Abstract: There is a controller connected to a plurality of probes via a transmission line. Each probe includes sensors which provide voltage signals related to the fluid status in the probe environment. The voltage signals are applied to a voltage controlled oscillator which in turn provides an oscillating signal for transmission to the controller. A counter in the controller is controlled by a timer to count the number of oscillations of the transmitted signal over a predetermined time period. The count is represented by a digital signal which is processed to provide an indication of the fluid status of the probes. Probe circuit temperature is also transmitted to the controller for use in correcting the received oscillator frequency.

Abstract: There is a controller having an identifier signal output and a status signal input, and a plurality of probes, each having an identifier signal input and output and a status signal input and output. The probes are connected in series with the inputs of one probe connected to the outputs of the next, with the identifier signal output of the controller connected to the identifier signal input of the first probe, and with the status signal input of the controller connected to the status signal output of the first probe. Each probe includes a decrementer which decrements the identifier signal received from the previous probe by one and retransmits it to the next probe. Each probe also includes a comparator and a switch configured so that when the comparator receives an identifier signal equal to zero, the switch switches the status signal output of the probe from its status signal input to a source of a local probe status signal.