Electrical resistance is a very suitable parameter, since in many cases it tends to increase linearly with temperature. It is said that a certain property X is thermometric, that is, it can be used to measure the temperature T, when the relationship between X and T is linear:
X = k ∙ Δ T
Where k is a constant of proportionality to be determined.
A well-known thermometric property is the expansion of mercury upon heating, used in a clinical thermometer. Other thermometers use gas, metal sheets that expand with increasing temperature, resistance or use the brightness of a filament, among other properties.
It is convenient to have this range of possibilities because temperature is one of the most characteristic quantities of any system, be it biological or inanimate. That is why it is the magnitude that is most measured in industrial processes, and for the ranges that are handled in each of them, certain thermometric properties are preferable to others.
Resistance Thermometer Features
Resistance thermometers have the following characteristics:
-They are very simple in operation. The sensor element consists of a wire made of metal, platinum, nickel, tungsten and copper being the most used.
-They offer a quick reading.
-Operate in a wide range of temperatures.
Resistivity, resistance and temperature
The materials used to make resistance thermometers are conductors whose resistivity almost always increases with temperature. Resistance and resistivity are not synonymous, but they are closely related.
The resistivity is the relationship between the electric field inside the material when current flows and the density of said stream. It is, therefore, a property of the material.
For certain materials, called ohmic , the relationship between electric field and current density is linear. As the temperature rises, the ions of the conductor increase their vibrations and with it the opposition to the passage of the current.
Instead, resistance is a property of the conductor, determined not only by the resistivity of the material, but by geometry: length and cross-sectional area.
If the cross section is kept constant, the relationship between these quantities is:
The unit for electrical resistance in the International System is the ohm (Ω), while resistivity comes in Ω ∙ m, although it is common to find Ω ∙ mm.
In metals, resistivity increases with temperature in a linear fashion:
ρ (T) = ρ or (1 + α ∙ ΔT)
Where ρ is the resistivity of the material at a certain temperature, ρ o is the resistivity at the reference temperature, generally 0ºC or 20 ºC, α is the thermal coefficient of the material and ΔT is the temperature variation.
Since the resistance depends on the resistivity of the material, if the temperature difference is not very large, it is true that:
R (T) = R or (1+ α ∙ ΔT)
Resistance is easy to measure, and since the relationship with temperature is linear, it is a good thermometric property.
The central element of the resistance thermometer is a metal wire that is wound around an insulating holder, usually made of mica, ceramic, or glass. It is enclosed in a tube filled with insulating powder and wrapped in insulating layers, sealed against moisture.
The pressure inside the tube is kept low, to avoid the formation of oxides that cause error in the readings. The set is small: between 1-5 mm in diameter and 10-50 mm long, covered in turn by an outer casing that serves to protect it, since the device is delicate and must be handled with care.
Platinum, a precious metal, is the material most used to manufacture resistance, as it is very stable over a wide range of temperatures and provides extremely precise measurements, to the point of serving as an international standard for temperature in the -260 ° C range. – 630 ° C. However, platinum resistance thermometers can be made with a much greater range.
To measure changes in the resistance of the wire, it must be incorporated into a special circuit called a Wheatstone bridge , used to measure unknown resistances or impedances.
This is done using thin copper wires (two, three or four copper wires, the more wires, the more accurate the thermometer, the three are the most common).
In order for the device to work, a small measurement current must be supplied, the value of which is close to 1 mA (the lower the better to avoid excessive heating) and the voltage drop produced is measured. Knowing the current and the voltage, the resistance of the sensor is determined with Ohm’s law and through it the temperature.
Characteristic curve of the platinum thermometer
The linearity of the relationship between resistance and temperature is not always fulfilled with total accuracy in all temperature ranges, this depends a lot on the material of the wire.
The problem of non-linearity can be solved using an additional circuit or simply by making use of the resistance versus temperature graph , called the characteristic curve , like the one shown:
Characteristic curve of the Pt-100 or 100 Ω platinum resistance thermometer.
Platinum resistance thermometers are manufactured according to the resistance of the coil: Pt-25, Pt-100 and Pt-1000 are the most used.
The letters “Pt” refer to the chemical symbol for platinum, and the number is the resistance of the wire at the reference temperature 0ºC. The higher the resistance, the more sensitive the thermometer is, as it offers a greater variation in resistance with the same temperature change. However, Pt-100 is the most widely used industrially, with a resolution of one tenth of a degree.
Instead of wire or coil windings, some manufacturers use a thin layer of platinum deposited on top of an insulating ceramic substrate. This decreases the size of the device and makes it even more accurate and faster.
Uses / Applications of Resistance Thermometer
The resistance thermometer is preferably used in the chemical, pharmaceutical and food industries, as well as in areas where high precision in temperature measurement is required to ensure quality products.
The instrument manufacturer indicates the range of temperatures that it can accurately measure. Outside of their range, the thermometers do not give accurate measurements and in the worst case the sensing element is damaged.
Measurement of ambient temperature
Accurately measuring ambient temperature is important in the automotive industry, whose assembly, welding and engine testing processes produce a lot of heat in the environment. In these cases, the copper resistance thermometer is generally preferred.
Automotive temperature sensor
To measure the temperature of a car engine, an electrical resistance is used as a thermometric element.
To determine the temperature of industrial smelting furnaces, boilers, refrigerators and nuclear reactors.
Also, precise temperature control is extremely important for the food industry, as it keeps food fresh and germ-free for longer.
Platinum resistance thermometers are used in the detection of gravitational waves. The device created for this purpose consists of two interferometers, which are optical instruments for measuring interference from light.
Interferometers use mirrors to properly direct laser beams, and their temperature is continuously monitored to ensure that they maintain the proper curvature and ensure accurate measurements.
Advantages and disadvantages
-Variety of uses.
-Wide measurement range that allows them to be used in various industries.
-They remain stable for a long time.
-They are linear or very close to linearity in a wide range of temperatures.
While the limitations include:
-They are not used for temperatures higher than 660ºC.
-Not below -270 ºC.
-They must be handled with care.
-They are less sensitive than other cheaper devices such as thermistors, and in some applications, their response time is greater than these.
-Thermometers made of platinum are expensive.