Guidelines for Proper Reading of the
Retort Condensed Liquid Meniscus in a J Tube
A meniscus is the curved surface formed by the contact of a liquid and air or between
any two fluids of different density that are contained by a third material.
This curved meniscus makes measuring the volume of liquids collected with a retort
“JP-Tube” graduated cylinder particularly hard due to differences in the way people read
the meniscus.
Always Read the meniscus with the interface at eye level !
When the meniscus and glassware are at eye level, the circumferential major
graduations appear as a single line, such as the 3 ml line shown below.
Figure 1: Water – Air Meniscus at Eye Level
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LIQUID – AIR MENISCUS
Water – Air Meniscus
The general recommendation for a liquid – air meniscus is to read the volume at the
meniscus between a liquid and air on its “lowest point”.
The lowest point is the air – liquid contact in the middle of the graduated cylinder at the
very top of the liquid, as shown by the dashed red line below.
To do this you need to look through the edge and see the liquid level at the middle of
the JP Tube.
Figure 2: Water – Air Meniscus Read at center of JP Tube at top of liquid
Correct Reading would be 2.73 ml
Always estimate between graduations to the next decimal place !
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Buret Card
To see the meniscus better, it is sometimes helpful to make a “buret card”.
A buret card is made from a white piece of heavy paper and a black marker as shown
below.
Figure 3: Buret Card
By holding the buret card directly behind the meniscus with the black mark just below,
the meniscus is easier to see, as shown below.
Figure 4: Using a Buret Card with Black Mark just below meniscus
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Oil – Air Meniscus
Opaque liquids or interfaces make it more difficult to see the meniscus in the middle of
the “JP-tube” so you have to estimate the top of the liquid in the middle of the JP Tube.
Figure 5: Oil – Air Meniscus with API Test Mud
While it is difficult to see the scale the correct line to read is shown below.
Figure 6: Oil – Air Meniscus with API Test Mud
While it is difficult to see the scale the Correct Reading would be 16.23 ml Total Volume
of recovered liquids
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Water – Oil Meniscus
The water – oil meniscus is even harder to read due to it being highly concave, as
shown below.
Figure 7: Water – Oil Meniscus API Test Mud
While it is difficult to give one recommendation for all examples of this kind of meniscus,
the correct reading is not the bottom of the meniscus.
Due to the highly curved nature of the meniscus with a significant amount of water
above the lowest point of the meniscus, it is necessary split the difference between the
top of the water at the edge of the JP Tube and the bottom of the oil in the middle to
obtain the correct water volume reading.
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The correct reading is to split the difference between the top of the water at the edge of
the JP Tube and the bottom of the oil in the middle.
Again, this has to be done differently for each example.
For this example it would be about 1/3 up from the bottom of this distance as shown by
the red dashed line in the Figure shown below.
Figure 8: Water – Oil Meniscus estimating the correct read line to give the actual
volume of water present
While it is difficult to see the scale, the Correct Reading would be 0.78 ml water
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OIL OR NAF VOLUME
So to get the Oil or NAF Volume take the difference between the Water Volume and the
Total Volume.
THESE VALUES ARE NOT THE EXACT OR CORRECT VALUES FOR ANY OF THE
TESTS AND ARE FOR ILLUSTRATIVE PURPOSES ONLY
Total Liquid Volume
Water Volume
NAF Volume
16.23 ml
- 0.78 ml
15.45 ml
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