What Is a Data Logger for Temperature?

What Is a Data Logger for Temperature?

ByKen

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What Is a Data Logger for Temperature?

A temperature data logger is a small, battery-powered instrument that measures temperature at set times and stores each reading in its own memory. Unlike a thermometer you read by eye, a logger keeps a record that you can review later, even when nobody was watching the process at the time. You can later download that record to a computer and review how temperature changed over hours, days, or weeks. That makes loggers useful for cold storage, food handling, shipping, building checks, and process work where someone needs proof of what happened while nobody was watching.

Quick answer

A data logger for temperature is a small battery-powered instrument that measures temperature at a set interval and stores each reading with a time stamp. You choose the sensor type, the recording interval and the memory, then download the record to a computer. Settings decide how long the run can last and how fast events are captured. Models differ in range and accuracy, so check the datasheet figures before you rely on the data. For related reading, see digital temperature and humidity meters and multimeters with temperature.

This guide explains how a temperature data logger works, which settings control what it records, and what limits to check before you rely on the data. It is a general explainer. Specific ranges, accuracy, and memory depend on the model, so read the datasheet for the logger you plan to use.

How a temperature data logger works

A logger has three basic parts: a sensor, a clock, and memory. The sensor produces a signal that changes with temperature. The internal electronics convert that signal into a temperature value. At each recording interval, the logger reads the value and writes it to memory with a time stamp. The logger then waits until the next interval and repeats the process.

Fluke describes its temperature data acquisition products as a mix of a thermometer, a multimeter, and a data logger. In general terms, they scan one or more sensors, perform calculations, convert the readings into engineering units, and store or transmit the data for analysis. A simple handheld temperature logger does the same job on a smaller scale.

Sensor types

The sensor is the part that most affects accuracy and range. Fluke’s data acquisition guidance groups common temperature sensors into three types. Thermocouples are robust, inexpensive, and cover a wide measurement range, which suits high-temperature processes. Resistance temperature detectors (RTDs), also called platinum resistance thermometers (PRTs), give strong accuracy and stability over a moderate range, and they are common in calibration and lab work. Thermistors are very sensitive at low temperatures and suit precision monitoring near room temperature.

Many everyday loggers use an internal sensor, which measures the temperature of the air inside the logger’s housing and the air around it. Others accept an external probe, which is placed in the material or space you want to monitor. Fluke’s 961 series, for example, has an internal sensor in the 961A, an external sensor for temperature only in the 961B, and an external sensor for temperature and humidity in the 961C. Choosing the right sensor location is often more important than choosing the brand.

Recording interval

The recording interval is the time between readings. A shorter interval captures quick changes but fills memory faster. A longer interval uses less memory and can run for longer, but it may miss short spikes. Setting the interval is a trade-off you make before the recording starts.

Interval ranges vary by model. The Extech RHT20, a humidity and temperature logger, lets you program the sampling interval from 1 second to 24 hours. The Fluke 961 series logs at intervals from 10 seconds to 24 hours. A quick interval is useful when you need to see a door opening or a compressor cycling, while a longer one suits a week-long storage check.

Memory and how long you can record

Memory sets how much data the logger can hold. Once memory is full, the logger either stops or overwrites old data, depending on its design, so check that before you start. The total recording time is the number of readings the memory holds multiplied by the interval. For example, a logger that stores 16,000 points and records once a minute will hold about eleven days of data. The same logger recording once every 15 minutes will hold about 166 days.

Memory specs differ by model. The Extech RHT20 lists 16,000 points, with up to 16,000 humidity and 16,000 temperature readings. The Fluke 961 series lists 32,000 data sets. Read the memory figure on the datasheet and do the math for your planned interval before you start a long run.

Accuracy and measurement range

Accuracy describes how close a reading is to the true temperature, and it is given as a tolerance. It can change across the measuring range. The Extech RHT20 lists a temperature range of -40 to 158 degrees F (-40 to 70 degrees C), with accuracy of plus or minus 1.0 degree C between -10 and 40 degrees C, and plus or minus 2.0 degrees C in other ranges. The Fluke 961A lists a range of -30.0 to 70.0 degrees C with plus or minus 0.4 degrees C accuracy. The 961B, with an external probe, lists a wider range of -85.0 to 150.0 degrees C, with accuracy given as plus or minus (0.3 + 0.005 times the reading) degrees C.

These figures show why you should match the logger to the job. A logger that is accurate at room temperature may be less accurate at the cold end of a freezer range. Check the accuracy at the temperature you actually need.

Downloading and reading the data

Most loggers store data internally and send it to a computer over USB. The Extech RHT20 connects to a PC over USB and uses included software to retrieve the readings. The software download requires a login on the manufacturer’s site. The Fluke 961 series can be read over USB, and its USB interface lets you read a PDF report from a PC without software, while Excel and TXT exports require the Flukelog PC software. Fluke’s data acquisition page also notes that its systems use software such as Fluke DAQ to set up and read the data.

Once downloaded, the data is a list of time stamps and values. Plot it on a chart to see the trend, and check for gaps or flat lines. A flat line can mean the logger stopped, the sensor came loose, or the value truly did not change. Look at the start and end times to make sure the record covers the period you care about.

Where loggers are used

Data loggers appear in many settings. Cold-chain and cold-storage teams use them to show that a refrigerator or freezer stayed within a range over time. Shipping teams place them in packages to record temperature along a route. Building and indoor air checks use them to track rooms, storage areas, or equipment spaces. Process and manufacturing teams use them to profile ovens, furnaces, and other heat processes, where the logger travels through the process with the product. The sensor, interval, and memory choices change from one setting to the next, but the basic idea is the same.

Common mistakes

The most common mistake is placing the sensor where it does not represent the product or space you care about. An internal sensor in a logger inside a box measures the air in the box, which may not match the temperature of the item inside. Another mistake is choosing an interval that is too long to catch the event you need. A third is running out of memory before the test ends, because the interval and memory were not matched to the run length.

It also helps to start the logger and confirm its time before you place it, and to note where each logger sits. Without that record, a set of readings is hard to interpret later.

What we could not verify

The sources above describe the logger types, sensor options, interval ranges, memory figures, and download methods for the named models. They do not set a general acceptance limit for stored temperatures, a standard for how long a record must be kept, or a rule for which sensor to use in every regulated setting. Those rules depend on the product, the industry, and local requirements, and this guide does not set them. We also did not verify the exact interval or memory figures for any logger not named here.

Summary

A temperature data logger measures temperature at a set interval and stores the readings with time stamps. Its sensor type, sensor placement, recording interval, memory size, and accuracy decide what the record can show. Choose those settings to match the length of the test and the event you need to catch, and check the datasheet figures for the range you will use. Once the run ends, download the data and review the trend, looking for gaps and flat lines that may signal a problem with the logger or the placement.

How we chose these sources

The logger explanation is based on the Extech and Fluke product and data acquisition pages listed below. This guide does not set storage or acceptance rules for any regulated use.

Sources

FAQ

How long can a temperature data logger record?

It depends on memory and interval. Multiply the number of readings the memory holds by the interval. For example, 16,000 points at one reading per minute is about eleven days, using the Extech RHT20 memory figure.

What is the difference between an internal and external sensor?

An internal sensor reads the air around the logger, and an external probe reads the material or space you place it in. The Fluke 961A uses an internal sensor, while the 961B and 961C use external sensors.

How do I read the stored data?

Most loggers download over USB. Fluke’s 961 series can give a PDF report over USB, while Excel and TXT exports need the Flukelog PC software. Plot the record to check for gaps and flat lines.

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