05 · Measurement ranges & flow
Which spans ring balances cover, how accuracy depends on the range — and why differential pressure is the classic method for measuring volume flow.
Real calibration data
Based on a sample of over 10,000 calibration records from transmitters with electrical output (4–20 mA, LCD) from recent years — real field data showing which spans customers actually order.
Source: Rixen Messtechnik · Sample of over 10,000 calibration records from transmitters with electrical output (4–20 mA, LCD) from recent years · Top 15
Available measuring ranges
Rixen ring balances are available from 40 Pa to 1,800 Pa full span. The scale can be offset symmetrically (e.g. −50…+50 Pa) or asymmetrically (e.g. −10…+40 Pa) — the measuring element remains the same.
| Full span | Typical ΔP | Accuracy | Typical application |
|---|---|---|---|
| 40 Pa | 5–25 Pa | ±1,5 Pa | Cleanroom, OR airlocks |
| 60 Pa | 10–40 Pa | ±1,5 Pa | Cleanroom, laboratories |
| 100 Pa | 20–80 Pa | ±1,5 % v. E. | HVAC, filter monitoring |
| 160 Pa | 30–120 Pa | ±1,5 % v. E. | HVAC, duct pressure control |
| 250 Pa | 50–200 Pa | ±1,5 % v. E. | Fans, filters, ventilation |
| 400 Pa | 80–300 Pa | ±1,5 % v. E. | Fans, volume flow measurement |
| 600 Pa | 100–500 Pa | ±1,5 % v. E. | Industrial furnaces, biogas |
| 1.000 Pa | 200–800 Pa | ±1,5 % v. E. | Draft measurement, pressure vessels |
| 1.800 Pa | 300–1.500 Pa | ±1,5 % v. E. | High-pressure filters, compressors |
Ranges < 100 Pa
±1,5 Pa
Fixed absolute accuracy. At 40 Pa full span this represents ±3.75 % FS — but in absolute terms 1.5 Pa is the relevant threshold, not the percentage.
Ranges > 100 Pa
±1,5 % v. E.
Relative accuracy based on full scale. At 1,000 Pa span this is ±15 Pa — still excellent for industrial draft and filter monitoring applications.
Accuracy in practice — spans below 100 Pa
This section applies to spans below 100 Pa. For spans of 100 Pa and above, the specification is ±1.5 % of full scale. Two figures are often cited together without explaining the difference — here is what each one means.
Specification
for spans < 100 Pa
Maximum possible error
This is the worst-case guarantee: at no single point within the measurement range will the error exceed ±1.5 Pa. In practice this maximum typically occurs only at specific points — for example near the endpoints of an asymmetric range — and not uniformly across the entire span.
Typical delivery value
Median error across all calibration points
This is the median of all individual measurement errors across all calibration points from a sample of over 10,000 transmitters delivered in recent years. It shows that most devices perform significantly better than the specification — but it does not mean that every individual point is within ±0.49 Pa. A device with a median of 0.49 Pa can still have individual points up to e.g. 1.3 Pa.
For dimensioning and system design, always use the specification of ±1.5 Pa (or ±1.5 % FS for ranges above 100 Pa) as the basis — not the median. The median shows that Rixen devices typically exceed the specification, but the worst-case guarantee is what counts for safety-relevant applications such as cleanroom pressure cascades.
Range selection
01
What differential pressure occurs during normal operation? What is the conceivable maximum? The range should show the normal value at 50–70 % of scale — not at the upper stop.
02
Pressure spikes during start-up, valve slams or filter changes can briefly reach 2–3× the nominal pressure. The ring balance survives mechanical overload without drift.
03
For cleanroom pressure cascades with 10–20 Pa difference between stages: choose the 40 Pa or 60 Pa range — ±1.5 Pa absolute applies there. For boiler draft measurement ±1.5 % FS at 600 Pa is sufficient.
Volume flow measurement
Wherever flow passes through a constriction — orifice plate, nozzle, Pitot tube, Venturi — Bernoulli's principle creates a differential pressure that is proportional to the square of the flow velocity.
Bernoulli / orifice equation
ΔP — differential pressure [Pa]
Q — volume flow [m³/h or m³/s]
k — device coefficient (geometry, fluid, temperature)
Halving the volume flow reduces the differential pressure to one quarter — not one half. This is the most common source of errors when selecting measurement ranges for flow applications.
| Flow Q [%] | ΔP [Pa] * | Scale position |
|---|---|---|
| 25 % | 6.25 Pa | 3 % |
| 50 % | 25 Pa | 11 % |
| 75 % | 56.25 Pa | 25 % |
| 100 % | 100 Pa | 44 % |
| 125 % | 156.25 Pa | 69 % |
| 150 % | 225 Pa | 100 % |
* Normalised: ΔP = 100 Pa at Q = 100 %
Lower range limit
In the lower 10 % of the flow range, differential pressure drops to just 1 % of full scale (e.g. 2.5 Pa on a 250 Pa instrument). At this level the reading falls within the instrument's hysteresis and accuracy band — results should be treated as indicative only. For reliable volume flow measurement, the minimum operating point should be at least 20–25 % of Qₐₐₘ (corresponding to approx. 4–6 % of full-scale ΔP). If the process regularly runs in this range, select a smaller measurement span.
Because ΔP = k·Q², the ring balance scale is linear in differential pressure — which corresponds to a quadratic relationship when read as volume flow. The transmitter output is also linear to ΔP. To display Q directly, the BMS or SCADA must apply the square root: Q = k·√ΔP.
Linear ΔP scale
The ring balance displays differential pressure linearly. The transmitter output is proportional to ΔP — volume flow Q = k·√ΔP is calculated in the BMS or SCADA.
Compatible with primary elements
Orifice plate, nozzle, Pitot tube or Venturi as primary element — the ring balance as secondary device.
Long-term stable under differential pressure
The mechanical measuring path without diaphragm remains calibration-stable under continuous differential pressure — no drift from diaphragm fatigue.
Practical example
Given
Result
Chosen range: 250 Pa. Nominal flow corresponds to ΔP = 150 Pa (60 % of scale — optimal), minimum flow at approx. 25 % (below 20 % readings become indicative only), peak covered with margin. Accuracy: ±1.5 % FS = ±3.75 Pa at nominal. Volume flow Q = k·√ΔP is calculated in the BMS.