Hydrometer temperature correction
A hydrometer reads true only at the temperature it was calibrated at. Here is how far off a warm or cold wash reads, and how to correct it.
Updated
In short
Check the calibration temperature printed on the hydrometer, usually 20 °C or 60 °F. A sample warmer than that reads low, and a cooler one reads high. Correct the reading with the formula below, or with the chart: a wash read at 30 °C on a 20 °C hydrometer is about 2.6 points higher than it shows, so 1.050 is really 1.053.
Why temperature changes the reading
A hydrometer floats higher in a denser liquid. A liquid gets less dense as it warms, so the hydrometer sinks a little and the reading drops, though nothing in the wash has changed. It reads true only at its calibration temperature, and the further the sample is from it, the bigger the error.
This guide is for gravity readings of a wash. Spirit is read with an alcoholmeter, which needs its own correction: try the alcoholmeter correction on the home page.
The formula
The usual correction scales the reading by the density of water at the two temperatures. With both temperatures in °F:
corrected = reading × f(sample) ÷ f(calibration)
f(t) = 1.00130346 − 0.000134722124 t + 0.00000204052596 t² − 0.00000000232820948 t³
For °C, convert first: t = °C × 9 ÷ 5 + 32. The fit follows the density of water to within about 0.0002 from 0 to 100 °C.
Correction chart
For a reading of about 1.050. The correction grows a little with the gravity, so treat these as a guide and use the formula for exact work.
| Sample at | Correction | 1.050 becomes |
|---|---|---|
| 10 °C | −1.5 points | 1.048 |
| 15 °C | −0.9 points | 1.049 |
| 20 °C | none | 1.050 |
| 25 °C | +1.2 points | 1.051 |
| 30 °C | +2.6 points | 1.053 |
| 35 °C | +4.3 points | 1.054 |
| 40 °C | +6.2 points | 1.056 |
| Sample at | Correction | 1.050 becomes |
|---|---|---|
| 50 °F | −0.7 points | 1.049 |
| 60 °F | none | 1.050 |
| 70 °F | +1.1 points | 1.051 |
| 80 °F | +2.4 points | 1.052 |
| 90 °F | +4.1 points | 1.054 |
| 100 °F | +6.1 points | 1.056 |
Worked examples
- A wash reads 1.050 at 80 °F on a 60 °F hydrometer: 1.050 × f(80) ÷ f(60) = 1.0524, so about 1.052.
- A starting reading of 1.060 taken at 30 °C on a 20 °C hydrometer: 1.060 × f(86) ÷ f(68) = 1.0626, so about 1.063. Left uncorrected, the strength worked out from it would come out low.
Getting a reading you can trust
- Cool the sample towards the calibration temperature. The formula assumes the sample behaves like water, and a sugary wash expands a little more, so it's best within about 10 °C.
- Spin the hydrometer and let the bubbles go. Gas from a fermenting wash lifts it and reads high.
- Read at eye level, at the flat surface of the liquid rather than the curve climbing the stem, unless your hydrometer's instructions say otherwise.
- Check it now and then in plain water at its calibration temperature. It should read 1.000; if not, allow for the difference.