PID settings

hi friends
can someone suggest me the best way to tune up PID.
In my sytem i am controlling negative pressure inside a vessel by adjusting the damper position in suction duct.For last few days i observe there are lot of fluctions in the system.It seems someone played with the PID setting.I dont remember the old settings.
How should i start to ,tune up the PID again.

Your help will really be appriciated.
thanx

Hello panthripu;

Here is a link to a few basic methods you can use to tune your process loop; please remember to write down you gain values and keep them somewhere safe!
http://www.jashaw.com/pid/tutorial/pid6.html
Hope this helps,
Daniel Chartier

thanks friend

I have tried to tune it and finally i am getting oscillations with upto 14% on positve side and 18 % negative side.for one oscillation it is taking constant around 18to 20 sec.
the gain is now at .42 and integral is 5sec.
it is possible to improve it more ?if yes how to tune it better?

In microwin under tools there is a pid tune control panel..

This may be of help ?

I have no expierence of it though, just thought id mention it in case you had missed it.