Adjusting the LI-8150 Flow Rate

Adjusting the LI-8150 Flow Rate

First generation LI-8150 multiplexers, which used a rotary vane pump, provided an adjustable flow rate of 2 to 4 LPM. This functionality was lost when the LI-8150 was upgraded to the more rugged diaphragm pump. However, control over the multiplexer flow rate can be restored through a simple hardware modification. This is accomplished by replacing the quick connect coupling on the LI-8150 pump inlet with a quick connect needle valve (part number 300-09658), installing a flow meter, and adjusting the pressure setting in the subsampling loop.

To modify an LI-8150 with the diaphragm pump for a low flow rate:

  1. Remove the two thumbscrews that secure the display panel inside the multiplexer. Set the panel aside but leave the cable plugged into it.
  2. Locate the quick connect coupler on the vacuum side of the multiplexer pump. Facing the pump with the hoses pointed toward you, the vacuum side of the pump is the top right and lower left connections. These connections are tied to a single coupler through a “Y” fitting and a few short lengths of tubing. Remove the quick connect coupler by pressing in on the orange rings at its ends and pulling out on the hoses.
  3. Install the needle valve with the outside connected to the hose going to the pump. Push the tubes into the connections firmly and give a slight tug to ensure they are locked in place.
  4. Connect a flow meter to one of the OUT connections at one of the ports.
  5. Power up the multiplexer and LI-8100. Set the port from step 4 as the active port.
  6. Loosen the locking ring on the needle valve and adjust the valve to the desired flow rate, as measured by the flow meter connected in step 5. The flow rate should be kept above 1.8 LPM to help ensure adequate mixing in the chambers.
  7. Once the desired flow rate is set, tighten the locking ring on the needle valve.
  8. Remove the splash guard covering the fuses inside the multiplexer and locate the “Pressure Adjust” potentiometer.
  9. Using a small flat head screwdriver, adjust the pressure adjust potentiometer so that the pressure in the analyzer’s optical bench is at ambient. Optical bench pressure is given by the pressure value reported in the LI-8100 interface software.
  10. Reinstall the splash guard and display panel, and remove the flow meter. The multiplexer is now ready for low flow operation.

If you have questions about the implications of lowering the flow rate, please contact LI-COR Environmental Technical Support at

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