03 · The measuring principle
A calibration weight.
No drift.
Decades of precision.
The ring balance is a measuring principle without springs, without diaphragms — calibration is determined solely by a fixed weight and unchanging geometry. The result: no drift, constant accuracy over the entire service life.
Fig. · Ring balance — construction & components
01 — Why ring balance
Where sensors drift, mechanics stays stable.
Typical differential pressure sensors age: springs fatigue, diaphragms lose their elasticity. The ring balance has no memory effect — its calibration is determined solely by a fixed weight and unchanging lever lengths.
No memory effect
A weight does not age. The calibration remains constant over the entire service life — even after prolonged overload, the display returns reliably to zero.
From 40 Pa span
The smallest buildable span is 40 Pa — e.g. ±20 Pa, 0…40 Pa, or asymmetric −10…+30 Pa. All intermediate values are calibratable without extra charge.
Without auxiliary power
The mechanical display works without electricity. Optionally with 4–20 mA, 0–20 mA, 0–10 V, or as an intrinsically safe ATEX variant for hazardous areas.
02 — The principle compared
From the U-tube to the ring body.
The U-tube manometer is intuitive: pressure differences shift the liquid level, Δh is the measure. In the ring balance, rotation converts exactly this height difference into a large rotation angle — a few millimetres of level shift become tens of degrees.
Classical
U-tube manometer
Δp shifts the liquid level (Δh)
Ring balance — same physics, different solution
Rixen ring balance
Δh → rotation angle — mechanical amplification
03 — Interactive measuring element
// Readout console
// Explore components
Move the slider → ring rotates. A small oil level shift becomes a large rotation angle — mechanical amplification in action. "Overload" shows when oil is significantly displaced. Click a component for details.
04 — The principle in words
A pressure differential, translated into rotation.
Imagine a ring-shaped tube, like a doughnut, suspended vertically and free to rotate. Inside it is half-filled with thin oil — and that is exactly the trick.
01The ring as a container with two chambers
At the very top sits a small dividing wall T that splits the upper hollow into a left and a right chamber. The oil at the bottom acts as a seal: gas cannot flow from one chamber to the other.
02At rest: nothing happens
When both pressures are equal, the ring stands upright. The oil level is balanced, the weight G hangs centrally below. The display shows zero.
03One side gets more pressure → ring rotates
If p₁ pushes harder than p₂, the gas shifts the dividing wall T to the side. Since T is rigidly attached to the ring, the entire ring rotates. Weight G is deflected sideways — and gravity pulls it back.
04Small Δh, large angle — mechanical amplification
The oil shifts like a U-tube: the level difference is small — just a few millimetres, depending on the measuring range. The ring balance converts this into a large angular deflection, making tiny pressures clearly visible and easy to measure electronically.
05Why this is so long-term stable
No spring, no diaphragm — only gravity and geometry. The calibration is determined solely by the mass G, the lever lengths, and the area of the dividing wall. These quantities do not change.
05 — The components
The ring body in detail.
The exploded view shows every individual component of the ring body — from the dividing wall and bearings to the connection plate. Each part has a defined function within the measuring principle. No electronics in the measuring path, no ageing components, no moving seals.
Fig. · Exploded view ring body · Rixen Messtechnik
06 — Dynamic behaviour
How quickly does the ring balance respond?
The ring balance exhibits first-order (PT1) settling behaviour — no overshoot, no oscillation. A smooth exponential approach to the final reading. This is an inherent characteristic of the measuring principle, not a defect.
Why PT1 behaviour?
When a pressure step occurs, the ring body does not jump instantly to its equilibrium angle but approaches it exponentially. The interplay of driving pressure torque, the restoring moment of the calibration weight, and the viscous damping of the sealing fluid produces the characteristic PT1 time constant. Larger pressure differentials drive the ring faster — at small measuring ranges the same mechanism results in a proportionally longer settling time.
Step response · ring balance · 50 Pa range · based on laboratory measurements
07 — Areas of application
Where ring balances have been working for decades.
From cleanroom pressure hold to ATEX — wherever very small pressure differentials must be measured reliably and permanently.
