Bio-Rad Model 583 Gel Drying Supports - Manual (Page 19)

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Vacuum Source Options
APPENDIX A. VACUUM SOURCE OPTIONS
A.1 VACUUM SOURCE REQUIREMENTS
Vacuum is commonly measured in inches of mercury (“ Hg) or millimeters of mercury (Torr). Atmospheric
pressure at sea level is 760 Torr (29.64” Hg), while a perfect vacuum measures 0 Torr (0” Hg). Both quanti-
ties assume ambient temperature is 0° C.
Standard pressure gauges are often designed with one end vented to the environment, while the other end
is the measuring port. These gauges indicate 0” Hg pressure when the absolute pressure is 29.64” Hg, and
they indicate -29.64” Hg for a perfect vacuum. The relationship between gauge pressure and absolute pres-
sure is:
Absolute Pressure = (Atmospheric Pressure, at Sea Level and 0° C) + (Gauge Pressure)
Absolute Pressure, in “ Hg = 29.64” Hg + (Gauge Pressure, in “ Hg)
Note: Gauge values are negative numbers, though the “-” sign is not marked on it’s face.
In practical terms, this means the larger the gauge value, the greater the vacuum, while the smaller the
absolute pressure value, the greater the vacuum. Thus, the weakest vacuum specified for proper operation
of the Model 583 dryer is a gauge pressure of 25” Hg, or an absolute pressure of 4.88” Hg.
A.2 TYPES OF VACUUM PUMPS
The types of vacuum sources discussed below each have unique advantages and disadvantages for gel dry-
ing. When choosing a vacuum source, keep in mind that it should be able to provide a vacuum pressure of
at least 25” Hg and a capacity of 20 to 30 liters/min.
For best results, use Bio-Rad’s HydroTech Vacuum Pump. The HydroTech is specially designed to meet all
the vacuum requirements of gel drying. It is low in maintenance, has a large capacity, and does not require
a vapor or liquid trap. The pump attains a vacuum of 29” Hg (23.4 Torr.) or better with a capacity of 28
liters/min (1 CFM).
When using “house vacuum” the actual vacuum pressure varies as demands on the system change during
the day, often dropping below 20” Hg. Variations of this magnitude can cause gels to crack, even though the
dryer’s vacuum is still sealed.
A vapor and liquid trap should be used with a mechanical vacuum source to protect the pump from the cor-
rosive effects of gel liquids and vapors, such as methanol, acetic acid, and water. A simple trap system con-
sists of a liquid trap followed by a vapor trap connected in-line between the dryer and pump. The vapor trap
is a 2 liter side-arm flask set into a bucket of dry ice/alcohol solution. The dry ice has to be replenished
often enough to keep all gel vapors in the side-arm flask. The liquid trap is a 2 liter side-arm flask without
the dry ice or alcohol solution. The pump oil should be checked and probably changed frequently.