What are my options for measuring temperature using a PC?
The three most common types of temperature sensors are thermocouples, resistance temperature detectors, and thermistors. Each offers its own set of advantages while imposing certain signal conditioning requirements. Thermocouples are popular because they are inexpensive and easy to use. Several established types collectively cover a range of about -200Â° to 1700Â° C.
A thermocouple consists of two wires of dissimilar metals joined at one end. This 'hot junction' forms the sensor, which is used to measure the temperature in question. The free ends of the wires connect to the data acquisition hardware and make up the 'cold junction,' which is affected by the temperature near the hardware. This circuit produces a low-level voltage (typically 50mV or less), so it requires a high-gain differential input channel for best results.
Note that the thermocouple generates a voltage that is nonlinearly proportional to the temperature difference between the hot and the cold junctions. This problem is solved through 'Cold Junction Compensation (CJC),' wherein a 'cold junction' reference on the data acquisition hardware senses the temperature near the terminals. This cold junction temperature and the measured voltage are both used to calculate the correct temperature at the hot junction. Ideally, the cold junction sensor and correction algorithm are built into the data acquisition hardware and software to provide seamless, automatic compensation.
The resistance temperature detector (RTD) is another popular method for measuring temperature. An RTD is a metallic device, the resistance of which rises more or less linearly with temperature. Compared to thermocouples, RTDs are relatively expensive, but are also very stable and accurate over a range of about 200Â° to 800Â°C. To be used with RTDs, the measurement hardware must be capable of measuring resistance; it should be able to supply a stable source of excitation current and support the 4-wire measurement technique to eliminate lead resistance.
Thermistors are a third method for temperature measurement; they're best suited for high accuracy measurements over a range of approximately -40Â° to +150Â°C. A thermistor is a semiconductor device the resistance of which decreases nonlinearly with temperature rise. Thermistors require data acquisition hardware with a regulated source of excitation current and support for the 4-wire measurement configuration.
Regardless of the temperature sensor chosen, the data acquisition software must be able to convert the sensor's output to temperature and to apply any linearization required by the particular sensor type. Linearization is typically based on established look-up tables and known equations.
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