3. Open all isolation valv e s (if present), if the enti re machine is to be dehydra ted.
4. With the machine ambien t temperature at 60 F (15. 6 C) or higher, operate the v ac uu m
pump until the manome te r reads 29.8 in. Hg vac, ref 30 in . ba r. (0.1 psia) (–100.61 kPa) or
a vacuum indicator rea ds 35 F (1.7 C). Operate the pum p an additional 2 hours.
Do not apply greater vacuum than 29.82 in. Hg vac (757.4 mm Hg) or go below 33 F (.56 C)
on the wet b ulb vacuum indica tor. At this t emperature/pressure , isolated pock ets of mois-
ture can turn into ice. Th e slow rate of evaporation ( s ub limination) of ice at the se low tem-
peratures/ pressures greatly increases dehydration time.
5. Valve off the vacuum pum p , stop the pump, and recor d th e instrument reading.
6. After a 2-hour wait, take another instrument rea ding. If the reading has not changed,
dehydration is comp let e. If the reading indic at es vacuum loss, repeat St eps 4 an d 5.
7. If the reading contin ues t o change after several at tempts, perform a leak tes t up to the
maximum 230 psig (1585 k P a) pr e ssure. Locate and repair t he leak, and repeat
dehydration.
Click here f or Figure 29 — Dehydration Co l d Trap