Abstract: The machine is of the kind comprising an elongate etch chamber, and a conveyor which is arranged to feed p.c.b's through said etch chamber. In accordance with the invention the etch chamber is divided into two spaced sections comprising a first relatively long main etch section (1) followed by a second relatively short control etch section (2), and two control stations are provided, the first (5) between the two spaced sections, and the second (6) subsequent to said second section, the first and second control stations providing locations for checking and comparing the etch conditions of the p.c.b's at the two stations in order to control the speed of the conveyor appropriately so that the amount of undercut of the p.c.b's exiting from said second etch chamber is kept to a minimum. Preferably, checking is carried out automatically by providing a light source (7,8) beneath the conveyor (1a) at each of the first and second control stations which direct light upwardly through p.c.

Abstract: A gas detector (1) to detect a predetermined gas of interest within an air sample, the detector (1) comprising a sampling tube (12), an infra-red radiation source (13) located at one end of the tube (12), an infrared sensing means (17) located at the other end of the tube, an optical filter (16) appropriate to the gas of interest located in the infra-red radiation beam, either adjacent the infra-red source (13), or adjacent the infra-red sensing means (17), an air sample inlet located at or towards one end of the tube (12) and an air sample outlet located at or towards the other end of the tube (12), with the detector (1) capable of emitting an electrical signal or a changed electrical signal in accordance with whether the gas of interest is or is not detected, a pure, reference gas source (26-30) and valve means (31, 32) to control admission of the reference gas to the sampling tube (12) at prescribed intervals of time.