Basic Thermometry Concepts: Speed

Speed, or "response time" as it is more often called, is another very important consideration when choosing a thermometer. Some thermometer technologies are faster than others and, depending upon the application, those additional seconds or fractions of a second can make all the difference.

Response time can be affected by many things, including the position of the sensor relative to the substance being measured, the mass of the sensor itself, the speed of the processor doing the calculations, the length of the wiring between the sensor and the processor and the type of technology used. In general, electronic thermometers are faster than mechanical thermometers (like liquid mercury or dial thermometers), thermocouple sensors are faster than resistance technologies (like the thermistor or the RTD) and reduced tip probes are faster than standard-diameter probes (because the sensor is closer to the material being measured and the mass of the sensor is smaller and therefore more responsive to changes in temperature).

Time Constants
In technical catalogs and websites, including ThermoWorks.com, response time is often listed in increments called "time constants." It can be a little confusing, but one time constant is the time it takes for a given instrument to get to 63% of a full reading. The practical equivalent of 100% full reading takes five time constants.

So, for example, if a technical spec table lists a given probe as having a time constant of 0.5 seconds, you can expect to get a full reading with that probe at 2.5 seconds (or five times the listed time constant). This is important to remember, so you are sure to be comparing apples to apples when comparing instruments with different specifications or brands.

Commercial claims like the one on our our Super-Fast Thermapen packaging—"reads to within 1°F of final temperature of an ice bath in 3 seconds"—are full reading claims. The technical "response time" of the Thermapen is 0.6 seconds, or 3 seconds divided by five. Don't be misled by competitive thermometers that claim a "response time" of 3 seconds "like a Thermapen" when their actual time to a full reading is 15 seconds. Technical specs—like those on our probe selection pages—use "time constants" and will need be multiplied by five in your head to approximate the time for a full reading.

Another number that can be misleading is the "reading update rate." This number refers only to the frequency with which the digital processor of a thermometer samples the sensor. The Super-Fast Thermapen has an update rate of 0.5 seconds. That means that the digital display will show changes in the temperature as measured by the sensor every half second but it has nothing to do with the speed with which the sensor will adjust to the temperature of the material being measured.

Finally, as with accuracy, the real response time of an instrument is actually quite variable depending upon the particular substance and the range of temperatures being measured. Spec tables give outside limits, not exact speeds.

It's also important to remember that, just as with accuracy, the total response time of a system (i.e. meter and probe) may well be the aggregate of the response times of the individual components (i.e. the meter response time plus the probe response time). Again, that's one of the things that makes integrated systems like the ThermoWorks Thermapen and the ThermoWorks FoodCheck appealing is that the response times listed are composite.