A temperature data logger is a small standalone device that records temperature at set intervals through a shipment or a storage cycle. It carries its own sensor, clock and memory, and needs no network connection while it runs. Open it after the trip and the record is complete: every reading, from the moment it started to the moment it was pulled, sits in memory or on a chip, ready to read out.
Every unit shipping through an insulated shipper or a refrigerated truck can carry one, taped or clipped next to the payload. The logger reports nothing while the shipment moves. It answers one question after the fact: did the product see a temperature it should not have seen, and for how long. That answer is what a quality team acts on, not a driver's word or a dashboard reading. It is one entry in the cold chain knowledge base, alongside the packaging it usually rides inside.
Single-use and reusable loggers
A single-use logger is a flat card or a small molded unit built around one battery and one memory chip, sold to run one trip and then be discarded or returned for data extraction only. It costs little per unit, needs no calibration between trips because there is no next trip, and suits any shipment where the receiving site has no reason to send equipment back. Most single-use programmes run through parcel networks and air cargo cold chain lanes for exactly that reason.
A reusable logger is a heavier unit built to survive years of trips, with a battery that gets replaced or recharged and a housing rated for repeated handling. It costs far more per unit but earns that cost back on a closed loop: a regular lane, a known number of sites, and a process for collecting the logger at the far end and sending it back. Break that loop and a reusable logger becomes an expensive single-use one.
USB, PDF and Bluetooth readout
Three readout methods cover most of the market. A USB logger plugs into a computer, and its software pulls the full record off; this suits a warehouse or a quality department with a fixed reader on hand. A PDF-generating logger prints its own report to a small screen or drops a finished file onto a USB port, so the person receiving the shipment reads a pass or fail result without installing anything. A Bluetooth logger pairs with a phone or tablet and pushes the record to an app, which suits a receiving dock handling shipments from many senders and unable to install a reader for each one.
None of the three read the shipment while it moves. All three store the full time and temperature record locally and hand it over on arrival, as a file, on a screen, or through an app. Reporting in transit, with alarms a person can act on, is a different function, built into powered trackers rather than a standalone logger.
Sample interval and memory limits
Sample interval is how often the logger takes a reading, from every few seconds to once an hour. A short interval catches brief spikes, such as a door left open for two minutes at a transfer point, that a long interval can average out or miss entirely. A long interval stretches the same memory over a longer trip and saves battery, which matters on a multi-week ocean lane or a slow distribution chain.
Memory is finite, and interval and duration trade against each other directly. A logger with room for ten thousand readings covers a ten-day trip at a fifteen-minute interval, or a one-day trip at a one-minute interval, not both. Set the interval before the trip starts, based on the duration and the kind of excursion the product is at risk from, not on a factory default tuned for a different lane.
Placement decides what the data means
A logger records the temperature at its own sensor, nothing more. Strap it to the outside of a box and it reads the truck or the aircraft hold, not the product. Drop it loose in a corner of a refrigerated shipper and it may read an air pocket that never touches the payload. The only reading that means anything is the one taken where the product itself sits, ideally at the point in the load most exposed to the outside temperature, since that point is the one most likely to fail first.
This is why pack-out instructions for a qualified box specify logger placement as precisely as they specify coolant quantity. A logger placed correctly on a qualification run and then placed carelessly on the live shipment produces a record of the wrong thing. The paperwork will still show a pass or a fail. It will not be the pass or fail that matters, because it never measured the payload.
Calibration and start delay
A logger's stated accuracy holds only if it has been calibrated against a known reference within a set period, typically once a year, with a certificate on file naming the reference standard and the date. An expired calibration does not necessarily mean a reading is wrong. It means nobody can say whether it is right, which is the same problem for a reviewer deciding whether to release a shipment.
Start delay is the gap between activating the logger and the trip actually beginning, and every extra minute of it sits inside the record as warehouse air, not transit conditions. Some loggers start on a button press, some on a programmed delay, some only once they detect movement. Whichever method is used, the person packing the shipment needs to know it. A logger started too early or too late shifts the whole record against the true chain of custody.