Mechanical pressure gauges convert fluid pressure into a visible pointer deflection on a calibrated dial — no electrical power required. Bourdon tube gauges (most common, 0.1–4,000 bar): a C-shaped, spiral, or helical hollow metallic tube straightens proportionally to applied pressure, rotating a pointer via a gear-and-sector linkage. Diaphragm gauges (<1 bar, or viscous/corrosive fluid service): a flexible metallic or elastomeric diaphragm deflects under pressure, transmitting motion through a push rod to the pointer. Capsule gauges (very low pressure, 0–25 mbar to 0–600 mbar): two corrugated diaphragms welded at the periphery form a sealed capsule that expands under differential pressure.
Bourdon tube: process pressure applied to the tube interior creates a net outward force on the curved tube wall, straightening it; the free end displacement (0.5–5 mm) is amplified by the Bourdon-to-pointer linkage to produce 270° of pointer arc. Diaphragm: process fluid contacts the primary diaphragm; pressure deflects it proportionally; a push rod transmits the deflection to a Bourdon tube or gear sector driving the pointer. Diaphragm gauges are preferred for viscous, crystallising, or corrosive fluids because the process fluid contacts only the diaphragm — not the Bourdon tube. Differential gauges have two process connections — the pointer reads the difference between inlet high-pressure and outlet low-pressure.
Challenges
Bourdon Tube Fatigue Cracking from Pulsating or Rapidly Oscillating Pressure
High-frequency pressure pulses fatigue the Bourdon tube at its maximum-stress point — tube cracks; hazardous fluid release.
Pointer Oscillation from Pressure Pulsations Making Reading Impossible
Pump pulsation drives pointer oscillation — reading is impossible; mean value obscured.
Diaphragm Rupture from Pressure Spike or Water Hammer
Sudden overpressure spike ruptures diaphragm — process fluid enters gauge body; measurement lost.
Gauge Freezing in Outdoor Installation (Water Ingress Through Case Seal)
Water enters through damaged case seal and freezes inside gauge body — pointer mechanism jams; gauge fails.
Socket Corrosion from Process Fluid Wetting the Bourdon Tube in Corrosive Service
Corrosive fluid entering through thread leak attacks thin-wall Bourdon tube — tube perforation.
Solutions
- Liquid-Filled (Glycerine or Silicone) Gauge — Dampens Pulsation and Lubricates Mechanism: Liquid-filled case absorbs vibration; lubricates gear mechanism; pointer moves smoothly in pulsating service.
- Pulsation Dampener (Snubber) on Gauge Connection: Sintered metal or needle valve snubber smooths pressure spikes — protects Bourdon tube from fatigue.
- Overpressure Protection: Use Gauge with 1.3× Full-Scale Over-Range Rating + Pressure Limiter: Select gauge rated 130% of max expected pressure; add pressure limiter valve on connection — protects diaphragm.
- IP 67 / NEMA 4X Stainless Case for Outdoor and Washdown Service: Hermetically sealed SS case prevents water ingress — rated for full immersion; freeze protection.
- Diaphragm Seal (Chemical Seal) + SS 316L Bourdon Tube for Corrosive Fluid: Diaphragm seal isolates process fluid; fill fluid (silicone, glycerine) transmits pressure — Bourdon tube never contacts process.
Applications
- Utilities: Steam, compressed air, water, nitrogen — standard Bourdon gauges on all pipelines and equipment.
- Chemical: Diaphragm seal gauges for corrosive, crystallising, or highly viscous process fluids.
- Pharmaceutical: Dry or glycerine-filled SS 316L gauges; validation-tagged; calibrated and certified for GMP service.
- Refrigeration: Compound gauges (positive and vacuum range) on refrigerant circuits — dual-scale standard.
- Hydraulics: High-pressure glycerine-filled gauges on hydraulic circuits — 400–700 bar rated.
Models & Capacities
Pressure Gauges — Models & Range Reference | |||||||||
Type | Pressure Range | Dial Size [mm] | Accuracy Class (EN 837) | Max Overpressure | Process Connection | Case Material | Wetted Material | IP Rating | Special Features |
Bourdon C-type (standard) | 0–0.6 bar to 0–600 bar | Ø40–Ø250 | Class 1.0 / 1.6 / 2.5 | 1.3× FS | 1/4″ BSP to 1″ BSP; G1/4-G1/2 | Steel / SS 316L | Brass or SS 316L | IP 65 | Standard industrial; dry or glycerine fill |
Bourdon spiral (high accuracy) | 0–10 bar to 0–1,000 bar | Ø100–Ø160 | Class 0.1 / 0.25 / 0.6 | 1.3× FS | G 1/4 to G 1/2 | SS 316L | SS 316L | IP 65 | High precision test/reference gauge; adjustable pointer |
Bourdon helical (high pressure) | 0–400 bar to 0–4,000 bar | Ø100–Ø160 | Class 0.6–1.0 | 1.3× FS | G 1/4 male | SS 316L / Alloy | SS 316L / alloy | IP 65 | Ultra-high pressure: hydraulics, hydraulic test rigs |
Diaphragm gauge | 0–25 mbar to 0–40 bar | Ø100–Ø160 | Class 1.0 / 1.6 | 1.5× FS | G 1/2 flange; Tri-Clamp | SS 316L | SS 316L / PTFE diaphragm | IP 65 | Viscous, crystallising, corrosive, hygienic service |
Capsule gauge (low pressure) | 0–1 mbar to 0–600 mbar | Ø100–Ø160 | Class 1.6 / 2.5 | 2× FS | G 1/4 male / flange | SS 316L | SS 316L capsule | IP 65 | Combustion control, filter ΔP indication, gas pressure |
Differential pressure gauge | 0–100 mbar to 0–40 bar ΔP | Ø100–Ø160 | Class 1.6 / 2.5 | Per side: 25–250 bar | Dual G 1/4 or flange | SS 316L / CS | Bourdon or diaphragm | IP 65 | Filter condition monitoring; level ΔH indication |
Compound gauge (vac + pos) | −1 to +3 bar, −1 to +15 bar | Ø100–Ø160 | Class 1.6 | 1.3× positive FS | G 1/4 to G 1/2 | SS 316L | Brass or SS 316L | IP 65 | Vacuum and positive pressure — evaporators, condensers, HVAC |
Test / calibration gauge | 0–2.5 bar to 0–1,000 bar | Ø100–Ø250 | Class 0.1 / 0.25 | 1.3× FS | G 1/2 or DIN 16282 male | SS 316L precision | SS 316L | IP 65 | Reference standard; UKAS/NIST traceable calibration |