D I F F E R E N T I A L T H E R M O S T A  v1
Basic operation
As its name suggests, an essential property of a differential thermometer is its ability to measure or calculate the relative difference between the two absolute temperatures. In order to make this process reliable, it is highly desirable that measurement of each particular temperature in interest be as linear as possible. DTE1 incorporates both specially made analog circuits and carefully designed software routines which make this possible. An inherent nonlinearity is less then 0,2'C over the whole temperature measurement range spanning -30'C - +130'C. Therefore, the calculated temperature difference is precisely as shown on the device display no matter whether absolute temperatures lie in sub-zero range or above the standard boiling point of water.
DTE1 enables setting the temperature difference thresholds with the resoluiton of 1'C which its designers believe to be accurate enough for the device intended purpose. As for actual threshold settings, only general guidelines can be provided as solar water heating systems vary enormously among each other. The differences are caused by variations in piping matherials and lengths, thermal insulation of solar panels, pipes and hot water tank, geographical and mycroclimate conditions of the exact location at which the system is installet etc. For example, hot water exiting solar panels looses some of its temperature on the way down to the collecting hot water tank; how much energy is being lost depends on several of the factors mentioned previously. Therefore, in some systems it is not wise to turn the circulating pump on if the registered temperature differences are below e.g. 10'C, while in others the temperature difference as low as 3'C might be economic.
The default upper and lower temperature difference thresholds as set in the production process according to a general rule of thumb, but each user is advised to adjust them according to their system needs and own preferences. Setting the thresholds is easy and intuitive. The procedure is explained on the next page of this article.
Protective functions
In addition to basic functions, there is an overhead logic built into DTE1 with the aim of prolonging the lifespan of the water circulating pump and piping installations. It is formed using a number of internal software timers which limit the maximal frequency by which the pump can be turned on and off. The time constants of those timers differ in accordance to the specific protective function each of them performs. For example, one such timer prevents the pump to be turned off over a period of two minutes once it has been turned on.
DTE1 monitors power suply fluctuations so that it can prevent turning on the circulating pump if grid voltage is not sufficient for its safe and reliable operaiton. This protective mechanism was necessary to be incorporated into the software as some pumps can be damaged if operated by underrated voltage for a prolonged period of time - due to their mechanical and electrical construction, such pumps put efforts to provide a constant pumping power which leads to an increased power supply current they use, which under particular unfavourable conditions might prove self destructive. Since solar water heating systems are sometimes installed at remote locations without human supervision where grid network is not as reliable as it is in cities, this protective feature is believed to be saving some users money.
DTE1 is able do recognise malfunctioniong sensors which it indicates using two graphical symbols. If a symbol "- -" is displayed over the period of time when a temperature reading from a particular sensor (panels/tank) should be displayed, that sensor is to be considered open circuit. Similarly, if a symbol "---" is displayed, that sensor should be considered short circuit. Both open and short circuited sensors are unable to provide temperature readings and consequently DTE1 preventively turns the circulating pump off. If unexpected pump turning off occurs, users are advised to check sensor connectors and replace them as soon as possible if that happens to be the cause of the problem.
Since low pressure water heating systems normally operate in the temperature range above 0'C and below 100'C, DTE1 provides means of indicating temperatures out of this safe range. Should the temperature in solar panels or hot water tank fall below +3'C or rise above 97'C, the digits on the display blink. This should be considered a warning to the user that system is reaching unsafe and potentially catastrophic operation. If the temperature in panels or tank is below freezing, ice might damage the installations because it occupies more space than liquid water. Similarly, if water surpasses the boiling point, uncontrolled steam venting might occur. DTE1 will not stop circulating pump operation under out-of-range temperature readings as circulating water might actually prevent system damage by providing heat transfer to/from endangered points, but users are strongly advised to take appropriate measures and prevent occurring of such dangerous situations.
designed by LP 2011