Introduction. This professional converter implements the NIST ftH2O (39.2°F / 4°C) standard with transparent formulas and precision suited for design checks, troubleshooting, and training.
Authoritative Data Source & Methodology
Primary authority: NIST Special Publication 811, Appendix B.9 (2016), conversion factors for “foot of water (39.2 °F)” and for psi.
Exact tables specify: 1 ftH₂O (39.2 °F) = 2,988.98 Pa and 1 psi = 6,894.76 Pa.
Therefore 1 ftH₂O = 2,988.98 / 6,894.76 = 0.4335275 psi and its inverse 1 psi = 2.3066587 ftH₂O.
“Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.”
- NIST SP 811, Appendix B.9 — ft of water (39.2°F) to Pa; psi to Pa.
- Engineering references confirm the same factors used in industry conversion charts.
We assume standard gravity (9.80665 m/s²) and water density at 4°C used by NIST tables. Temperature-dependent density will change results slightly.
The Formula Explained
Hydrostatic pressure: \( P = \rho \, g \, h \)
Where \( \rho \) is water density at 4°C, \( g \) is standard gravity, \( h \) is height of water column.
With units conversion: \( \displaystyle P_{\mathrm{psi}} = \frac{\rho\,g\,(h_{\mathrm{ft}}\times 0.3048)}{6{,}894.76} \)
NIST table equivalence yields constants: \( \displaystyle 1~\mathrm{ftH_2O} = 0.4335275~\mathrm{psi} \quad \text{and} \quad 1~\mathrm{psi} = 2.3066587~\mathrm{ftH_2O}. \)
Glossary of Variables
| Symbol / Field | Meaning | Units |
|---|---|---|
h (Feet of water) | Height of water column at 39.2°F used as a pressure unit | ftH2O |
P (Pressure) | Pressure corresponding to water head | psi |
k (Factor) | Exact conversion from ftH2O to psi | k = 0.4335275 psi/ft |
How It Works: A Step-by-Step Example
Goal: Convert 120 ftH₂O to psi.
- Choose “ftH₂O → psi”.
- Enter
120in the Feet of water field. - Apply \( \text{psi} = h \times 0.4335275 \).
- Compute: \( 120 \times 0.4335275 = 52.0233~\text{psi} \).
- Rounded for display: 52.02 psi.
Frequently Asked Questions (FAQ)
Is 0.4335 psi per foot always correct?
It is correct for the NIST ftH₂O unit (39.2°F, standard gravity). If water temperature or local gravity differs, the factor changes slightly.
How many feet of water are in 1 psi?
Approximately \( 2.3066587 \) ftH₂O per psi.
What about inches of water column (inH₂O)?
1 psi ≈ 27.6799 inH₂O at 39.2°F. Multiply psi by 27.6799 to get inH₂O, or divide inH₂O by 27.6799 to get psi.
Can I account for other temperatures?
Yes, by using \( P=\rho(T)\,g\,h \). This page targets the standard ftH₂O definition; future versions may add a “custom density” option.
Why does some literature show 2.31 ft per psi?
2.31 is a rounded rule-of-thumb for quick mental math. The precise factor is 2.3066587.