10
Proper Positioning of Probe into the Flow Cell Probe Chamber
Once the probe is assembled, insert the probe through the compression fitting and
tighten the fitting around the probe so it is snug but still allows the probe to slide
through when pressure is applied.
Lower the probe into the flow chamber and slide the probe through the fitting until the
tip of the probe tip is aligned with the small horizontal tube that connects to the
adjacent chamber (see pictures below). This position allows the water that is exiting
this tube to flow across the surface of the membrane which keeps air bubbles from
forming on the membrane tip (where the white dot is located). This is where chlorine
passes through the membrane. Readings can become unstable and drop off towards
zero if bubbles form on the membrane cap tip (white dot). If bubbles are seen forming
on the tip, ensure the probe is properly positioned and increase the flow until bubbles
dislodge. With the probe properly positioned, a flow rate of 500 to 750 mL/min is
typically adequate to keep air bubbles from forming on the membrane.
Sample Flow Considerations
For proper operation, a minimum of 250 ml/min sample flow is required. Large changes in flow (e.g., from 250 to
1000 ml/min) can impact the reading. If flow exceeds 1000 ml/min, the probe chamber may overflow. If overflow
cannot be tolerated, or if sample flow is too variable due to changes in line pressure, then a pressure regulator,
constant head pressure device, or Dole valve (flow regulator) is recommended.
Drain Line Considerations
Sample flow through the probe chamber will be restricted if there is any backpressure on the drain line. To avoid
backpressure, make sure to keep drain lines short and as vertical as possible, and make sure the end of the drain
tubing is not submerged under water. To test for proper draining, set the flow for 1000 ml/min. If the probe
chamber overflows at this flow rate, then there is backpressure on the drain line. This can be verified by unhooking
the drain tubing from the flow cell. If the flow cell stops overflowing when the drain line is disconnected, then the
drain line has too much back pressure.
Note: An air bubble covering this white dot will cause the chlorine reading to drop off.
