Air: temperature at the server inlet
The temperature that matters is the air entering the servers, not the average of the room. The ASHRAE thermal guidelines for data processing environments recommend an inlet temperature of 18 to 27 °C for air-cooled equipment, with wider allowable ranges by equipment class (15 to 32 °C for class A1). Facilities measure it with rows of sensors at rack inlets: thermistors and digital sensors on the rack, PT100 or PT1000 probes where the signal goes to the building management system.
A 10 kΩ thermistor with β = 3950 K is at 10 kΩ at 25 °C and changes by roughly 4 % per degree near room temperature, which is why it is the usual choice for cheap, dense monitoring. The NTC calculator converts resistance and temperature for any part; take the β value from the datasheet, since a wrong one moves every reading away from 25 °C.
Humidity is a dew point question
Relative humidity alone is a poor control variable, because it changes with temperature while the amount of water in the air stays the same. What decides whether water condenses is the dew point. Air at 24 °C and 50 % relative humidity has a dew point of 12.9 °C; at 27 °C and 60 % it is 18.6 °C. Any surface colder than the dew point collects water, which is the practical limit on how cold the facility water for rear-door or direct liquid cooling may be supplied without insulation or control. The current edition of the ASHRAE guidelines gives the humidity limits for each class in terms of dew point and relative humidity. The dew point calculator converts between them and reports absolute humidity as well.
Water and coolant: temperature difference and pressure
Cooling capacity is calculated from flow and the temperature difference between supply and return, often only 5 to 10 K, so an error of a few tenths of a degree in either sensor matters. Over the cable lengths of a plant room, how the resistance thermometer is wired decides the result. A PT100 on 80 m of 0.5 mm² cable reads about 14 °C high if connected with two wires and about 0.35 °C high with three wires and a typical 5 % mismatch between the leads; with four wires the leads drop out. The RTD lead-wire error calculator shows the error and the longest cable for a given budget.
Liquid-cooling loops add pressure measurements: loop pressure, and differential pressure across filters and heat exchangers as a sign of fouling. A transmitter with a 10 bar upper range limit calibrated for 0 to 6 bar does not keep its datasheet accuracy as a percentage of that span, because temperature effect and drift are specified against the upper range limit. The transmitter accuracy calculator combines the terms.
Air management uses very small differential pressures between the raised floor, the cold aisle and the hot aisle, a few tens of pascals: 25 Pa is 0.1004 inH₂O. The pressure converter handles the units.
Loops, tanks and rotating machines
Sensors in a large hall are far from the controller. A two-wire 4–20 mA transmitter needs its minimum voltage at the terminals at the highest current it can drive, not at 20 mA. With a 24 V supply, a 250 Ω input and 400 m of 0.5 mm² cable, the cable adds 27.5 Ω and a transmitter needing 12 V keeps a margin of 5.9 V at 22 mA; the loop power calculator checks any combination, including a warm cable tray.
Generator day tanks and water storage are measured by hydrostatic pressure. Two metres of diesel at 840 kg/m³ is 164.8 mbar; the density varies with the fuel and its temperature, and the reading with it. The level to pressure calculator gives the range to order.
Pumps, fans, chillers and generators are monitored for vibration. Severity is judged as RMS velocity in mm/s against the zones of ISO 10816-3 and its successor ISO 20816-3, while accelerometers report in g; the vibration converter converts between them at a given frequency.
Outside the scope of this site
Electrical power and energy metering, leak detection, airflow measurement and the control strategy itself are not covered here. The calculators convert and check sensor-level quantities; they do not size a cooling plant.
Updated October 11, 2026