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Problems Associated with Low Humidity
and Sealing Pouches with RTV

The most common problem associated with low humidity is the pouch-to-valve stem clearance. Since all leather pouches 'self-adjust' to an optimum clearance when they are new, only minimal care is required to insure that the pouch-to-stem clearance is adequate (about 1/16") for correct performance. However, as the pouch leather ages, it shrinks a bit and it becomes a bit stiff. These two factors reduce the pouch-to-stem clearance to a critical level. So much so that vacuum is required to 'suck' the valve closed when it first starts up. Then when the lower humidity of Winter comes along, the pouches are actually 'holding' (or 'pushing') many of the valves open a slight amount, to the point where the foot pumps can't produce enough air flow to overcome the valve leakage -and suck the valves closed. (BTW, sucking the valves closed 'pushes' the pouch further into the pouch well, and stretches them ever so slightly.) Then, when the humidity level rises again, the pouches become a little softer and a little less stretched, to the point where the valves can now seat (or close) without much effort, and the system seems to 'come alive'.

The best corrective measure is to replace the pouches. Next to that is stretching them slightly by pressing them down into the pouch well with your finger. Another option is to stretch and then seal the pouches with a sealer like Dow Corning 3140. See:

Sealing Pouches with Dow Corning 3140 RTV


John A Tuttle

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This page was last revised May 9, 2019 by John A. Tuttle, who Assumes No Liability
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