Sensor Times
Monday, February 24, 2025
ARTICLES
 
   

RTD Sensors

 

Resistance Temperature Detectors (RTDs) are one type of temperature sensors. RTD changes the resistance with respect to temperature. It measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance.


The RTD element consists of a thin film of platinum or nickel which is deposited onto a ceramic substrate and laser trimmed to the desired resistance. Thin-film elements attain higher resistances with less metal and, thus, tend to be less costly then the equivalent wire-wound element. RTDs also called resistance thermometer.


Resistance thermometers require a small current to be passed through in order to determine the resistance. This can cause resistive heating, and manufacturers' limits should always be followed along with heat path considerations in design. Lead wire resistance should be considered, and adopting three and four wire connections can eliminate connection lead resistance effects from measurements. Industrial practice is almost universally to use 3-wire connection. 4-wire connection needs to be used for precise application.


Some metals have a very predictable change of resistance for a given change of temperature. A precision resistor is made from one of these metals to a nominal ohmic value at a specified temperature. By measuring its resistance at some unknown temperature and comparing this value to the resistor's nominal value, the change in resistance is determined. Because the temperature vs. resistance characteristics are also known, the change in temperature from the point initially specified can be calculated.

 

Common Resistance Materials for RTDs

 

• Platinum (most popular and accurate)
• Nickel
• Copper
• Balco (rare)
• Tungsten (rare)

 

What are the parameters to select a right RTD sensor?

 

• Sensor material
• Temperature coefficient
• Nominal resistance
• Wiring configuration

 

 

Temperature Range

 

Platinum RTD's can measure temperatures from -200°C to 650°C. You must consider the temperature limitations of all the materials involved, where they are applied, and the temperatures to which each will be exposed.

 

Temperature Coefficient

 

The temperature coefficient (TC), or alpha of an RTD is a physical and electrical property of the metal alloy and the method by which the element was fabricated. The alpha describes the average resistance change per unit temperature from the ice point to the boiling point of water.
RTDs are manufactured from metals whose resistance increases with temperature. Within a limited temperature range, the resistivity increases linearly with temperature. Each metals specific, and unique resistivity, can be determined experimentally. This resistance is directly proportional to a metal wire's length, and inversely proportional to the cross-sectional area.
R = k L / A

where
R = resistance (ohm, O)
k = constant of proportionality or resistivity of the material (ohm, O)
L = length of conductor (m)
A = cross sectional area of conductor (m2)


Resistivity and temperature can be expressed as
kt = ko [1 + a (t - to)]
where
kt = resistivity at temperature t (ohm, O)
ko = resistivity at standard temperature to (ohm, O)
a = temperature coefficient of resistance (1/oC)
t = temperature (oC)
to = standard temperature (oC)

 

Resistance thermometer elements

 

Resistance thermometer elements are available in a number of forms. The most common are:
• Wire wound in a ceramic insulator - wire spiral within sealed ceramic cylinder, works with temperatures to 850 °C
• Wire encapsulated in glass - wire around glass core with glass fused homogenously around, resists vibration, more protection to the detecting wire but smaller usable range
• Thin film - platinum film on ceramic substrate, small and inexpensive to mass produce, fast response to temperature change

 

Resistance thermometer construction

 

 

These elements nearly always require insulated leads attached. At low temperatures PVC, silicon rubber or PTFE insulators are common to 250°C. Above this, glass fiber or ceramic are used. The measuring point and usually most of the leads require a housing or protection sleeve. This is often a metal alloy which is inert to a particular process. Often more consideration goes in to selecting and designing protection sheaths than sensors as this is the layer that must withstand chemical or physical attack and offer convenient process attachment points.

 

Resistance thermometer 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 leading to errors in the signal. Using this configuration you will be able to use 100 meters of cable. This applies equally to balanced bridge and fixed bridge system.

 

Three-wire configuration

 

In order to minimize the effects of the lead resistances a three wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out. High quality connection cables should be used for this type of configuration because an assumption is made that the two lead resistances are the same. This configuration allows for up to 600 meters of cable.

 

Four-wire configuration

 

The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured. In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better alternative configuration four-wire Kelvin connection that should be used in all RTDs. It provides full cancellation of spurious effects and cable resistance of up to 15 O can be handled. Actually in four wire measurement the resistance error due to lead wire resistance is zero.

 

SPECIFICATION

Range: -220 to 850°C for platinum RTDs and -200 to 340°C for Nickel RTDs
Linearity: Platinum and copper are more linear. Nickel and Balco are less.
Sensitivity: -10 to 100 ohms/degree

 

Advantages

 

• High accuracy
• Low drift
• Wide operating range
• Suitability for precision applications
• Stable output for long period of time
• Ease of recalibration
• Repeatability
• Not affected by the corrosion or oxidation

 

Limitations

 

• RTDs in industrial applications are rarely used above 660 °C. At temperatures above 660 °C it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. So that laboratory standard thermometers replace the metal sheath with a glass construction. At very low temperatures, say below -270 °C (or 3 K), there are very few phonons, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.
• Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However, thermistors have a smaller temperature range and stability.

 

 

 

 
 
E-mail info@sensortimes.com ---------- Telephone 216-916-6766