Hi guys,
I have a X525 quadcopter that uses Crius Multiwii SE 2.5. Acro and angle modes are working well with very little drift in angle mode. However, altitude hold sucks. I know that the barometer in this board has a low resolution and most people think it's crappy. But I have isolated the baro using a black plastic cap with a small hole on top and stuffed it tightly with black foam. Now it's not affected significantly by prop wash or sunlight. It's working fine when the motors are off and I lift the quad in my hand. However, after I arm and push the throttle to more than 40%, the readings are fluctuating rapidly causing a very bad altitude hold. What could be the reason behind this?
For vibration dampening I have nothing but regular foam which I use under the control board. Also, I have 10 inch Gemfan plastic props whose hubs are almost impossible to balance!
Please help me to get a good altitude hold. I have been struggling with this for months!

barometer should not be affected by vibration - if vibration is an issue your acc/gyro will be more affected.

regarding the poor alt hold it could be noise on the i2c bus.  route all power wires away from the fc and twist all +ve and gnd wires. 

what version of multiwii are you on ?

please consider sunlight too ,
direct sunlight to barometer can affect it ,
try covering the barometer with some kind of form

I have twisted the ESC wires...if radiated interference is the problem then I shall cover the power wires with some aluminium foils too. So far, twisting has helped somewhat...although I am not very certain...

Well...it seems you are right...interference does affect all sensors...but as far as I have observed...vibrations too seem to affect the barometer...I switched to carbon fiber props today and vibration damping plates underneath the FC...the fluctuations have reduced even further...now I just get a fluctuation of around +/- 1ft with a gradual drift in the reference point(where it was powered on)...I would think that temperature could be affecting the reference point...what are your thoughts?
Thank you for your valuable advice btw...performance of barometer has improved significantly since....I'm using Multiwii 2.4 btw...