Dear Steve:
Thank you very much for your reply. do you mean you have used ideal PID
Out = Gain(Error+a*dError/dt + b*integral(Error dt)) ? If yes what was the 8051 derivative ?

works with a sample time of 100msec just fine

I want to control the heater with varying duty cycle of pulses, I was thinking in past hour that I really don't need high frequency of pulses because my system is slow and even if I try to use high sampling rate I only wast microcontroller's processing time because my Error dosn't change in matter of even 100msec significantly. So I'm thinking about a very slow sampling rate (for example 1 Hz), in that way I can happily implement even Ideal PID :)

thank you very much indeed