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Precision Temperature Logger & Controller, Part 1

This Temperature Logger & Controller is based on the Dick Smith Electronics Q1437 digital thermometer. It records & displays two temperature channels once a second on a PC over a period of up to 12 hours. With appropriate thermocouples, temperatures from -200?C to +1300?C can be recorded, with the display range, resolution, and temperature program adjustable in real time while the data is being logged.

By Leonid Lerner

As well as providing precise temperature logging, the project will control a 230V AC heater rated up to 10A, in response to the temperature readings in on/off mode and a time-temperature regime set with up to four set points.

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The accuracy of the instrument is 0.1% (excluding probe error) and its precision is 0.1°C over the entire range. The logger/controller interfaces to your PC through the parallel port using a standard DB25 connector.

The precursor to this project was the PID Temperature Controller featured in the July 2007 issue which in turn was based on the Digital Thermometer/Thermostat featured in the August 2002 issue of SILICON CHIP and previously available as a kit from Dick Smith Electronics.

That kit project has long been discontinued which made this comprehensive update necessary.

This new project dispenses with the original microcontroller and its associated circuitry from the July 2007 design, with all functions now performed by the attached PC. This significantly simplifies hardware construction which now involves soldering less than a dozen discrete components.

The down-side in eliminating the microcontroller is that the device can no longer be operated stand-alone.

In practice, this is not a serious inconvenience since the controller will mostly be used in the temperature logging mode, with the operator observing variations in real time. In this mode, the role of the microcontroller was, for the main part, that of a communications device.

Its other functions, such as controlling the analog-to-digital converter (ADC), are now performed by the DSE Q1437 digital thermometer, while the Triac control signal is generated by the PC.

The improvements arise from the fact that temperature readings are now logged with the accuracy and precision of the DSE Q1437 digital thermometer. This is a professional instrument and is based on modern microprocessor technology with a super low-noise ADC and custom ambient temperature compensation circuitry, which will be described in greater detail below.

The accuracy, reproducibility and noise level of this instrument have been found to be much superior to those available with the previous circuitry. Temperature control to within fractions of a degree is now possible in many cases.

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