How to use a multimeter to convert the resistance - temperature detector (RTD) signal into a rough temperature value?
Both commonly used analog multimeters and digital multimeters can roughly estimate the approximate temperature range of the resistance temperature detector (RTD).
Commonly used resistance temperature detectors include platinum resistors (Pt100, Pt1000) and copper resistors (Cu50, Cu100).
The measurement range of the Pt100 resistance temperature detector is from -200°C to 850°C. The minimum range is 50°C, the absolute error is ±0.2°C, and the basic error is ±0.1%. For the Pt1000 platinum resistor, its measurement range is only from -200°C to 250°C, and other parameters are exactly the same as those of the Pt100.
The measurement range of both the Cu50 and Cu100 is from -50°C to 150°C. The minimum range is 50°C, the absolute error is ±0.4°C, and the basic error is ±0.1%.
Let's take the PT100 resistance temperature detector as an example.
The Pt100 is just a detection element. When it works, it must be equipped with an auxiliary DC single power supply of 5V to 24V. Using the principle of the Wheatstone bridge, the electrical signal that changes according to a linear law is sent to the integrated operational amplifier block or the isolation transmitter, and then processed by the single-chip microcomputer to truly reflect the temperature value of the measured object. The temperature controller sends corresponding commands to control the temperature of the controlled object.
The commonly used PT100 resistance temperature detector is divided into two-wire, three-wire, and four-wire systems. Judging from its graduation table, its measurement range is relatively large, ranging from -200°C to +600°C.
The so-called PT100 actually means that its resistance value at the standard 0°C is 100 ohms. And when the temperature is below zero, its resistance value gradually decreases. When the temperature is -200°C, the resistance value is approximately 18.5 ohms. When the temperature rises from 0°C, its resistance value increases. For example, when the temperature rises to 50°C, its resistance value is approximately 119 ohms. When the temperature is 100°C, its resistance value is approximately 138 ohms. When the temperature is 200°C, its resistance value is approximately 176 ohms, and when the temperature is 600°C, its resistance value is approximately 313 ohms.
Based on the above, for the Cu50 resistance temperature detector, 50 ohms refers to its resistance value at 0°C. When the temperature is -50°C, its resistance value decreases from 50 ohms to 39.2 ohms. When the temperature rises from 0°C to 50°C, its resistance value increases to 60.7 ohms. By analogy, when the temperature reaches 150°C, its resistance value rises to 82.13 ohms.
From the above, both the PT100 resistance temperature detector and the Cu50 resistance temperature detector have a large dynamic range and a regular change in resistance value in a linear manner. When they are matched with many types of temperature controllers for temperature collection and control, the effect is good. Therefore, they are widely used in high-precision temperature equipment such as medical treatment, motor manufacturing, cold storage, industrial control, temperature calculation, and resistance calculation of the Wheatstone bridge, with a very wide application range.
