Abstract: An apparatus for controllably varying the sample volume for each of a plurality of sample lines used with air monitoring equipment is disclosed. The sample volume is varied depending on the compound or compounds of interest sampled through each line. The flow volume from each sample line is determined and controlled by proportional time that an electrically actuated valve is open for that sampling line.

Abstract: An apparatus for controllably varying the sample volume for each of a plurality of sample lines used with air monitoring equipment is disclosed. The sample volume is varied depending on the compound or compounds of interest sampled through each line. The flow volume from each sample line is determined and controlled by proportional time that an electrically actuated valve is open for that sampling line.

Abstract: A single-coil wound on a ferrite core surrounding a current path is driven from a voltage controlled constant current source. The coil drive current is modulated with a fixed frequency squarewave. The average voltage present across the coil is sensed and integrated to provide an output signal which is proportional to the current flow and also provides a negative feedback to the voltage controlled current source to bias the modulated drive current in a direction which compensates for the biasing effects of the current flowing in the conductive path.