.5% to ±1.5% |
| Resolution at low pressure (<100 mbar) | Excellent — 0.5 Pa achievable | Poor — pointer barely moves | Good |
| Overpressure tolerance | 1.5-2× full scale before permanent set | 3-5× full scale | 2-3× full scale |
| Response time | 100-500 ms (mass of capsule) | 50-200 ms | 10-50 ms (lowest mass) |
| Hysteresis | 0.3-1% of span | 0.5-2% of span | 0.2-0.8% of span |
| Relative cost | Medium-high | Low | Medium |
| Typical lifespan (cycles) | 105-106 | 106-107 | 105-106 |
Frequently Asked Questions About Corrugated Tube Pressure Gauge
That's classic hysteresis from the capsule walls, and almost always points to one of two things. First, the capsule has been overpressured at some point and the corrugations have work-hardened — the metal no longer relaxes elastically on the down-stroke. Second, the linkage hairspring has weakened, so on falling pressure there isn't enough return force to overcome friction in the sector-pinion mesh.
Quick diagnostic: pressurise to 80% of full scale, hold 30 seconds, vent slowly, and watch the pointer return. If it stops 3-5% above zero and a gentle tap on the case drops it to zero, the hairspring is the culprit. If tapping does nothing, the capsule itself is set and the gauge is scrap.
Stack them if you need resolution better than 1% of span. A single 50 mm capsule at 50 mbar gives you maybe 1.5 mm of travel, and after the linkage you're looking at roughly 0.5° of pointer sweep per mbar — readable but not precise. A 3-capsule stack triples the travel, so you get 1.5° per mbar, which is what altimeters and precision lab barometers need.
The tradeoff is response time and overpressure margin. Stacked capsules have 3× the internal volume, so they fill more slowly, and any one capsule failing leaks the whole stack. For most HVAC and process work, a single capsule is fine and the stack adds cost you don't need.
Temperature change in the case. Beryllium copper capsules have a measurable thermal expansion coefficient, and if the gauge is mounted on an outdoor wall or near a chiller, a 15°C swing between day and night can shift the apparent zero by 1-2% of span. Sealed-case absolute gauges are even worse because the trapped reference gas inside the case expands too.
Fix is either a temperature-compensating bimetal strip in the linkage (most quality gauges have one — look for a curved bimetal element near the pivot) or relocation of the gauge to a thermally stable spot. If the bimetal is bent or broken, the gauge will drift roughly with ambient temperature in a near-linear way and that's how you confirm it.
Not directly — and this is where many installers get burned. The capsule's natural frequency is typically 30-80 Hz, and a diaphragm pump pulsing at 5-10 Hz with sharp pressure spikes will excite the capsule into resonance. The pointer hunts wildly and within a few thousand hours the corrugations crack at the inner crests from fatigue.
Either install a snubber (a capillary or porous bronze restrictor) ahead of the gauge to damp the pulses, or switch to a diaphragm gauge with a viscous fill. Glycerine-filled cases are common but they only damp pointer motion, not capsule fatigue — you still need the snubber upstream.
That's a non-linearity error and it almost always traces back to the lever-and-sector geometry, not the capsule itself. When the lever pivot is set to give correct zero and span, but the geometry isn't tangent at mid-range, the pointer over-reads in the middle of the dial. Calibration shops call this an S-shaped error curve.
The fix is mid-range calibration: most quality gauges have a third adjuster that shifts the sector-pinion engagement point to flatten the curve. If your gauge only has zero and span adjustments, mid-range error of ±2% is the practical floor and you can't trim it further. For tighter performance you need a 3-point calibrated instrument.
Altitude and weather measurements need absolute. The capsule must be evacuated and sealed at the factory so that what flexes it is the difference between vacuum and ambient atmospheric pressure. A gauge-pressure capsule references the case interior, which is at local atmospheric — meaning it always reads zero regardless of altitude.
Easy field check: take a working gauge into a sealed bag and squeeze it. A gauge-pressure capsule responds to your squeeze; an absolute capsule does not, because squeezing the bag raises both case and process pressure equally. If you bought what you thought was an altimeter capsule and it ignores the bag test, it's actually a differential or gauge type.
References & Further Reading
- Wikipedia contributors. Pressure measurement. Wikipedia
Building or designing a mechanism like this?
Explore the precision-engineered motion control hardware used by mechanical engineers, makers, and product designers.
