[TerraMex]

i'll just throw in a few things around , hardware related :

Accurately reading temperatures with a thermistor or a resistor type probe is a very difficult thing. Most people dont realize how non linear , or time invariant it can actually be, specially with low temperatures, or very high ones. Measuring temperature in real time in a process is an actual pain in the @ss. Either with a thermocouple, thermistor or a Pt-X . They have a very slow response time due to the process of thermal transfer, and its variation. However you can have readings with less accuracy of more than 0.5 º Celcius that it wont matter that much. If it's 60 º or 60.5 º , it's not a big deal.

The level meter im trying to develop suffers the same. But because the variations are so small in the time frame , it's really not that important.

To deal with the first part, the linearity, you need to stabilize the readings by using a wheastone brige (preferably), reading the value between the two resistor divisors. Then you need to determine which voltage variation you want (Vin), and a correlation to white noise (you will have WN, but can be dealt with, more later). Lets assume WN is small , althou when you're trying to read a 1mV, it can be a nuisance . Then you need to linearize the readings into a more reliable signal, and more defined (and hopefully with again, low noise). So enter the OpAmp (non inverting). You will have some amout of troubles in the CMRR (Common Mode Rejection Rate) , but that can be regulated, by gain, and maybe an offset voltage in the inverting side, to give an ajustable interval of values in the output . Preferably an Instrumentation OpAmp setting, that can cut most of the noise factor.

So, lets (again) assume a somewhat linear output, with ajustment of offset . Ok, now you have a linear output of a temperature reading ready to be used in whatever you want.

Then you need to correlate the value read, to the value it's supposed to have to do X or W . Enter another OpAmp , a comparator with histherisis (err, dont know the correct word in english, sorry) , to switch at the given voltage . This will change the state it is on to a state that can feed a relay or an digital circuit (example) . It can go (lets say) from 0v (aproxim.) to , lets say 5v. The temp reaches the T1 value (a predetermined voltage) and it goes into High(5v). If the temp drops below T2 , it goes into Low (0v).

You can even use two, and a second temp reading from the hot side. Just a few ideias anyway.

PS: you can measure flow indirectly by several methods : with light refraction , magnetic displacement (if the liquid "cooperates"), Dopple effect or ultrasounds.