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How does a resistance thermometer work?

The electrical resistance of a

resistance thermometer's sensor changes with the temperature. As the resistance of measuring resistors to EN 60751 (2009-05) increases with rising temperature, we refer to it as PTC (Positive Temperature Coefficient). Pt100 or Pt1000 measuring resistors are normally used for industrial applications. The thermometers based around EN 60751 are defined in DIN 43735.

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What are 2-, 3- and 4-wire circuits?

They describe the number of wires with which the measuring resistor (e.g. a Pt100) is connected. While with the simplest 2-wire connection, the lead resistance can falsify the measuring result, this negative influence can be compensated within the 3- or 4-wire connection, and thus the accuracy of the measurement improved.

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What are fluctuations and overshoots?

Two-point control often involves undulation. A temperature rise above the set point, after the start of the temperature control, is referred to as overshoot. Temperature changes around the set point are referred to as fluctuations. A better quality of control is expected when the level of overshoot and fluctuation is low.

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What does „Pt100“ mean?

Pt stands for Platinum with a nominal resistance of 100 Ohm at 0 °C (EN 60751).

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What happens with a high ambient temperature > 85 °C?

High TC error and damage to the electronic components.

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What is a PID controller?

PID control is a combination of proportional, integral and derivative control. Thus, through proportional control, the temperature is controlled smoothly without fluctuations. Through integral control, automatic offset-matching is made and through derivative control, a fast reaction to external disturbances is possible.

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What is a two-point controller?

A two-point controller is a controller that works discontinuously, with two output states. Depending on whether the actual value is above or below the set point, the upper or lower output state is active. Two-point controllers are used when the actuating variable is not a continuous variable, rather it can switch between two states, e.g. On/Off. Though the two-point controller achieves a steady state, it never stops working. With strong changes in the set point it can certainly control quicker than is possible with other control processes.

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What is D-control?

D-function (or derivative control function) is used in order to hold an output in proportion to a time derivative function of the input.

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What is P-control?

P-control (proportional control) is used to hold an output in proportion to the deviation between the set point and the actual value.

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What is self-optimisation or autotuning?

The PID terms for temperature control vary in value and combination, depending on the characteristics of the controlled system. In current use, there are a number of traditional methods proposed and implemented for determining the PID terms from the waveform of the temperatures to be controlled by the temperature controller. This enables self-optimisation (e.g. the determination of the PID terms used for a variety of controlled systems). Among the self-optimisation processes are step response, limit sensitivity and limit cycle processes.

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What is the derivative time?

The derivative time is the time required when controlling a differential until a ramp deviation corresponds with the control output in proportional control. The longer the derivative time, the stronger is the derivative component of the output signal.

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What is the hysteresis?

Two-point control switches the output on or off depending on the deviation form the set point. This means that the output can frequently change with the smallest temperature changes. This can shorten the service life of the output relay and it can have a negative effect on the service life of the power switch. Therefore, a separation is created between the points for switching ON and OFF. This difference between the switching points is known as the hysteresis.

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What is the integral time?

The integral time is the defined time in which the integrator must reach the value of the step response of the P-controller. The shorter the integral time, the stronger the effect of the integral component. If the integral time is too short, however, this can lead to fluctuations.

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What is the set point?

The set point is the parameter that the temperature controller should react to. The time required to achieve stable control varies for each controlled system.

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What is thermal voltage (or the Seebeck effect)?

The effect, named after Thomas Johann Seebeck, describes the fact that an electric voltage exists when two different metallic conductors are connected at two different points, if there is a temperature difference between the connected and the open end of the „thermocouple“.