How to choose a temperature data logger for mapping and monitoring

Specification checklist

• Range: covers the application with margin (a −80 °C freezer needs a probe rated well below −80 °C).
• Resolution: 0.01 °C for calibration-grade work; 0.1 °C is often the limiting uncertainty term.
• Accuracy specification and — separately — the calibration uncertainty the supplier can actually certify.
• Sampling interval range and memory for the study duration at that interval.
• Battery life at the interval and temperature of use.
• Response time; internal vs external probe.
• Time-stamping, clock accuracy and synchronisation between loggers.
• Export: CSV or equivalent with timestamps, values and units explicit; audit trail if used for monitoring.
• Physical: ingress rating, mounting, size for the positions planned.

For mapping studies, uniformity between loggers matters more than absolute accuracy of any one: calibrate them together against the same reference.

Start from the acceptance criteria, not the datasheet

The specification you need is set by the limit you have to police, and the arithmetic runs backwards from it. To demonstrate a ± 2 °C band convincingly, the conventional target is a logger whose expanded uncertainty in service is no more than a quarter of it — 0.5 °C. That number then decides the resolution, because resolution alone contributes 0.29 d and puts a hard floor under everything else.

The specifications people skip and then regret

• Export format. A device that only exports a proprietary binary, or a PDF, is a records problem under ISO/IEC 17025 clause 7.5 and a nuisance in every analysis. Insist on CSV with a declared unit and ISO timestamps.
• Declared unit in the file. A logger that exports bare numbers forces you or your software to infer °C or °F, and inferring it is how confident nonsense gets produced.
• Battery life at the temperature of use, not at 23 °C. A logger rated for twelve months may manage weeks at −80 °C.
• Response time, and whether the sensor is internal or on a probe. It decides what the logger is actually measuring and how long it must equilibrate before a study starts.
• Memory depth against interval and duration. A 72-hour study at one-minute logging is 4,320 readings per device; check the device does not wrap around and overwrite.
• Clock accuracy and whether it can be synchronised. Drift across a set of twelve loggers turns one door event into six.
• Whether the supplier offers calibration at your points, and whether that calibration is accredited for the medium you will use.

For mapping, buy a matched set

A mapping study measures differences between positions, so uniformity across the set matters more than the absolute accuracy of any single device. Buy them together, calibrate them together against the same reference in the same run, and the shared reference error becomes common to all of them and cancels in every gradient the study reports. A mixed collection of loggers accumulated over five years, calibrated at different times by different laboratories, gives away that advantage entirely.