# Control system - Thermistor, thermocouple, PT100

• Femme_physics
In summary, Thermistor is a resistor that changes its resistance depending on temperature, Thermocouple is a sensor for measuring temperature, the bimetal-principle is a compound strip is formed by riveting or welding two layers of metals, chosen so as to have very different values of linear expansivity, and the conventional type of bimetallic strip element is still to be found in some thermostats.
Femme_physics
Gold Member
I'm 95% sure I got it all correct, but I'd appeciate another set of eyes

## Homework Statement

1) What's a thermistor? What is its principle of action?

2) What's a thermocouple, and what is its principle of action?

3) How does temperature transducer act based on the bimetal principle? Demonstrate your explanation with a drawing.

4) In a certain system given a temperature sensor of the type PT100 that behaves according to the formula

R=R0(1+a(T-T0)

whereas:

a = 0.00385 1/Celsius
R0 = 100 ohms
T0 = The relative temperature = 0 Celsius

A) In a certain temperature R = 215.5 ohms, calculate that temperature.
B) Calculate the temperature you got in clause A in units of Kelvin and Ferheneit.

## The Attempt at a Solution

1) Thermistor - a resistor that changes its resistance depending on temperature due to the crystaline structure of the substance the thermistor is made of.

2) Thermocouple - sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end.

3) The bimetal-principle: A compound strip is formed by riveting or welding two layers of metals, chosen so as to have very different values of linear expansivity. The linear expansivity (old name, expansion coefficient) is the fractional change of length per degree change of temperature and for all metals is positive, meaning that the strip expands as the temperature increases. This bending action can be sensed by a displacement transducer. The conventional type of bimetallic strip element is still to be found in some thermostats, although the strip is very often arranged into a spiral. This allows for much greater sensitivity, since the sensitivity depends on the length of the strip. The amount of deflection can be fairly precisely proportional to temperature change if the temperature range is small.

4) http://img201.imageshack.us/img201/2306/frkh.jpg

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Femme_physics said:
2) Thermocouple - sensor for measuring temperature. It consists of two dissimilar metals, joined together at one end.
No interesting transistors or op-amps, so I'll confine myself to just one.

I think that should be "joined together at the point where temperature is to be measured." Maybe you should mention that a small potential difference develops dependent on the temperature difference between that junction and the other (usually, colder) junction of those same two metal wires.

Good luck with your control exams. http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon14.gif

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Great! You got everything right!

And with NascO's improvement it's even better!

## 1. What is a thermistor and how does it work?

A thermistor is a type of temperature sensor that works by changing its resistance in response to changes in temperature. It consists of a material with a high temperature coefficient of resistance, meaning its resistance changes significantly with temperature. As the temperature increases, the resistance of the thermistor decreases, and vice versa. This change in resistance can be measured and used to determine the temperature.

## 2. What is a thermocouple and how does it differ from a thermistor?

A thermocouple is also a type of temperature sensor, but it works on the principle of the Seebeck effect. It consists of two dissimilar metals joined together at one end, which creates a voltage difference when there is a temperature gradient between the two ends. This voltage difference can be measured and used to determine the temperature. Unlike a thermistor, a thermocouple does not change its resistance with temperature and can measure a wider range of temperatures.

## 3. What is a PT100 and why is it used?

A PT100 is a type of platinum resistance thermometer that uses a platinum element with a resistance of 100 ohms at 0 degrees Celsius. It works on the same principle as a thermistor, but it is more accurate and stable over a wider temperature range. It is often used in industrial and scientific applications where precise temperature measurements are required.

## 4. How are these control systems used in industry?

Thermistors, thermocouples, and PT100s are commonly used in temperature control systems in various industries such as manufacturing, food processing, and HVAC. They can be used to monitor and control the temperature of equipment, processes, and products. They are also used in safety systems to prevent overheating and potential hazards.

## 5. How do I choose the right type of temperature sensor for my application?

The choice of temperature sensor depends on factors such as the temperature range, accuracy, response time, and cost. Thermistors are suitable for a limited temperature range and offer high sensitivity, while thermocouples have a wider temperature range but lower accuracy. PT100s offer high accuracy and stability over a wider temperature range but are more expensive. It is important to consider the specific requirements of your application before choosing a temperature sensor.