Abstract: An apparatus for blending two gases together in precisely adjustable proportions. The apparatus includes a mechanically adjustable, blending subassembly adapted to receive the first gas at a predetermined pressure and a one-to-one force balanced pneumatic relay which is operated by the first gas and which meters the flow of the second gas to the blending subassembly at a pressure substantially equal to the pressure of the first gas being supplied to the blending subassembly. Check valves are strategically located to prevent back flow of either of the gases being mixed when the device is being operated in the low-flow mode, thereby eliminating the requirement for time consuming balancing of the gas flow pressures.