Hi,
My plan is to install a temperature sensor grid under the (soon to be installed) styrofoam insulation of my house, to monitor the thermal performance. The sensors would be installed between the brick wall and a styrofoam in a 1x1m pattern.
In total 7 rows and 40 columns. I will try and equalize the wire length to cancel out errors due to cable resistance difference, and will use a somewhat large cross section (0.75mm is around 350 miliohm, 2.5mm around 100) wire.
In order to use a single channel ADC I'll feed all the wires trough a reed relay multiplexer.
The resolution I want is 0.1K or better, but I'll be satisfied with less (up to 0.5K) if it's budget breaker. The reading period not less than 1 per minute, probably once per 15 minutes. Ideally, the temperature range will sit between 21-24C, absolute min-max is -15..30C.
As you can see, there is a lot of sensors (280) to measure, measuring time, accuracy and precission are not critical, but price is. I cannot afford 2-3$ per piece, so I bought the cheap ones from ebay (MF58 3950 B 10K 5% NTC). The same goes for the relays, I'll use 47x SIL05-BV07056 in the multiplexer (they are cheap, with around 150 miliohm contact resistance).
The sensors will be mounted on a simple +/- 20x20 mm PCB with a screw hole and wire solder points. All enclosed in some sort of goo. Attached is a sketch of the board.
For readout I'll use a dedicated ADC and a micro, but for now I'll just put my Keithley 2015 to cicle through the sensors (in sync with the multiplexer).
My main concern is the reliability of the used sensors and relays. Never did something like this and as it is a single shot job, after the insulation is glued in there will be no more access except via the dangling wire in the atic. And any mistakes stay buried and unrepairable.
All the suggestions are welcome. Just keep in mind, it is a low budget project, I can't afford to put hundreds or thousands of euros in this.