Quote:
Originally Posted by hans.schaaper
An audible alarm will affect the the board size and the cost.
Plus the issue of the set point which will be fixed and as a consequence must be chosen rather conservative to avoid going off on a warm day in summer traffic. Please advise.
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Regarding an alarm set point. It would seem logical to me to have this at the temperature reached when the pressure relief valve is about to open. This is the most common mechanism by which coolant is lost. After that stage, more serious events could follow if the coolant isn't replaced, so a warning at that point would seem best. If the coolant level was already low, but an overheat hasn't occured, then the same criterion would apply. It will still blow at the same temperature.
The pressure required to open the relief valve is known. I think we have to assume all caps are rated at 140 mPa (1.4 bar approx). It's easy enough to find the temperature at which the vapour pressure is 1.4 bar, again with an assumption thrown in. This is that the coolant is simply water. The presence of 50% ethylene glycol will depress the vapour pressure somewhat, but the calculation should give a 'safe side' value.
Looking at a pressure/temperature curve (for example:
http://en.wikipedia.org/wiki/File:Wa...sure_graph.jpg), it appears to be around 112-115C. This temperature also seems to correspond to the start of the emergency cooling fan band, when the highest fan speed available is tripped.
TC