Capacitive Moisture Sensor - Is AC or DC better/more accurate?

Question

I am looking into making a capacitive moisture sensor. This has been discussed many times already, but I am not sure what is the better option, AC or DC.

I want to run this off of a battery, so DC should be easier, but I read the following paper (tried to understand as best I could) and they used AC. From looking at it they did this so they could adjust the frequency and found that they had better readings for lower moisture levels at a specific frequency.

http://ourspace.uregina.ca/bitstream/handle/10294/5765/Knutson_Kevin_198302207_MASC_ESE_Fall2014.pdf?sequence=1

Does this make sense? Can anything like this be done with DC?

Or should I just follow this lead and try something like this myself?

I'm new to electronics so bare with me. Thank you in advance.

Answer 1

I'll resist baring anything with you, but inexpensive RH sensors are usually capacitive- using a hygroscopic (moisture absorbing) polymer. Water is a polar molecule and causes a large increase in the dielectric constant of the polymer. This same method can be used to measure moisture in a bulk material that absorbs water.

Yes, you have to use AC to measure capacitance. There are simple circuits that can be used for this, or, as this is 2016, you could leave the hard work to IC engineers and buy a capacitance-to-digital converter chip. They will typically work up to maybe 5-10pF full scale range (changing capacitance) so the measurement cell can't be too large. Analog Devices makes some that are not EOL.