Data logger calibration: how temperature loggers are calibrated

A data logger's reading is the output of its sensor, its analogue electronics and its firmware. Calibrating the sensor alone would miss the rest, so loggers are normally calibrated as a whole, reading their logged values against a reference thermometer placed alongside them.

Typical approach

• Environmental chamber (or bath for probe-type loggers) held at each calibration point until stable.
• Reference thermometer close to the loggers, at the same height and airflow.
• Calibration points spanning the use range — for 2–8 °C use, typically around 2, 5 and 8 °C or wider.
• Sufficient dwell time for the logger's response, then several logged readings compared with the reference.
• Corrections and uncertainty reported per point.

Watch for

Chamber uniformity and stability dominate the uncertainty; the logger's own resolution (often 0.1 °C) and response time matter; and the calibration must be at the points the logger will be used at — a calibration at 25 °C says little about 4 °C.

Calibrate as a system, in the medium it lives in

A logger's reading is the output of a sensor, some analogue electronics and firmware, and each contributes. Calibrating the bare sensor characterises a third of the instrument. This is why loggers are calibrated whole, reading their own logged values rather than an external measurement of their probe.

It also decides the medium. A logger with an internal sensor measures the air around its body, so it is calibrated in a chamber where the air around its body is controlled. Putting it in a dry-block well characterises a snug metal hole it will never occupy, at an uncertainty the well fit does not actually support. Probe-type loggers are the exception and belong in a bath, at the immersion they will be used at.

Response time is where studies go wrong

Loggers are slow. A body-mounted sensor in still air can take 15 to 30 minutes to reach 90 % of a step, and a buffered probe in glycol far longer by design. If readings are taken before the logger has settled, the error appears as a calibration correction and gets carried into every measurement afterwards. Wait for a stable trace rather than for a timer, and record the stabilisation time used.

For a mapping study, calibrate them together

• Uniformity between loggers matters more than the absolute accuracy of any one, because a mapping study is fundamentally a set of differences between positions.
• Calibrating the whole set against the same reference in the same run makes the shared reference error common to all of them, so it cancels in every gradient the study reports.
• Calibrate at the points the study will police — 2 °C and 8 °C for a cold room, not 25 °C because it is convenient to generate.
• Do a post-study check as well as the pre-study calibration. It replaces an estimated drift term with a measured as-found error, and it catches the logger that failed silently halfway through.
• Record which logger is which. A calibration set with no mapping to the sensor identifiers in the data file is unusable.