Date of manufacture:
This spring loaded unidirectional valve was introduced to prevent the variable back pressure of certain ventilators from causing inaccurate performance from some vaporisers. The so called 'pumping effect' caused unprotected vaporisers to deliver higher than calibrated concentrations. The valve, placed after the vaporisers but before the common gas outlet, raised the operating pressure of the vaporisers to above the maximum pressure fluctuation likely to be caused by the succeeding device. Modern plenum vaporisers are protected from this effect and the device is therefore no longer used. The Blease Manley ventilator has a variable back pressure characteristic which might cause the effect.