3.2 Temperature Sensors

Temperature Sensors

Temperature is one of the most frequently measured process variables in industry. Whether monitoring an exothermic chemical reaction, controlling a distillation column, or ensuring food safety during pasteurization, accurate temperature measurement is critical. Industrial temperature measurement primarily relies on electrical sensors, specifically Resistance Temperature Detectors (RTDs), Thermocouples, and Thermistors, as well as non-contact methods like Pyrometers.

Resistance Temperature Detectors (RTDs)

RTDs operate on the principle that the electrical resistance of certain metals changes in a predictable, highly linear manner with temperature. As the temperature of the metal increases, its resistance increases (a positive temperature coefficient).

Pt100 Characteristics

The most common industrial RTD is the Pt100. "Pt" stands for Platinum, the metal used for the sensing element due to its exceptional stability, linearity, and resistance to corrosion. "100" indicates that the sensor has a nominal electrical resistance of exactly 100 ohms at 0°C (32°F).

Platinum RTDs conform to standard curves defining their resistance-temperature relationship, defined by the alpha coefficient ($\alpha$). The industry standard alpha for a Pt100 RTD is 0.00385 ohms/ohm/°C. This means that for every degree Celsius increase in temperature, the resistance of a 100-ohm platinum element increases by 0.385 ohms. For example, at 100°C, a Pt100 RTD will have a resistance of 138.5 ohms.

Lead Wire Resistance Compensation

Because RTDs measure tiny changes in resistance, the electrical resistance of the copper lead wires connecting the sensor to the transmitter can introduce significant measurement errors. If a lead wire has 2 ohms of resistance, the transmitter will read the sensor as being roughly 5°C hotter than it actually is. To mitigate this, different wiring configurations are used:

1. 2-Wire RTD: The simplest configuration. Lead resistance adds directly to the measurement unless the system characterizes or otherwise compensates it, so suitability depends on lead resistance and the required uncertainty.
2. 3-Wire RTD: The most prevalent configuration in industrial settings. It uses three wires (usually two red and one white). A 3-wire measuring circuit compares the lead paths and cancels their resistance when the corresponding leads are closely matched. Unequal lead resistance remains as error, which is why the same cable type, length, gauge, and terminal quality matter.
3. 4-Wire RTD: Provides true lead wire compensation and the highest accuracy. The transmitter supplies a constant current through two leads and measures the voltage drop across the sensor using the other two leads. Since the voltage measurement circuit has high impedance, virtually no current flows through the sensing leads, eliminating any error caused by the lead wires.

Thermocouples

Thermocouples are the workhorses of industrial high-temperature measurement. They are rugged, inexpensive, and capable of measuring extreme temperatures where RTDs would melt.

The Seebeck Effect

Thermocouples operate on the Seebeck effect. In 1821, Thomas Seebeck discovered that when two dissimilar metal wires are joined at both ends to form a loop, and a temperature difference exists between the two junctions, a small thermoelectric voltage (measured in millivolts, mV) is generated in the loop. In practice, one junction is the "hot junction" (measuring the process), and the other is the "cold junction" (at the transmitter or control system).

Common Thermocouple Types

Different pairs of metals generate different millivolt profiles and are suited for different temperature ranges and atmospheres.

• Type J: Iron (+) and Constantan (-). Suitable for vacuum and reducing atmospheres. Has a limited upper temperature range due to the iron oxidizing.
• Type K: Chromel (+) and Alumel (-). The most common general-purpose thermocouple. It has a wide temperature range and is highly resistant to oxidation at high temperatures.
• Type T: Copper (+) and Constantan (-). Excellent for cryogenic and low-temperature applications (-200°C to 350°C). It is highly stable and resists moisture.
• Type E: Chromel (+) and Constantan (-). Provides the highest millivolt output per degree of temperature change, making it highly sensitive.
• Type N: Nicrosil (+) and Nisil (-). Designed to be an improved Type K, offering better stability and resistance to high-temperature oxidation.
• Type S, R, B: Made of Platinum and Platinum-Rhodium alloys. These are noble metal thermocouples used for extreme high-temperature applications (up to 1700°C), such as glass manufacturing and metal smelting. They are expensive but highly stable.

Cold Junction Compensation (CJC)

A thermocouple measures the temperature difference between the hot junction and the cold junction. To know the absolute temperature at the hot junction, the transmitter must know the exact temperature of the cold junction (where the thermocouple wires connect to the transmitter terminals). This is achieved using Cold Junction Compensation (CJC). The transmitter incorporates an internal temperature sensor (like a thermistor or RTD) right at the terminal block to measure the ambient temperature. It then electronically adds the corresponding millivolt equivalent to the raw thermocouple signal before calculating the final process temperature.

Extension Wires

When extending a thermocouple circuit, you cannot use standard copper wire, as this would create unwanted secondary junctions that alter the millivolt signal. You must use designated thermocouple extension wire, which is made from the same (or similar thermoelectric) materials as the thermocouple itself.

Thermistors and Pyrometers

Thermistors (Thermal Resistors) are highly sensitive semiconductor devices. Most industrial thermistors are NTC (Negative Temperature Coefficient), meaning their resistance decreases dramatically as temperature increases. While they offer incredibly high sensitivity over narrow temperature ranges, their response is highly non-linear, limiting their general industrial use compared to RTDs.

Pyrometers measure temperature without physical contact by analyzing the thermal radiation (infrared or optical light) emitted by an object. Infrared pyrometers are ideal for moving targets (like a conveyor belt of baked goods), corrosive processes, or extremely high temperatures where physical sensors would be destroyed.

Thermowells

Temperature sensors are rarely inserted directly into the process fluid. Instead, they are housed inside a thermowell—a sealed metallic tube protruding into the pipe or tank. The thermowell protects the delicate sensor from high pressures, corrosive chemicals, and physical damage from flowing fluids, while allowing the sensor to be removed or replaced without shutting down the process.

When selecting a thermowell, the technician must balance thermal response time with structural integrity. A crucial engineering factor is the wake frequency. As fluid flows past a cylindrical thermowell, it creates alternating vortices (Von Kármán vortex street) on the downstream side. This causes the thermowell to vibrate. If the frequency of these shedding vortices matches the natural resonant frequency of the thermowell, catastrophic mechanical failure can occur due to resonance. Therefore, calculating wake frequency limits is essential for safe thermowell insertion length and diameter design.

Summary Table: Temperature Sensors