Reading a Scale

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Correctly reading a scale is a skill that is important to master. The Spectronic 20™, spectroscope, pipet, buret, graduated cylinder, and many other instruments and devices utlilize scales that must be read properly for successful laboratory work.

When measuring the volume of a liquid, always read the scale from the bottom of the meniscus. The meniscus is the curved surface of a liquid in a narrow cylindrical container.

When reading a scale, always strive to avoid parallax errors. Parallax errors arise when a meniscus or needle is viewed from an angle rather than from straight-on at eye level.

Correct: Viewing the meniscus at eye level	Incorrect: viewing the meniscus from an angle

When measuring absorbance on a Spectronic 20™ (or taking other measurements on an instrument with a needle that has a mirrored plate behind it), view the scale so that the needle's reflection is hidden behind the needle itself.

Straight-on reading	An incorrect reading from an angle - note the reflection

Step 1: Determine the scale increment

A scale is made up of a series of graduations. Usually, some of the graduations are labeled at regular intervals, with smaller, unlabeled graduations between them.

To read the scale, you must first determine the scale increment. The scale increment is the quantity between any two adjacent graduations. To find the scale increment, subtract the values of any two adjacent labeled graduations and divide by the number of intervals between them.

In the graduated cylinder above, first subtract 60 mL - 50 mL = 10 mL. Next, count that there are ten intervals between the labeled graduations. Therefore, the scale increment is 10 mL/10 graduations = 1 mL/graduation.

Step 2: Use the graduations to find all certain digits.

Now determine the reading. Initially, use the labeled graduations and the scale increment to find the certain digits in the measurement. For example, in the 100 mL graduated cylinder above, the first digit of the reading is 5, since the last labeled graduation below the meniscus is 50. Next, use the scale increment. There are two unlabeled graduations below the meniscus, and each graduation represents 1 mL, so the certain digits of the reading are 52 mL.

Step 3: Estimate the uncertain digit and take a reading

Finally, estimate the distance that the meniscus lies between the two graduations as a decimal fraction and multiply by the scale increment. Above, the meniscus is about eight tenths of the way to the next graduation, so the final digit in the reading is (0.8 increment) * (1 mL/increment) = 0.8 mL. The uncertain digit in the reading is always the last digit, so the volume in the graduated cylinder is 52.8 mL.

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What is the volume of liquid in the graduated cylinder? Answer

What is the volume of liquid in the graduated cylinder? Answer

A Vernier scale is an auxiliary sliding scale used to more easily read the values on a fixed main scale. Its purpose is to allow accurate readings, rather than estimations, between the smallest graduations on the fixed scale. A vernier scale commonly has ten graduation marks. Each division on the Vernier scale is nine-tenths of the size of the finest division on the main scale.

To use the Vernier scale, read the main scale to the last certain digit. The last certain digit on the main scale is the graduation just below the zero on the vernier scale. The mark on the vernier scale that directly lines up with a graduation mark on the main scale is the last digit in your reading.

Video: Using the Vernier scale ( 3.54 M )

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