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FAQ

Why does the cumulative pressure in the module drop off gradually, unexpectedly decrease or go negative?

There are several possible reasons for a cumulative pressure drop or negative pressure readings. Refer to RF Service Procedure Curve Identification Chart (RFS010) for curve illustrations. Possible causes include:

  • A leak in the system. First, be sure the system is not leaking. If there is a slow steady drop in pressure and the drop is isolated to one, or perhaps a few modules, check for leaks.
    • First, confirm that Synthetic Grease w/PTFE (Part #: RF41) was used on the lip of the bottle. This also applies to the lip of the septa port on bottles so equipped. This aids in making a tight seal between the module top or septa cap, and the glass bottle.  
    • Second, inspect the gasket which seals the module top to the bottle. It is important to do this before each run commences. If the gasket is loose, has moved to one side, or is showing signs of wear or deterioration, replace the gasket. Refer to RF Service Procedure Gasket Replacement (RFS011)  

After inspection and, if necessary, replacing the gasket, test the module according to RF Service Procedure Module Testing (RFS001). For bottles with septa ports, ensure that the septa connection is tight, and the septum does not have holes.

  • Temperature drop. Be sure that the temperature, over the pressure drop, has not also been dropping. For instance, if a water bath was turned off or unplugged or if an oven door was opened, the resulting temperature drop would correspond to a pressure drop within the module.
  • Absorption of CO2 in the substrate liquid. Refer to the CO2 Absorption chart in RF Service Procedure CO2 Absorption (RFS009). At the start of a study, CO2 in the bottle's head space will dissolve into the liquid until saturated with CO2. If CO2 absorption happens faster than CO2 production, then the net result will be negative. After the fluid becomes saturated and/or CO2 production surpasses absorption, the pressure will begin rising. This can be avoided in one of two ways:
    • Purge the head space with CO2 and let it sit under 3-4 psi pressure to allow time for the CO2 to absorb into the liquid.  Monitor the pressure and observe when the pressure decline levels off. Release the remaining pressure and immediately add the sample/substrate and start the experiment.
    • Bubble O2 into the solution until it is completely saturated. Purge the bottle head space with CO2 and start the experiment.
  • CO2 permeability through internal tubing. For reliable valve operations, silicone tubing is used in the valve assembly. This component is slightly CO2 permeable. In a pure CO2 environment at 2 psi, pressure studies show that the permeation rate is approximately 0.020-0.025 psi/hr. As a result of this phenomenon, a slight loss of pressure may be evident at the end of the study when gas production rates are leveling off. This is a normal occurrence and can be accounted for by running a blank in your study. See the RF Getting Started Guide for more information.
  • Temporary communication loss. Though rare, a module can lose communication with the coordinator at the time of a valve vent cycle. When the module re-establishes communication, the actual pressure may be lower or higher than what it was before the communication loss. This will be recorded as a typically sharp, cumulative pressure drop or spike. The cumulative curve can be adjusted by adding an offset from the point of pressure loss. This offset is the difference between the pressure just before the communication loss and the pressure immediately following the communication loss.