Voltage doubler 0.7V to 1.4V DC output from air flow sensor
If the input signal is 0.7 volts DC and you want 1.4 volts DC then I'd consider using an op-amp circuit. It can be powered from 12V. You'd need a non-inverting gain of 2 configuration like this: -



Rf and Rg would typically be in the order of 1kohm to 100kohm (both the same value). The op-amp needs to connect to the 12V power rail and ground. You need to choose an op-amp whose input lines can work close to the ground. The LM324 sounds an obvious choice. It's a quad package op-amp but there are plenty of single op-amp versions aka LM321. Also don't forget to put 100nF across the supply pins to the chip.