... How many times do we have to squeeze a bellows to remove 5% of the air
from a window cavity, with a check valve and a vacuum breaker that allow
dry air to replace withdrawn air when the bellows is released, if we want
to lower the cavity dew point from 50 to 10 F?

.... 31 times.

10 P=.5*EXP(17.863-9621/(460+70))'initial cavity vapor pressure ("Hg)
20 W=.62198/(29.921/P-1)'initial humidity ratio (#w/#a)
30 PRINT P,W
40 P10=EXP(17.863-9621/(460+10))'vapor pressure at 10 F and 100% RH
50 W10=.62198/(29.921/P10-1)'10 F humidity ratio
60 W=.95*W'squeeze bellows
70 IF WW10 THEN N=N+1:GOTO 60
80 PRINT N,W,W10

..3741957 7.87707E-03

31 1.525878E-03 1.536647E-03

Nick