Abstract: A circuit is provided for detecting an open condition in a thermocouple or thermocouple leads connected to a preamplifier even though the thermocouple is grounded or connected to ground through some resistance. A high frequency AC test signal is applied to both leads of the thermocouple and one of the leads is connected to a low pass filter. When the thermocouple circuit is closed, the AC signal on both leads will be substantially equal. If, however, either of the leads or the thermocouple itself is open, the AC signal will be different due to the low pass filter connected to one of the leads. That filter attenutes the signal on the associated lead as well as causing a phase shift of the two AC signals. A differential amplifier connected to the thermocouple leads detects the amplitude attenuation or the phase shift to provide an output whenever the thermocouple circuit is opened.

Abstract: A vehicle fuel economy calculator including a digital circuit for generating a first series of pulse groups with the number of pulses in each group representing a calculation of instantaneous vehicle fuel economy. A digital averaging circuit generates a second series of pulse groups with the number of pulses in each group representing an average vehicle fuel economy determined by the average number of pulses in each pulse group in a predetermined number of consecutive pulse groups in the first series. A gating and indicating circuit is responsive to the vehicle speed and fuel consumption rate for providing an indication of zero fuel economy when the vehicle speed is below a predetermined speed, for coupling the first series of pulse groups to the indicator during a specified period of time after the vehicle reaches the predetermined speed or when the fuel consumption exceeds a predetermined rate, and for coupling the second series of pulse groups to the indicator circuit at all other times.