A liquid measuring device is described for carrying out the assay using a gas or vapor permeable but liquid impermeable membrane barrier to control the volume of liquid to be measured or transferred. The charge pump may include a plurality of boost circuits to boost the output of the charge pump during a fast start up. The print heads are arranged so that the unloaded receiver length between two print heads is an integral multiple of the unloaded receiver length transported for each revolution of the encoder roller. When the core component is connected to another component including a display screen, the core component may operate in a second mode in which input received through the touch screen is provided to the other component.