Abstract: A constant current is applied through a heater by means of an electrical connector which mates with terminal pins of the tube. The applied current causes a voltage across the heater. A first voltage measurement is made across the heater at a first time. After a predetermined time interval has elapsed, a second voltage measurement is made across the heater. The value of the first voltage measurement is substracted from the value of the second voltage measurement and the value of the difference obtained is compared with a predetermined reference value. If the difference value is less than the reference value, a first nominal value of heater resistance is indicated. If the difference value is greater than the reference value, a second nominal value is indicated. The nominal heater resistance values are detected independent of the electrical connector contact resistance.