Variable Area Meter Accuracy Explained
Understanding rotameter accuracy specs — full-scale vs. reading accuracy, the effect of fluid properties on indicated flow, and how to apply correction factors for non-water fluids.
Full-Scale vs. Reading Accuracy
Rotameter accuracy is typically stated as a percentage of full scale (FS), not a percentage of reading. This distinction matters significantly at low flow rates.
Example: ±2% FS Accuracy on a 0–10 GPM Meter
Practical implication: Rotameters should be sized so that the normal operating flow is between 40% and 80% of full scale. Operating below 20% FS significantly degrades accuracy.
Effect of Fluid Properties
Rotameters are calibrated for a specific fluid (typically water for liquids, air for gases) at standard conditions. When used with a different fluid, the indicated flow must be corrected.
Liquid Correction (Density)
For liquids, the float equilibrium depends on the density difference between the float and the fluid. The correction factor is:
Gas Correction (Density)
For gases, the correction accounts for both specific gravity and operating conditions (pressure and temperature):
Common Correction Factors
| Fluid | SG / Density | Correction Factor | Effect |
|---|---|---|---|
| Water (reference) | 1.00 | 1.00 | No correction needed |
| Sulfuric acid (98%) | 1.84 | 0.74 | Meter reads 35% high |
| Ethanol | 0.79 | 1.12 | Meter reads 11% low |
| Glycol (50%) | 1.07 | 0.97 | Meter reads 3% high |
| Air (reference) | SG = 1.00 | 1.00 | No correction needed |
| Natural gas | SG ≈ 0.60 | 1.29 | Meter reads 22% low |
| Nitrogen | SG = 0.97 | 1.02 | Meter reads 2% low |
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