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I have an ESP32, and use capacitive soil humidity sensors to check the humidity of my plants. They get shipped with about 15 cm of cable attached.
What is a reasonable/safe cable length I can use before it gives unrealistic readings? It is powered with 3.3 V.
The longer you make your wire, the more voltage drop the analogue signal will suffer along the wire. This will show itself as decrease in the maximum value you can see appearing on your input pin, where a short wire may get up to 3.2v, but a long wire might only get up to 3.1v . Therefore for the most part you could fix this in software by re-scaling your readings as needed, and you can have pretty much as long a wire as you would like.
However, the longer the wire the more noise you will pick up, so you may need to do some software de-noising to fix this.
In general, you should be fine to make it up to a few meters long without any problems
From this "capacitive soil moisture sensor" you will get a voltage and if you measure the voltage with the high resistance of the ADC and a 10-100nF capacitor directly close to the ADC-pin every second, then you can use in theory a very long wire.
The solution is a twisted wire (GND+Vout) or a shielded wire. You can use a coaxial cable or simply twist your wire like they have done it with each pair of wires in a network cable.
If you still have noise, then you should use a little LC- or RC-filter. You can use a 10k Ohm resistor from Vout of the sensor to the ADC-pin and directly very close to the ADC-pin a 100nF capacitor.
The cutoff frequency is here: f0 = 1 / (2 * Pi * R * C) = 1/(2 * Pi * 10000 Ohm * 0,0000001F) = 159Hz
If you only want to measure every minute, then you can use higher values for a lower cutoff frequency.
