Patent ID: 7655887

Claim:
A method for controlling electrical heating of an element to maintain a constant electrical resistance R S , comprising: (a) supplying electrical power to said element in an amount sufficient for heating same and increasing its electrical resistance to R S , while concurrently monitoring real time electrical resistance R of said element for detection of any difference between R and R S ; (b) upon detection of a difference between R and R S , adjusting the electrical power supplied to said element by an amount ΔW determined approximately by: Δ ⁢ ⁢ W = m α ρ × t × R 0 · ( R s - R ) ; ( i ) Δ ⁢ ⁢ W = m α ρ × t × R 0 · [ R s + R ⁡ ( 0 ) - 2 ⁢ R ] ; ⁢ or ( ii ) Δ ⁢ ⁢ W = m α ρ × R 0 · [ f s ⁡ ( R s - R ) - R - R ⁡ ( 0 ) t ] , ( iii ) wherein m is the thermal mass of said element, α p is the temperature coefficient of electrical resistance of said element, R 0 is the standard electrical resistance of said element measured at a reference temperature, t is the time interval between current detection of electrical resistance difference and last adjustment of electric power, R( 0 ) is the electrical resistance of said element measured at last adjustment of electric power, and f S is a predetermined frequency at which the adjustment of electric power is periodically carried out; wherein the element is heated under a constant power flux to cause the element to reach steady state.