The RTD is the go-to sensor when a temperature measurement has to be accurate and stable - and in oil and gas that covers most process points, from separators to custody metering. This guide explains what an RTD is, how it turns temperature into resistance, why the wiring configuration matters, and how an RTD differs from a thermocouple.
RTD Sensor in one line: An RTD (Resistance Temperature Detector) is a temperature sensor whose electrical resistance changes predictably with temperature. The most common type, Pt100, is a platinum element that measures 100 ohms at 0 degrees C, and its resistance rises linearly and repeatably as temperature increases.
An RTD exploits a simple physical fact: the electrical resistance of a pure metal rises as it gets hotter. Platinum is the metal of choice because its resistance-versus-temperature curve is highly linear, stable over years, and repeatable between units. A Pt100 element reads 100 ohms at 0 degrees C; a Pt1000 reads 1,000 ohms and is favored where higher resistance reduces lead-wire error. To read the temperature, measurement electronics pass a small, precise current through the element and measure the resulting voltage to derive resistance, then convert that to temperature.
Because the element's resistance is being measured, the resistance of the connecting wires adds directly to the reading as an error - which is why RTD wiring is deliberately designed to cancel it.
A 2-wire RTD is the simplest but least accurate: lead-wire resistance is added straight into the measurement and cannot be separated out, so it is used only on very short runs or where precision is not critical. A 3-wire RTD, the workhorse of industrial installations, uses a third lead so the electronics can measure and subtract most of the lead-wire resistance. A 4-wire RTD fully separates the current-carrying and voltage-sensing paths, eliminating lead-wire error almost entirely - reserved for laboratory-grade and custody measurement.
In practice, most field RTDs feed a nearby temperature transmitter that performs the lead-wire compensation and outputs a clean 4-20 mA or HART signal, keeping the sensitive resistance measurement short and local.
RTDs win on accuracy, stability, and linearity across the moderate temperature ranges typical of process measurement, which is why they dominate separators, heaters, and metering skids. Thermocouples win on temperature range (they read far hotter), ruggedness, faster response, and lower cost, making them the choice for flares, furnaces, and exhaust. The rule of thumb: reach for an RTD when accuracy matters and temperatures are moderate; reach for a thermocouple when it is very hot or a fast, cheap point is enough.
RTD stands for Resistance Temperature Detector - a sensor that measures temperature by the change in electrical resistance of a metal element, most commonly platinum (Pt100 or Pt1000).
A Pt100 is the most common RTD: a platinum element with a resistance of 100 ohms at 0 degrees C. Its resistance rises predictably and near-linearly with temperature, making it accurate and stable for industrial process measurement.
A 2-wire RTD adds the resistance of the lead wires directly into the reading as error. A 3-wire RTD lets the electronics measure and cancel most of that lead-wire resistance, giving a substantially more accurate reading over typical field cable runs.
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