How to Read a Pipette's Water Level
At its most basic, a pipette is a graduated tube that draws in liquids for precise volume measurement and then transfers the liquids to other containers. Many types are used for different applications. Volumetric, graduated, mechanical action and micro-liter pipettes are among those familiar to workers in medical and scientific research environments.
Inexpensive pipettes are often used once and then discarded. High-tech models are microprocessor-controlled, some featuring multi-channel (multiple tube) automation and LCD menus that enable a range of user-defined transfer modes. These expensive instruments are designed for high-volume laboratory work such as media preparation and sample analysis.
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Instructions
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1
Examine the pipette closely and note the calibrated volume of the pipette in milliliter units (ml). This number is clearly etched near the top. The evenly spaced numbers and marks along the length of the pipette are calibration marks that correspond to precise volumes. The volume mark that corresponds to the liquid level is the liquid volume delivered when the pipette is completely drained (by gravity).
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2
Draw water into the pipette using the suction of the pipette bulb, or by mouth, to any level in the top half of the calibration range. Stabilize the water level before proceeding. You can do this with the bulb, or by tightly stopping-up the top of the pipette with a thumb or finger.
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3
Raise the pipette to eye level and note the curved (concave) surface that forms the top of the liquid column. This surface, called the meniscus, is an effect of surface tension in liquids.
The bottom of the meniscus is your point of reference for reading volumes.
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Place the meniscus directly in front of your best-trained eye and note the calibration numbers that the meniscus falls between. These numbers represent fractions of the calibrated volume. For example: the calibrated volume is 10 ml, the pipette has calibration numbers 1 to 10 and the meniscus lies between 7 and 8, therefore adjacent numbers represent divisions of 1 ml and the liquid volume is between 7 and 8 ml.
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Note the 10 marks between adjacent calibration numbers. These marks represent subdivisions of 0.1 ml. Carefully note the two marks that the meniscus falls between.
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Note the liquid volume. For example, if the bottom of the meniscus sits precisely on the seventh subdivision then the volume is 7.70 ml.
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7
Estimate the "fractional subdivision". In the example, if you judged that the bottom of the meniscus lies precisely midway between subdivisions 7 and 8 then the volume is 7.75 ml.
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Tips & Warnings
Most pipettes have tolerances of 1 -- 2 percent. For example, 1 percent tolerance means actual volumes delivered differ from calibrated volumes by no more than 1 percent (plus or minus).
Estimates between the smallest calibration marks are called uncertain digits. One uncertain digit can be included as the last digit in the volume.
Because liquids expand or contract with changing temperature, most pipettes are calibrated at a standard temperature of 20 degrees Celsius (68 degrees Fahrenheit).
High quality pipettes are made of low thermal expansion glass to minimize temperature-induced errors.
References
Resources
- Photo Credit graduations de pipette image by pgm from Fotolia.com
