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.