# Humidity sensor

Discussion in 'General Electronics Chat' started by dariush, Jul 8, 2013.

1. ### dariush Thread Starter New Member

Jun 25, 2013
8
0
whats the difference of Capacitive and resistive humidity sensor?
which one is your choice if you want to make a humidity sensing board? and why?
tell me which one is better for me through rational reason. in a paper i read an ideal sensor should have these factor:
linear response
from 0% to 100% RH, short response time, high
selectivity (i.e., low or no cross-sensitivity), and
high long-term stability
what does these mean and when are they important?. tell me which one is more suitable resistive or capacitive ?

2. ### LDC3 Active Member

Apr 27, 2013
920
160
Linear response: For every equal change in relative humidity, the output changes by a corresponding amount. I.E. if the RH changes by 1% (at 5% or at 95%), the output changes by a fixed amount (say 5mV). The equation is a linear equation (out = k*RH). A resistor is linear if kept at one temperature.
Short response time: The device can determine the RH quickly. The RH usually doesn't change very fast unless the weather changes quickly.
High selectivity: The device only measures RH and not influenced by the temperature.
High long-term stability: The device does not change it's output over long periods (unless the RH actually changes).

I don't know what the difference is between a capacitance RH sensor and a resistive RH sensor, other than the output is either a change in capacitance or a change in resistance. A capacitive RH sensor could be used to generate a frequency.

3. ### MrChips Moderator

Oct 2, 2009
12,622
3,451
Humidity sensors are often capacitive.
You can look up Honeywell, Vaisala and Sensirion.

4. ### dariush Thread Starter New Member

Jun 25, 2013
8
0
so what is the problem if the equation be not linear what is wrong and what will happen for my circuit? i wanna know if i use a nonlinear sensor what will happen?

5. ### wayneh Expert

Sep 9, 2010
12,365
3,220
Two points make a line. You need more points to define a curve. In other words, calibration.

6. ### LDC3 Active Member

Apr 27, 2013
920
160
It means that some of readings over the entire range will be slightly inaccurate. The datasheet will tell you how much you may be off. If your error is 0.5% at 40% RH, then the actually value would be 40.2% RH.