3.61. An open-end mercury manometer is to be used to measure the pressure in an
ID: 883884 • Letter: 3
Question
3.61. An open-end mercury manometer is to be used to measure the pressure in an apparatus containing a vapor that reacts with mercury. A 10-cm layer of silicon oil (SG 0.92) is placed on top of the mercury in the arm attached to the apparatus. Atmospheric pressure is 765 mm Hg. (a) If the level of mercury in the open end is 365 mm below the mercury level in the other arm, what is the pressure (mm Hg) in the apparatus? (b) When the instrumentation specialist was deciding on a liquid to put in the manometer, she listed several properties the fluid should have and eventually selected silicon oil. What might the listed properties have been?Explanation / Answer
The equation to solve this is:
Pgas = Patm - h; ----> where h is the height difference of mercury and the gas.
To determinate h, we already know the SG of the silicon oil, and according to several sources, the SG of mercury is 13.54 so, we need the relation between those SG and the layer of silicon oil, so we can know the height difference. This is because there are two gases in the open tube:
13.54 / 0.92 = 14.7174.
The layer is 10 cm or simply 100 mm so:
10 /14.7174 = 6.795 mm
This value is the height difference in mercury, at equal pressure both sides of manometer, to balance the oil. Now we need to know the actual difference in height of the manometer with both of them:
365 + 6.795 = 371.795mm. This is the actual difference in height and below the atmospheric. Finally, applying the first equation, we get the pressuer This is BELOW atmospheric.
Pgas = 765 - 371.795 = 393.205 mmHg
b) I'm not quite sure about this, but let me tell you the properties of silicone oil:
Heat Resistance, because the flud have to be extremely stable against thermal oxidation.
Low-Temperature Resistance
Viscosity stability because the fluid has to shows little change in viscosity due to changes in temperature.
Chemical stability because it has to be nearly chemically inert and virtually unaffected by low concentration alkaline aqueous solutions or acid aqueous solutions at ambient temperatures.
It does not need to have adverse effect on metals, nor on most other substances.
Water repellency.
Good electric properties.
If you need anything else, tell me in a comment.
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