1. Why Reference Everything to 60 °F?
Density is temperature-dependent: a warm liquid is less dense, so a hydrometer floating in it reads a higher API gravity than the true value, and a cold liquid reads lower. To compare densities, settle custody tickets, and apply the petroleum measurement tables, every density must be expressed at a single reference temperature. In US practice that reference is 60 °F (15 °C internationally).
2. The Hydrometer Method (API MPMS 9.1)
API MPMS Chapter 9.1 (ASTM D1298) is the field/lab method for measuring density, relative density or API gravity with a glass hydrometer. The sample is brought to a stable temperature, the hydrometer reading and temperature are recorded, and corrections are applied for the meniscus and for the thermal expansion of the glass hydrometer itself (which is calibrated at the reference temperature). The temperature-corrected reading then feeds the reduction to 60 °F.
Read
Reading + Temp
Record the scale value and the observed temperature together.
Correct
Glass & Meniscus
Adjust for hydrometer glass expansion and meniscus per 9.1.
Reduce
To 60 °F
Apply the 11.1 temperature correction to the corrected reading.
3. The Base-Density Iteration (API MPMS 11.1 §11.1.3.5)
The reduction to 60 °F is not a single equation because the thermal-expansion coefficient α₆₀ itself depends on the 60 °F density we are solving for. The standard resolves this with a short iteration based on mass conservation (a fixed mass of liquid occupies less volume — and is therefore denser — at 60 °F than when warm):
Because CTL < 1 for a warm sample, dividing the observed density by CTL increases it toward the (denser) 60 °F value. The commodity coefficients K0/K1/K2 are the same generalized values used in the volume correction factor.
4. Worked Example
A crude oil reads a density of 840.95 kg/m³ at 80 °F. What is its API gravity at 60 °F?
The hydrometer would have read about 35.9 °API at 80 °F; the true 60 °F value is 35.0 °API — a ~0.9 °API correction that materially changes the volume correction factor and the settled quantity.
5. Standards & References
| Standard | Scope |
|---|---|
| API MPMS Ch. 9.1 / ASTM D1298 | Density, Relative Density or API Gravity by Hydrometer |
| API MPMS Ch. 11.1 / ASTM D1250 | Temperature volume correction; base-density iteration §11.1.3.5 |
| API MPMS Ch. 9.3 | Density by digital density analyzer (alternative to hydrometer) |