Temperatursensor ( NTC / FTE ) concept, development and classification

I. Basic concepts of temperature sensor
1. Temperatur
Temperature is a physical quantity that indicates the degree of hotness or coldness of an object. Microscopically, it is the intensity of the thermal motion of the molecules of an object. The higher the temperature, the more intense the thermal motion of the molecules inside the object.

Temperature can only be measured indirectly through certain characteristics of an object that change with temperature, and the scale used to measure the temperature value of an object is called a temperature scale. It specifies the starting point (zero point) of the temperature reading and the basic unit for measuring temperature. The international unit is the thermodynamic scale (K). Other temperature scales that are currently used more internationally are the Fahrenheit scale (°F), the Celsius scale (°C) and the international practical temperature scale.

From the perspective of molecular motion theory, temperature is a sign of the average kinetic energy of the molecular motion of an object. Temperature is the collective expression of the thermal motion of a large number of molecules and contains statistical significance.

Simulation diagram: In a closed space, the movement speed of gas molecules at high temperatures is faster than that at low temperatures!

NTC temperature sensor with stainless steel tube probe kit

NTC temperature sensor with ABS housing probe wire 105°

NTC temperature sensor with SEMITEC thermistor

2. Temperatursensor
A temperature sensor refers to a sensor that can sense temperature and convert it into a usable output signal. It is an important device for realizing temperature detection and control. Among the wide variety of sensors, temperature sensors are one of the most widely used and fastest-growing sensors. In the automation process of industrial production, temperature measurement points account for about half of all measurement points.

3. Composition of temperature sensors

II. Development of temperature sensors
The perception of heat and cold is the basis of human experience, but finding a way to measure temperature has stumped many great men. It is not clear whether the ancient Greeks or the Chinese first found a way to measure temperature, but there are records that the history of temperature sensors began in the Renaissance.

We start with the challenges faced by temperature measurement, and then introduce the development history of temperature sensors from different aspects [Source: OMEGA Industrial Measurement White Paper Document]:

1. Challenges of measurement
Heat is used to measure the energy contained in a whole or object. The greater the energy, the higher the temperature. Jedoch, unlike physical properties such as mass and length, heat is difficult to measure directly, so most measurement methods are indirect, and the temperature is inferred by observing the effect of heating the object. daher, the measurement standard of heat has always been a challenge.

In 1664, Robert Hooke proposed using the freezing point of water as the reference point for temperature. Ole Reimer believed that two fixed points should be determined, and he chose Hooke’s freezing point and the boiling point of water. Jedoch, how to measure the temperature of hot and cold objects has always been a problem. In the 19th century, scientists such as Gay-Lussac, who studied the gas law, found that when a gas is heated under constant pressure, the temperature rises by 1 degree Celsius and the volume increases by 1/267 (later revised to 1/273.15), and the concept of 0 degrees -273.15℃ was derived.

2. Observe expansion: liquids and bimetals
According to reports, Galileo is believed to have made a device that shows temperature changes around 1592. This device affects the water column by controlling the contraction of air in a container, and the height of the water column indicates the degree of cooling. But because this device is easily affected by air pressure, it can only be regarded as a novel toy.

The thermometer as we know it was invented by Santorio Santorii in Italy in 1612. He sealed the liquid in a glass tube and observed its movement when it expanded.

Putting some scales on the tube made it easier to see the changes, but the system still lacked precise units. Working with Reimer was Gabriel Fahrenheit. He started to produce thermometers using alcohol and mercury as liquids. Mercury was perfect because it had a linear response to temperature changes over a large range, but it was highly toxic, so it is now used less and less. Other alternative liquids are being studied, but it is still widely used.

The bimetallic temperature sensor was invented in the late 1800s. It takes advantage of the uneven expansion of two metal sheets when they are joined. The temperature change causes the metal sheets to bend, which can be used to activate a thermostat or meter similar to those used in gas grilles. The accuracy of this sensor is not high, maybe plus or minus two degrees, but it is also widely used because of its low price.

3. Thermoelectric effect
In the early 1800s, electricity was an exciting field. Scientists discovered that different metals have different resistance and conductivity. In 1821, Thomas Johann Seebeck discovered the thermoelectric effect, which is that different metals can be connected together and placed at different temperatures to generate voltage. Davy demonstrated the correlation between metal resistivity and temperature. Becquerel proposed the use of platinum-platinum thermocouples for temperature measurement, and the actual device was created by Leopold in 1829. Platinum can also be used in resistance temperature detectors, invented by Myers in 1932. It is one of the most accurate sensors for measuring temperature.

