The cheapest and easiest way to do this is to use two precision thermistors. Choose ones which are 30k ohm at 25 degrees C (fairly standard). Connect them each across one input to ground. Use two 10k ohm resistors to pull-up from each input to the +2.5V reference supply on the terminal board. This will give you +2.242V at 0 degrees C and +0.430V at 100 degrees C. The resolution of t he PicoLog 1012 should give you better than 0.25 degree C resolution at the ends of the range and almost 0.1 degrees C in the middle. Subtract one reading from the other to get the difference.
There are other, better ways to measure a temperature difference but they are all more complicated and not suitable for the PicoLog 1012 without amplification.
old_robin wrote:A year has gone from last-years attempt by and I have decided I REALLY ought to finally do this project this summer.
My water-heating solar panels have a water flow and return, and a pump on/off. When the pump is on the flow is almost completely constant at (from memory) 5 Litres/min so I do not need a flowmeter.
I have bought a Picolog 1012 and terminal board. It is 8-bit so if I measure temperatures that vary from 5 degrees to over 100 degrees the resolution is about a degree. When I take the difference in the temperature of the flow and return I can then be up to 2 degrees wrong due to rounding errors. fairly pointless as most of the time the difference will be between 5 and 10 degrees.
As the Irish says whe giving directions " I wouldn't have started from here if I were you". But I am, and want to inexpensively if poss monitor my panels.
The motor on/off is no issue, and is logging nicely on the Picolog software on the laptop.
What I need is to quite accurately measure and log the DIFFERENCE in temperaure between the flow and return to work out the KWHrs that I am getting. This difference will be from 5 degress (when the pump comes "on" because there is "energy to be had from the roof panels" to probably no more than 35 degrees absolute max (more probably 10 to 15 degrees max on a baking hot day in reality). So I need to measure the temperature difference between two pipes which are within 150 mm or each other and record it on the Picolog 1012 occasssionally (say every 5 minutes at the very fastest.
Can you advise or even supply the sensors and recommendations to do this?
Many thanks - Old_robin
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