Abstract: A switch interface for a remote device includes circuitry coupled to at least first and second terminals of the switch interface, configured to receive power over a two-wire connection through the at least first and second terminals. The circuitry is configured to send information regarding a parameter sensed by the remote device through the at least first and second terminals over the two-wire connection. The circuitry is operable to provide the power to the remote device and send the information when first and second wires of the two-wire connection are connected to the at least first and second terminals in a first polarity and a second polarity different from the first polarity.

Abstract: A monitoring instrument, such as a gas detection instrument, an easily replaceable sensor, and a mounting structure for the monitoring instrument. The monitoring instrument has a sensor attachment member that facilitates installation of the sensor. Alignment of the sensor during installation is facilitated by any of a number of alignment structures.

Abstract: A method and apparatus for detecting and clearing a plugged sensor port in a system containing a fluid being monitored through the sensor port. The method comprises establishing a range beyond which a fluid variable is expected to vary within a predetermined time interval, measuring the fluid variable, and indicating a plugged sensor port in response to the measured fluid variable remaining within the range for the predetermined time interval. The apparatus includes a sensor mechanism that senses a fluid variable, and a controller. The controller establishes the range beyond which the fluid variable is expected to vary within a predetermined time interval, determines whether the fluid variable is within the range, and indicates a plugged sensor port in response to the fluid variable remaining within the range for the predetermined time interval. The apparatus may further include a mechanism that clears the plugged sensor port.

Abstract: An interface that provides selected information from a remote device to a utilization mechanism over a two-wire connection, wherein all power for the interface and the remote device is received from the utilization mechanism over the same two-wire connection. A switch circuit provides a high impedance between first and second wires of the two-wire connection when open and a low impedance between the wires when closed. Current flow through the switch circuit is sensed by the utilization mechanism as indicative of information from the remote device. The interface comprises a voltage regulator that provides a regulated voltage to components of the interface and the remote device, and a current regulator that regulates current provided to the voltage regulator from the two-wire connection, when the switch circuit is open, to below a predetermined level at which the utilization mechanism interprets the switch circuit as being in the open state.