Science · Scientific investigation
Scientific Measurement, Units, and Tools
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Use a tool that matches the quantity being measured.
- SI units make measurements easy to compare.
- Read one estimated digit beyond the smallest marked division when appropriate.
- Read the bottom of a water meniscus at eye level.
- Convert units by multiplying by conversion factors.
What you’ll be able to do
- Match quantities with tools and SI units.
- Read a marked scale and liquid meniscus.
- Tell accuracy from precision.
- Convert between metric units.
Choose measurement tools and SI units, read scales correctly, and convert between metric units.
Match tool, quantity, and unit
Choose a tool that measures the needed quantity. Use a ruler for length, a balance for mass, a graduated cylinder for liquid volume, a thermometer for temperature, and a stopwatch for time.
Common SI units include meter (m), kilogram (kg), second (s), kelvin (K), and liter (L) for common lab volume. Prefixes show powers of ten: kilo means 1,000; centi means 1/100; milli means 1/1,000.
Match the needed precision to the tool. A meter stick can measure a room, but a caliper may be better for a small diameter. A beaker can hold liquid but a graduated cylinder usually measures volume more precisely.
The liter is accepted for use with SI; the SI derived unit of volume is the cubic meter. A light microscope can view many living cells with suitable preparation; electron microscopes resolve much smaller structures but conventional electron-microscope preparation does not keep a sample alive.
Micro (µ) means one millionth, 10−6, of the base unit. A microliter (µL) is one millionth of a liter. Therefore 1,000 µL = 1 mL. Example: 2,500 µL × (1 mL / 1,000 µL) = 2.5 mL.
Volume examples: a submerged solid raises water from 20 to 27 mL, so its volume is 7 mL. For a rectangular solid, volume = length × width × height; use matching length units, giving cubic units.
Scales, menisci, and precision
Read a scale at eye level. First find the value between labeled marks. Then count the smaller divisions. Record all certain digits and one reasonable estimated digit when the tool supports it.
Water in a narrow cylinder forms a curved meniscus. Read the bottom of the curve at eye level. Accuracy means closeness to the accepted value. Precision means close agreement among repeated readings.
Worked example. If marked lines are 1 mL apart, a liquid halfway between 24 and 25 mL can be recorded as about 24.5 mL. Writing 24.537 mL would claim more precision than the scale provides.
Read a display, not its decoration: identify the unit, determine each division, find the liquid surface at eye level, and report only supported digits. Repeated values clustered far from a reference suggest a systematic offset: repetition alone cannot remove it.
A measured value includes an estimate
Check the units and the gap between marks. Read the bottom of water’s curved surface at eye level. That curve is the meniscus. On this marked scale, record the digits you can read and one more digit you can reasonably estimate.
A measured value includes an estimate — OpenStax College, Chemistry (CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
Convert by canceling units
Use a conversion factor so unwanted units cancel. Example: 250 cm × (1 m / 100 cm) = 2.5 m. The centimeters cancel.
Metric prefixes also allow decimal movement. Because 1 L = 1,000 mL, 0.75 L = 750 mL. Always include units so the direction of conversion is clear.
Supporting note: justified digits
For multiplication or division, the least precise measurement limits how many meaningful digits should be reported. On TEAS questions, first use the instrument markings and answer choices; do not invent digits the tool cannot show.
Another conversion: 3.2 kg × (1,000 g / 1 kg) = 3,200 g.
Terms to remember
- SI units
- A shared measurement system based on powers of ten.
- precision
- How closely repeated measurements agree or how finely a tool reads.
- accuracy
- How close a measurement is to the accepted value.
- meniscus
- The curved surface of a liquid in a container.
- conversion factor
- A ratio equal to one that changes units without changing quantity.
- graduated cylinder
- A tool used to measure liquid volume.
Your quick summary
- Length uses a ruler or meter stick.
- Mass uses a balance.
- Liquid volume often uses a graduated cylinder.
- Read the bottom of a water meniscus at eye level.
- Accuracy and precision are different.
- Conversion factors cancel units.
Lesson quiz
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