A lactometer does not “prove purity” by itself. It estimates density. Combined with temperature correction and a few kitchen checks, it is the fastest household audit for the Purity Trust Score.
What you need
A calibrated lactometer (Quevenne or Indian ISI type), a tall glass or steel cylinder, a thermometer, and milk that has been mixed — cream rises, and an unstirred sample reads falsely thin or thick.
Let the sample sit 2–3 minutes after pouring so foam collapses. Foam lifts the lactometer and inflates the reading.
Take the lactometer reading (LR)
Lower the instrument gently. Do not drop it. Read at the milk meniscus, at eye level. Typical cow milk at 15°C sits near 28–32 LR; buffalo often reads higher because of solids.
If the stem is dirty or the glass is scratched, discard the instrument. A 1-degree error moves households into the wrong trust band.
Correct for temperature (CLR)
Lactometers are usually calibrated at 15.5°C or 20°C — check the printed mark. For every 1°C above the calibration temperature, add 0.2 to the observed LR. For every 1°C below, subtract 0.2. The result is the Corrected Lactometer Reading (CLR).
Example: observed LR 28.0 at 20°C on a 15.5°C instrument. Difference ≈ 4.5°C × 0.2 = 0.9. CLR ≈ 28.9.
Use the ready chart below instead of doing the arithmetic each morning. If your stem says 20°C calibration, subtract 0.9 from every “Correction” value in the 15.5°C table (or use (T − 20) × 0.2).
Temperature correction chart (instrument calibrated at 15.5°C)
CLR = observed LR + correction. Field screening only — not a NABL certificate. Round to one decimal.
| Milk temp (°C) | Correction to LR | How to apply |
|---|---|---|
| 10 | −1.1 | Subtract 1.1 from LR |
| 12 | −0.7 | Subtract 0.7 from LR |
| 15.5 | 0.0 | No change (calibration point) |
| 18 | +0.5 | Add 0.5 to LR |
| 20 | +0.9 | Add 0.9 to LR |
| 22 | +1.3 | Add 1.3 to LR |
| 25 | +1.9 | Add 1.9 to LR |
| 28 | +2.5 | Add 2.5 to LR |
| 30 | +2.9 | Add 2.9 to LR |
| 32 | +3.3 | Add 3.3 to LR |
| 35 | +3.9 | Add 3.9 to LR |
Typical CLR density bands (after temperature correction)
Bands vary by breed and season. Consistency for the claimed type matters more than one absolute number. Pair with strips.
| Claimed type | Typical CLR band | Red-flag screen |
|---|---|---|
| Cow / toned-style claim | ≈ 28 – 32 | Repeated CLR < 26 on “full cream” talk |
| Buffalo / richer claim | ≈ 30 – 34+ | Repeated CLR < 28 with thick sales talk |
| Full-cream marketing | Stay in type band above | Clean strips + collapsing CLR = classic water story |
Estimate Total Solids (TS)
A widely used field approximation in Indian collection centres is: TS% ≈ CLR/4 + 0.2 × Fat% + 0.14 (local SOPs vary; treat this as a screening formula, not a lab certificate).
If you do not have a fat test, log CLR + boiling behaviour (skin thickness, leftover residue) as the household audit. PTS crowdsourced weight is 40% and expects CLR, not a full Gerber fat test.
How we score it on DoodhwalaRating
CLR consistently at or above the local cow/buffalo band, with no urea/starch/detergent hits, supports a high purity tier. Repeated CLR below ~26 on claimed “full cream” supply is a red flag for dilution.
Always pair the number with smell, delayed curdling, and soapiness. Density can be gamed; strips and boiling tests catch what a lactometer misses.