Measuring the subsample loop flow rate

Follow this procedure if you wish to have a more precise measurement of the subsample flow rate. See Mass balance in an open system for more details.

  1. Ensure the aquatic chamber pump is set at the speed you will use for measurements.
  2. Pump speed in the aquatic chamber is set via a control voltage under Constants > Aquatic Chamber > Pump Speed. The factory default value for this is 4 VDC. It can be set between 2 and 5 VDC.
  3. Constants include pump speed and spectral blend
  4. Turn the aquatic chamber pump on (Environment > Flow). Set the chamber overpressure to off (Environment > Flow Valve = Manual and 0%).
  5. Fill the chamber with a volume of water comparable to your typical measurement sample volume. Insert the top plug.
  6. 15 mL is the nominal sample volume.

Note: Ensure that the chamber fill plug is in place. The chamber needs to be sealed and leak free for this test.

  1. Run the Flow Valve Fct test under Start Up > Warmup Tests.
  2. Set the instrument flow rate (Environment > Flow) to 800 µmol s-1.
  3. Using a sharp knife or clippers, cut the tube that connects the water vapor equilibrator to the mixing fan volume.
  4. This tube connects to the hosebarb labeled 2 on the side of the water vapor equilibrator.
  5. Install quick connect fittings (300-07124 and 300-07125 in the spares kit) on the tube
  6. Connect the barbed end of the male fitting to the mixing fan side of the tube and the female fitting to the equilibrator side of the tube.
  7. Separate the connectors.
  8. Seal the end of the tube coming from the mixing fan volume with a piece of tape or just cover it with your finger. Leave the tube coming from the water vapor equilibrator vented to the atmosphere.
  9. Observe Flow_s reported on the flow control screen (Environment > Flow).
  10. Ensure that it is reporting a value above 300 µmol s-1. If not, make sure the tube from the mixing fan is sealed. If the tube is sealed and Flow_s is less than 300 µmol s-1, increase the flow rate a little at a time until the flow rate exceeds 300 µmol s-1.
  11. Observe the leak rate reported on the flow control screen.
  12. The reported leak rate (in µmol s-1) is the flow rate through the subsample loop of the aquatic chamber. This value is based on an internal calibration between Flow and Flow_s and gives a good estimate of the subsample flow rate (within ~3% compared to an independent measurement) when Flow_s is greater than 300 µmol s-1.
  13. Note that the absolute reading reported for Flow_s is uncalibrated and the sensor suffers significant uncertainties with slight pressure pulsations. Do not rely on its reported value to estimate the subsample flow rate.
  14. Reconnect the fittings on the tubes.