A differential thermometer is a device that measures the difference between two temperaturtes rather than the exact single temperature. It does so by subtracting values of temperature readings that it gets from two separate sensors. There are various differential thermemoter desings ranging from mechanical to analog electronic to digital electronic. This article presents a sofisticated digital electronic device based on ATTINY26 microcontroler which is able to efficiently control thermal flow in solar water heating systems.

DTE1 is packed with numerous functions specific to the target application - controlling of home solar water heating systems. More precisely, DTE1 should be refered to as a "differential thermostat" as it has an output TRIAC by which it controls operation of water heating system in accordance to the difference between the temperature in the solar water heating panels and the hot water tank - if it is above the desired threshold set by the user, it powers on the water circulating pump in order to enable heat energy transfer from solar panels to the tank. Should the temperature difference fall below the threshold, the pump is turned off stopping unwanted heat transfer from in this case hotter water tank to colder solar panels.

In order to provide stable system operation, there are in fact not one but two temperature difference thresholds. One of them (larger difference value) is used when the circulating pump is off and a decision is being made by a differential thermometer whether to turn it on, while the other one (smaller difference value) is used if the pump is already on and a decision is being made whether to turn it off. This sort of decision making algorithms is usually refereed to as "hysteretic decision making". It ensures that switching of the controlled component, the circulating pump in this case, is not being done too often which otherwise might have detrimental effects on its lifespan etc. DTE1 provides a means to easily set both temperature difference thresholds so that device operation can be optimally tailored to each individual solar water heating system. Read more about this procedure on the next page.

The diagram on the right shows a typical home solar water heating system installation and the role of a differential thermometer within it. As can be observed, there are two temperature sensors. One of them should be placed onto a solar panel outlet tube while the other one should be placed onto the hot water storing tank outlet tube. This way, thanks to the water flow, the thermometer is able to measure water temperatures inside two major system blocks and based on those measurements it can decide whether water should be circulated by the circulating pump with the aim of transferring thermal energy from solar panel into the tank. It is important to note that a differential thermometer needs to be flexible enough in its operation as solar water heating systems differ significantly among themselves. This is caused by the differences in piping lenght and thermal insulation, solar panels azumuth and geographical position etc. Therefore, each differential thermometer representative must provide means for adequate adjustments in order to enable optimal exploitation of the given solar system into which it was built in.

The rest of this article describes features DTE1 posesses which make it a flexible solution for home solar water heating system control. Basic and additional features are explained on the next page, while the method of DTE1 programming is explained later.