Cleanrooms & operating theatres
Pressure hold against the corridor (typ. 5–30 Pa). No drift = no false alarms = no unwanted shutdowns.
Filter monitoring
Contamination indication in HEPA and submicron filters. Contact output or transmitter signal (4–20 mA, 0–20 mA, 0–10 V) signals replacement need.
Industrial furnaces & boilers
Draft measurement in flue duct. Insensitive to aggressive gases — ideal for corrosive atmospheres.
Volume flow measurement
Primary element (orifice, nozzle, Pitot) + ring balance. The transmitter output is linear to ΔP — since ΔP = k·Q², the BMS or SCADA applies the square root to derive volume flow.
Hazardous areas (ATEX)
RW65-Ex-II: ATEX II 1 GD, intrinsically safe 4–20 mA loop, stainless steel housing on request.
Building automation
Ventilation, smoke-control stairwells, climate regulation — everywhere small pressure differentials need to be permanently documented.
08 — Datasheet
Specification RW65 / MU-Analog
Every device is calibrated per order. All measuring spans between 40 Pa and ±1,800 Pa are available without extra charge. Factory calibration certificate included.
| Property | Value / range |
|---|---|
| Minimum range (span) | 40 Pa — e.g. ±20 Pa, 0…40 Pa, or −10…+30 Pa |
| Maximum range | ±1,800 Pa (±18 mbar) |
| Intermediate values | all freely selectable, calibrated per order, no extra charge |
| Accuracy | ±1.5 % FS (> 100 Pa) or ±1.5 Pa (< 100 Pa) |
| Display units | Pa · daPa · kPa · mbar · mmWS · in.W.C. |
| Sealing fluid mineral oil | ρ ≈ 0.8 kg/L, for ranges ≤ ±700 Pa |
| Sealing fluid Galden® | ρ ≈ 1.8 kg/L, for ranges > ±700 Pa |
| Overload (mineral oil) | continuous up to ±700 Pa |
| Overload (Galden®) | up to ±2.1 kPa |
| Ambient temperature | −10 … +50 °C |
| Protection class | IP65 (standard) / IP66 (stainless steel) |
| Scale size | 150 × 150 mm |
| Electrical output (active, 4-wire) | 4–20 mA · 0–20 mA · 0–10 V |
| Electrical output (passive, 2-wire) | 4–20 mA loop-powered (MU-Analog-65-2L) |
| Ex approval (RW65-Ex-II) | ATEX II 1 GD Ex ia IIC/IIIC T4 Ga Da |
| ISO certification | DIN EN ISO 9001:2015 |
09 — Overload protection
DZ1 and DZ2 — optional protection against pressure surges
Every ring balance has a mechanical end stop. If the differential pressure exceeds the measuring range, the ring reaches that stop. DZ1 and DZ2 are optional factory-fitted accessories — useful when occasional pressure surges are expected in the application.
When the ring is on its stop and pressure continues to rise, the sealing fluid level climbs. As soon as it reaches the inlet of the DZ tube, oil is redirected to the opposite chamber — or air passes directly from one chamber to the other. The ring stays on its stop; once pressure returns to the measuring range, measurement resumes normally.
Fig. · Bypass open: excess pressure can escape · Rixen Messtechnik
DZ1 — single-sided
One diagonal tube connects one chamber to the other. Protects against occasional pressure surges in one direction — either overpressure or underpressure. Suitable when peaks are expected from one side only, for example in exhaust air systems or clean-room supply.
DZ2 — double-sided
Two crossed tubes, one for each direction. Protects against surges in both directions. Recommended when pressure can reverse — for example during start-up and shut-down phases or in processes with alternating pressure conditions.
⚠ The DZ is designed for occasional pressure surges in the region of the overload limit — not for sustained or extreme overpressure. If pressure significantly exceeding the measuring range is applied, the sealing fluid will be expelled regardless of the DZ. Correct instrument selection requires that operating pressure remains well within the measuring range.
DZ1 and DZ2 must be specified at the time of order and cannot be retrofitted.