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4.1: Units of Measure

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    The Metric System

    The metric system is an internationally agreed-upon measurement system based on decimals or powers of 10. Scientists use a refined version called the International System of Units (abbreviated SI). In biology, you will often find a need to describe measurements of length, volume, mass, time, temperature or amount of substance.

    International System of Units

    SI Units

    Metric Units

    • length: meter (m)
    • volume: liter (L)
    • mass: gram (g)
    • time: second (s)
    • temperature: Celsius (°C)
      • Kelvin (K) is a unit of thermodynamic temperature and is the SI unit. The Kelvin scale in the same as the Celsius or centigrade scale but offset by 273.16
      • Biology uses Celsius predominantly because of the range in which organisms live.
    • amount of substance: mole (mol)
      • A mole is a number representing 6.022×1023 of something
      • Just as a pair of shoes equals 2 shoes, a mole of shoes is 6.022×1023 shoes
      • Just as a dozen eggs equals 12 eggs, a mole of eggs is 6.022×1023 eggs


    Strategy for Conversions

    1. What unit is being asked for?
      • 500ml = ____L → liters
    2. What unit are you starting from?
      • 500ml = ____L → milliliters
    3. Which unit is larger? By how much is that unit larger?
      • Liters are the larger unit. Liters are 1,000X (103) greater than milliliters.
    4. Which direction are we moving?
      • Since we are moving to a larger unit, our value will be smaller. In this case, the value is smaller by 1,000X
      • In other words, the value is 1/1000 or 0.001 the value.
      • So what is the answer?

    Factoring Out

    Using the idea of factors of ten, you can assess the difference between the two units and cancel out the original unit algebraically to reach the desired final unit.

    • 500ml=_____L
      • 1ml=\frac{1}{1000}L or, \frac{1L}{1000ml}
        • which states 1000 milliliter in every 1 liter
      • 500ml\times\frac{1L}{1000ml}=\frac{500L}{1000}=0.5L
        • pay attention to the units and how we’ve canceled out the ml in the numerator of 500ml and in the denominator in the conversion of 1L in 1000ml

    Additional Resources

    Accuracy and Precision

    Accuracy refers to how closely a measured value agrees with the correct or target value.
    Precision refers to how closely individual measurements agree with each other and reflects the repeatability in those measurements.

    File:High accuracy Low precision.svg

    This illustrates accuracy. Measurements are on target.

    File:High precision Low accuracy.svg

    This illustrates precision. Measurements are very close to each other and repeatable.


    This illustrates Accuracy AND Precision. Each measurement is on target and also highly repeatable.

    Instruments have a finite amount of accuracy and it is important to report measurements within that level of accuracy. Significant figures, report the number of digits that are known to some degree of confidence with the measuring device. With the increased sensitivity of the equipment, the number of significant figures increases.

    This page titled 4.1: Units of Measure is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Bio-OER.

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