What can cause a negative span check error?
If you see large negative span check errors in an otherwise functioning neph, have the tech search for the most likely problems. From my experience, these are: • A faulty NC solenoid We have had problems in the past where a valve that was supposed to be closed for a span check was actually slightly open, due to a bad seating of the plunger. This could be due to dirt in the valve body or just a bad valve mechanism. Unfortunately, the impactor box is not the easiest thing in the world to disassemble, so a valve replacement here will take some effort. One easy test to try first is to disconnect the vacuum line that runs between the impactor box and the neph outlet right at the neph outlet fitting. During a span check, there should be NO suction in this line. Place your finger over the line to check this. The valve should be competely closed so that the the CO2 that is being fed into the neph stays in the neph (or at least only exits under its own pressure from the inlet). If you feel ANY
