Flow Meter

Flow in the LI-6400 is measured with an electronic mass flow meter in the console.

Factory Calibration

The flow meter is calibrated at three temperatures using a mass flow controller. (The flow controller is, in turn, periodically calibrated with a precision device that operates on a volumetric displacement principle.) The flow data for all temperatures is fit with a straight line, whose slope becomes the af term in Equation (14-16) on page 14-9 in the instruction manual. This value is shown on the LI-6400 calibration sheet, and is viewable. See View Current on page 18-3 in the instruction manual.

Zeroing the Flow meter

The flow meter zero program is run at the user’s discretion. The program is fully automatic. When you select "Flow meter zero" in the Calib Menu, the flow will be shut off, and the system will begin a 10 second count down (Figure 18‑14). After ten seconds, the flow meter signal should read within 1 mV of zero. Press OK.

The flow meter’s zero has a slight temperature sensitivity, but the drift is typically less than 1 μmol s^-1 per 10 °C. The flow meter zero is controlled by an analog output channel, and shouldn’t change much from day to day. Since checking and setting it is so easy, it’s worth the effort to make sure.