Hello,
You can use:
An oscillator schematic and calculta the frequency with the timer of the pic.
You can use a controlled constant current generator to supply your probe. You command the generator with a constant time and you'll use a CAN to convert the voltage of the probe. U=(I*t)/C.
you can also use a sensirion sht11 digital sensor, which has a far better accuracy and is in fact not so expensive and easier to interface (all sources codes provided on the sensirion web site)
try to find uc examples that have builtin comparator like 89C2051 and some picks too. on www.atmel.com and www.microchip.com. The comparator user in examples for determining unknown input, value of cap and value of resistor. Examples like find value of pot also......
This may also help
using SHT11 is too expensive for me.
Now I’m trying to measure capacitance using R-C network, described by xirix. Similar method was described in microchip’s AN512.
Alternatively I will try to feed sensor with 1 kHz /from pin of the pic/ and measure voltage drop over it.
yes it is for industrial use. probably I’ll need about 20 sensors monthly… i don't know for sure.
Sensirion doesn’t send samples for my country
The only option is farnell
here are some cheap sensors http://www.k-won.com/english/products/prod_rhk1an.html