Search Product Finder

Products

Video

Data Acquisition

Accessories

Search

Corporate

Support

Contact

Sales

New s

Employment

Sensoray

7313 SW Tech Center Dr.

Tigard, Oregon 97223

503.684.8005

Comparison of Thermocouples, RTDs, and Thermistors

Measuring temperature electrically

Electrical temperature measurement is used in a wide varietyof industries. From melting steel to baking cookies, the rangeof temperatures can vary greatly and the type of sensorrequired to measure these temperatures must be chosenaccordingly.

In industrial and embedded environments, it is not alwayspractical to measure temperature locally. Often, temperaturemust be measured remotely at one location and then relayedback to a computer for processing and recording. This is oftenaccomplished by connecting thermocouples, RTDs, andthermistors to a remote measurement system (such asSensoray's model 2600 Ethernet data acquisition system, oran easily embedded interface such as the model 518 PC/104intelligent sensor interface) that will measure the sensorsignals and send the resulting data to the computer.

Thermocouples

A thermocouple is a temperature-sensing element that converts thermal energy directly into electrical energy. Inits basic form it consists of two dissimilar metallic conductors connected in a closed loop. Each junction forms athermocouple. If one thermocouple is maintained at a temperature different from that of the other, an electricalvoltage proportional to this temperature difference will be produced by the circuit. Thermocouples areinterchangeable, cheap, have standard connectors, and can measure a wide range of temperatures. The mainlimitation of thermocouples is their accuracy; system errors of less than 1°C can be difficult to achieve.

Thermoelectric Effect

In 1822, physicist Thomas Johann Seebeck accidentally discovered that when any conductor is subjected to athermal gradient, it will generate a voltage. Any attempt to measure this voltage involves connecting anotherconductor to the "hot" end. This additional conductor will then also experience the temperature gradient, anddevelop a voltage of its own which will oppose the original; the magnitude of this effect depends upon the metal inuse. If there is the same temperature at the two junctions there is no flow of current since the partial voltagesproduced at the two points cancel each other. Using a dissimilar metal to complete the circuit will have adifferent voltage generated, leaving a small difference voltage available for measurement, which increases withtemperature. This difference can typically be between 1 to 70 µV/°C for the available range of metalcombinations. Certain combinations have become popular as industry standards, driven by cost, availability,convenience, melting point, chemical properties, stability, and output.

Connecting Thermocouples

When choosing a thermocouple, consideration should be given to the insulation, thermocouple type, and probeconstruction.

RTDs

Resistance Temperature Detectors (RTDs), also referred to as platinum resistance thermometers (PRTs) orresistance thermometers, are temperature sensors that change resistance at a predetermined rate in responseto variation in temperatures. RTDs are rapidly replacing thermocouples in many industrial applications below600°C due to their higher sensitivity and accuracy over thermocouples.

RTD Types

Film thermometers

Film thermometers have a thin layer of platinum (as thin as 1 µm) on a substrate. Advantages of this

type of RTD are relatively low cost and fast response. Such devices have improved in performance

although the different expansion rates of the substrate and platinum give "strain gauge" effects and

stability problems.

Wire-wound thermometers

Have greater accuracy, especially for wide temperature ranges. The coil diameter provides a compromise

between mechanical stability and allowing expansion of the wire to minimize strain and consequential

drift.

How do RTDs work?

Resistance thermometers offer greater stability, accuracy and in some cases repeatability than thermocouples.While thermocouples use the thermoelectric effect to generate a voltage, RTDs require a power source tooperate and use electrical resistance. Ideally, the resistance will vary linearly with temperature.

Resistance thermocouples are most often made using platinum, due to its linear resistance-temperaturerelationship as well as its chemical inertness. The platinum detecting wire needs to be guarded fromcontamination to remain stable. Commercial platinum grades are produced which exhibit a change of resistanceof 0.385 Ω/°C (European Fundamental Interval) The sensor is usually made to have a resistance of 100 Ω at 0°C.

Resistance thermometers require a small current to be passed through in order to determine the resistance. Thiscan cause self-heating so it is important to minimize the current to reduce self-heating errors. Care should alsobe taken to avoid any strains on the resistance thermometer in its application. Lead wire resistance should beconsidered, and adopting three and four wire connections can eliminate connection lead resistance effects frommeasurements.

RTD Wiring Configurations

Two-wire configuration

The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is

not required as the resistance of the connecting wires is always included with that of the sensor, thus

leading to errors in the signal. This applies equally to balanced bridge and fixed bridge systems. The

leading to errors in the signal. This applies equally to balanced bridge and fixed bridge systems. The

values of the lead resistance can only be determined in a separate measurement without the resistance

thermometer sensor and therefore a continuous correction during the temperature measurement is not

possible.

Four-wire configuration

The four wire resistance thermometer configuration further increases the accuracy and reliability of the

resistance being measured. A standard two terminal RTD is used with another pair of wires. One pair

carries the excitation current while the other pair carries the resulting RTD voltage back to the

measurement system. No current flows in the second pair, so lead resistance has no effect on accuracy.

Thermistors

Thermistors are a type of resistor with an electrical resistance that possesses either a negative or positivetemperature coefficient of resistivity. Thermistors are composed of solid semiconducting materials with aresistance that decreases 4% per °C. They are constructed in a variety of sizes and may be obtained withthermal time constants of a millisecond or less. Thermistors produce a non-linear voltage and because of this arelimited to a useful temperature span of only about 100°C.

Thermistors are the most accurate of the temperature sensors, ∼±0.02°C, as well as the most sensitive. Theirresponse time is short in relation to RTDs, and about the same as thermocouples.

Connecting Thermistors

The resistance of thermistors is normally several orders of magnitude greater than any lead resistance. The leadresistance therefore, has a negligible effect on the temperature reading and thermistors are almost alwaysconnected in a 2-wire configuration.