Heidolph Vacuum Switchbox - Manual (Page 59)

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reference to reference chart (page 60) and nomogram (page 60).
- hysteresis Δp defines temperature rise in mbar, required to open the vacuum valve.
Extended valve opening times equals high losses due to repeated gas transport.
- Low-boiling substances prefer a higher Δp (5 - 10 mbar), whereas we recommend a lower
value (1 - 5 mbar) for high-boiling substances.
11.2. "Auto Mode" vacuum regulation mode
The "Auto Mode" is distinguished by the following parameters:
- Find distillation point automatically
- Distillation pressure once found will be maintained permanently, integrating hysteresis
Δp (h).
- Separate mixes within physical limits
- No need to know about thermal dynamic substance data.
11.2.1. "Auto Mode" settings
- The only item to be set is h hysteresis
Δp (h).
12. Solvent data
Examples for using reference table and nomogram:
This reference table details substance data needed for distillation, whereas the nomogram reflects
relationship between pressure and boil temperature of some solvents. Go by the Clausius-
Clapeyron equation and plot 1/T against log p.
12.1. at boil temperature of 40°C
LABOROTA 4002/4003 control: in column "vacuum for boil temperature" use the "Set Vac"
nominal value as depicted for 40°C.
12.2. at boil temperatures deviating from 40°C
4. On temperature axis of the nomogram mark boil point wanted.
5. Draw a line to the right and determine intersection with the solvent line.
6. From this intersection draw a straight line down to determine the vacuum pressure needed.
12.3. solvents not depicted
- The following items might help in determining the right vacuum pressure:
1. Rise of the straight line is a function of evaporation enthalpy. Hence, straight lines depicted
may be used for rough reference to a lightly deviating boil point.
2. A water jet pump or diaphragm-type pump may lower boil temperature by about 100°C.
3. Rule of thumb: decrease pressure to ½ will decrease boil point by about 15 %.