Depending on the measurement mode you choose, the amount of reference A/B fluid used is either 9ml or 13ml. As long as the pump heads are calibrated as precisely as possible, a 1L bottle can give you about 111 days of testing at 1x per day. This does not include how many times the "prepare measurement cell" function is used. Each time the function is used, a little bit more Ref B is used as this is what the sensor sits in when the IOND is not measuring.