Wirewound RTDs are fragile and therefore unsuitable for industrial applications. Recent years have seen the development of thin film RTDs, which are not as accurate as wirewound RTDs, but are more robust. The 20th century also saw the invention of semiconductor temperature measurement devices. Semiconductor temperature measurement devices respond to temperature changes and have high accuracy, but until recently, they lack linearity.

4. Thermal radiation
Very hot metals and molten metals generate heat, emitting heat and visible light. At lower temperatures, they also radiate thermal energy, but with longer wavelengths. British astronomer William Herschel discovered in 1800 that this “fuzzy” light or infrared light generates heat.

Working with compatriot Meloni, Robelli discovered a way to detect this radiant energy by connecting thermocouples in series to create a thermopile. This was followed in 1878 by the bolometer. Invented by American Samuel Langley, this used two platinum strips, one blackened in a single-arm bridge arrangement. Heating by infrared radiation produced a measurable change in resistance. Bolometers are sensitive to a wide range of infrared wavelengths.

In contrast, devices of the radiation quantum detector type, which had been developed since the 1940s, responded only to infrared light in a limited band. Today, inexpensive pyrometers are widely used, and will become more so as the price of thermal imaging cameras falls.

5. Temperature scale
When Fahrenheit made the thermometer, he realized that he needed a temperature scale. He set 30 degrees salt water as the freezing point and over 180 degrees salt water as the boiling point. 25 years later, Anders Celsius proposed to use a scale of 0-100, and today’s “Celsius” is also named after him.

Später, William Thomson discovered the benefits of setting a fixed point at one end of the scale, and then Kelvin proposed to set 0 degrees as the starting point of the Celsius system. This formed the Kelvin temperature scale used in science today.

III. Classification of temperature sensors
There are many types of temperature sensors, and they have different names according to different classification standards.

1. Classification by measurement method
According to the measurement method, they can be divided into two categories: contact and non-contact.

(1) Contact temperature sensor:

The sensor directly contacts the object to be measured to measure the temperature. As the heat of the object to be measured is transferred to the sensor, the temperature of the object to be measured is reduced. In particular, when the heat capacity of the object to be measured is small, the measurement accuracy is low. daher, the prerequisite for measuring the true temperature of an object in this way is that the heat capacity of the object being measured is large enough.

(2) Non-contact temperature sensor:
It mainly uses the infrared radiation emitted by the thermal radiation of the object being measured to measure the temperature of the object, and can be remotely measured. Its manufacturing cost is high, but the measurement accuracy is low. The advantages are that it does not absorb heat from the object being measured; it does not interfere with the temperature field of the object being measured; continuous measurement does not generate consumption; it has a fast response, usw.

2. Classification according to different physical phenomena
Zusätzlich, there are microwave temperature sensors, noise temperature sensors, temperature map temperature sensors, heat flow meters, jet thermometers, nuclear magnetic resonance thermometers, Mossbauer effect thermometers, Josephson effect thermometers, low-temperature superconducting conversion thermometers, optical fiber temperature sensors, usw. Some of these temperature sensors have been applied, and some are still under development.

Waterproof, anti-corrosion RTD PT100 temperature sensor

RTD PT100 Temperature sensor with 1-2 NPT external threaded connection

PT100 Temperature sensor RTD probe with 6 inch probe length

100 Ohm Class A Platinum Element (PT100)
Temperature Coefficient, α = 0.00385.
304 Stainless Steel Sheath
Rugged Transition Junction with Strain Relief
Probe Length – 6 Inches (152 mm) oder 12 Inches (305mm)
Probe Diameter 1/8 Zoll (3 mm)
Three Wire 72 Inch (1.8m) Lead Wire Terminating in Spade Lugs
Temperaturbewertung : 660°F (350°C)

The PT100 series are RTD probes with stainless steel sheath and 100 ohm platinum RTD element. The PT100-11 are available with 6 oder 12 inch probe length. These probes features a 3mm diameter sheath constructed from 304 Edelstahl, a heavy duty transition joint which connects the probe to the lead wires and 72 inches of lead wire terminating in color coded spade lugs. A Class A sensor element is used to provide high accuracy measurements.

The PT100 probe is well suited for industrial environments. RTDs are resistance based sensors so electrical noise has a minimum effect on the signal quality. The three wire lead design compensates for the lead wire resistance allowing longer wire runs without a significant impact on accuracy. The rugged transition joint with spring wire strain relief makes for a highly mechanically sound connection between the wire and the probe.