Recording a household thermometer's reading in an ice-water bath: an offset log per instrument

Goal

Keep, for every thermometer in the household (probe, dial, glass, oven, room), a dated record of what it reads in an ice-water bath, so that readings logged elsewhere can be read with the instrument's offset beside them and so that a change in the offset over months is visible.

Prerequisites

NIST describes the ice melting point as a verification point: when ice and water are packed together in an insulated container the mixture is at nearly 0 °C. Its preparation notes: water that is distilled, de-ionised or purified by reverse osmosis for both the ice and the water; ice pieces no bigger than about 1 cm; the container packed so that there is an ice-water mixture from top to bottom; the thermometer clean, immersed at least 10 cm to 15 cm where possible, with the tip at least 2 cm from the wall and about 5 cm from the bottom. NIST's SI page states that zero degrees Celsius is 273.15 K. A household that departs from any of this (tap-water ice, a short probe) records the departure rather than pretending to the NIST bath.

Steps

1. Header per thermometer: type, model, display resolution, probe length, and the container used.
2. Prepare the bath as above and note the ice source (distilled or tap), the container (insulated cup or flask) and the room temperature.
3. Insert the thermometer to the stated depth without touching the walls; do not stir with it. Start a timer.
4. Record the reading every 30 s until three consecutive readings are identical; record that final reading and the time it took to settle.
5. Remove, dry, wait until the display shows room temperature, and repeat once. Record both readings.
6. Enter the difference from 0 °C as the offset of the day in a separate column; keep the raw readings.
7. Repeat the sheet on a fixed cadence (for example quarterly) and after any drop or battery change, each time as a new row.

Expected result

A per-instrument table of offsets with dates, settle times and preparation notes, from which one can see whether an instrument's reading at 0 °C is stable, drifting, or simply coarser than its display suggests.

Limits and test basis

Proposed protocol; no offset or drift is claimed. A single point near 0 °C says nothing about the reading at other temperatures; NIST notes that a single-point ice check is a verification for narrow-range use. Tap-water ice and short immersion move the bath away from the NIST conditions by an amount this protocol does not quantify. The protocol records offsets; it does not adjust instruments and makes no statement about food, medical or safety use.