Temperature sensor [/ˈtɛmpərəʧər ˈsɛnsər/] n - Temperature is the quantity most oft measured in a data center, yet the instrument chosen decides what the number truly means. A cheap thermistor, a precision RTD, and an infrared camera all report temperature; they are not interchangeable.

The Kinds Commonly Kept

• Thermistor. [/ˈthɛrmɪstɒr/] n A resistor whose value changes with temperature. Cheap, fast, and suited to a narrow range. Its output is nonlinear, so it asks for calibration or a lookup table.
• RTD (resistance temperature detector). [/ˈrtd ˈrɛsɪstænkɛ ˈtɛmpɛrætʌrɛ ˈdɛtɛktɒr/] n Usually platinum wire. More accurate and stable than a thermistor, but slower and dearer. Reserved for the readings one actually trusts.
• Thermocouple. [/ˈthɛrmɒkæʊplɛ/] n Two dissimilar metals joined at a junction. Very wide range, though the output is a tiny voltage that needs careful amplification. Useful for extreme spots such as exhaust ducts.
• Digital sensors (I2C/SPI). [/ˈdɪgɪtæl ˈsɛnsɒrs ˈɪ2kspɪ/] n Chips like the DS18B20 or BME280 yield a calibrated digital value over a bus. Easier for microcontrollers and Pis, but the sensor and its enclosure bound the accuracy.
• Infrared. [/ˈɪnfrærɛd/] n Non-contact surface temperature. Good for hot spots, though the reading depends upon the emissivity of the surface at which you point it.

What Matters in the Field

• Where you put it. [/ˈwhɛrɛ ˈiæʊ ˈpʌt ˈɪt/] n A sensor wrongly placed is worse than none. Inlet, outlet, and hotspot readings each tell a different story.
• Response time. [/ˈrɛspɒnsɛ ˈtɪmɛ/] n A slow sensor smooths away real spikes. A fast one can be noisy.
• Calibration. [/ˈkælɪbrætɪɒn/] n Sensors drift with time. The field note [[Right sensor, wrong air]] reminds us that the sensor can be right while the airflow is not.

In These Pages

Temperature sensors appear in Raspberry Pi in the datacenter, [[Building custom datacenter devices]], and On Early Detection. See also [[Current transformer]] for the other common sensor in a custom build.