Measuring Instruments
Every day the Bureau of Meteorology reports the temperature in Sydney to the nearest tenth of a degree, the rainfall to the nearest millimetre, and the wind speed in kilometres per hour. None of those numbers come from a guess. They come from instruments, carefully chosen tools that turn what is happening in the world into a number you can trust. In this lesson you will meet the everyday measuring instruments of the science lab, learn which unit each one reads, and learn how to compare instruments by their range, sensitivity and accuracy.
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Q1 Ā· List as many measuring instruments as you can think of that you might find in a science lab or around the home. Next to each one, write what it measures.
Q2 Ā· You want to measure exactly how much water is in a small medicine cup. Would you use a kitchen measuring jug or a measuring cylinder? Explain your choice.
ā Know
- The common lab instruments and the quantity and unit each one measures: ruler or tape (length, m, cm, mm), measuring cylinder (volume, mL), balance (mass, g, kg), thermometer (temperature, Ā°C), stopwatch (time, s)
- The difference between an analog instrument (a scale and a pointer) and a digital instrument (a number readout) (NESA SC4-WS-05)
- The meaning of range, sensitivity and accuracy when describing an instrument (NESA SC4-WS-05)
ā Understand
- Why you must choose the right instrument and the right unit for each job
- How to read a scale correctly, at eye level and using the meniscus for liquids
- Why a more sensitive instrument can detect smaller changes than a less sensitive one (NESA SC4-OTU-01)
ā Can do
- Use a variety of analog and digital measuring devices to make observations
- Compare two instruments by their range, sensitivity and accuracy
- Choose a suitable instrument and unit for a given measuring task
- Ruler
- Measuring cylinder
- Balance
- Thermometer
- Length, in metres, centimetres or millimetres
- Volume of a liquid, in millilitres
- Mass, in grams or kilograms
- Temperature, in degrees Celsius
A measuring instrument is a tool that turns something happening in the world into a number, with a unit. The syllabus asks you to use a variety of measuring devices to make observations, so the first job is knowing which tool measures which quantity, and in which unit. Australia uses the metric system, also called the SI units, so all of these are metric.
- A ruler or tape measure measures length in metres (m), centimetres (cm) or millimetres (mm). Use a ruler for a pencil, a tape measure for a room.
- A measuring cylinder measures the volume of a liquid in millilitres (mL). A kitchen measuring jug does the same job, but more roughly.
- A balance or set of scales measures mass (how much matter is in an object) in grams (g) or kilograms (kg).
- A thermometer measures temperature in degrees Celsius (Ā°C). Pure water freezes at 0 Ā°C and boils at 100 Ā°C.
- A stopwatch or timer measures time in seconds (s) and minutes.
Choosing the right instrument matters. You would not weigh a feather on bathroom scales, and you would not measure a swimming pool with a 30 cm ruler. Match the tool to the size of the job.
The syllabus asks you to use both analog and digital measuring devices. The difference is in how they show you the reading.
- An analog instrument shows the reading on a marked scale, and you read where a pointer, a line of liquid, or a needle sits. A liquid thermometer, a ruler, and a clock with hands are all analog. You have to read the scale yourself, so reading carefully matters.
- A digital instrument shows the reading as a number on a screen. Digital kitchen scales, a digital thermometer, and a stopwatch on a phone are all digital. The number is read for you, which is often quicker and easier.
Neither type is always better. A digital scale is fast and easy to read, but an analog thermometer needs no batteries and keeps working in the bush. Both can be accurate if they are good quality and read properly.
The syllabus asks you to compare the range, sensitivity and accuracy of observations made with different instruments. These are three different ideas, so be careful not to mix them up.
- Range is the smallest to the largest value an instrument can measure. A lab thermometer might have a range of 10 Ā°C to 110 Ā°C. You cannot use it to measure inside a freezer at -18 Ā°C, because that is below its range.
- Sensitivity is the smallest change an instrument can detect, sometimes called its resolution. A ruler marked in millimetres is more sensitive than one marked only in centimetres, because it can pick up smaller differences. Digital scales reading to 0.1 g are more sensitive than ones reading to 1 g.
- Accuracy is how close a reading is to the true value. An accurate instrument gives a reading near the real answer. A cheap scale that always reads 50 g too high is not accurate, even if its numbers look precise.
A quick way to remember: range is how far it can measure, sensitivity is how small a change it can spot, and accuracy is how close it is to the truth. A good instrument for a job has a suitable range, enough sensitivity, and good accuracy.
Even a great instrument gives a wrong number if you read it badly. Two rules help you read analog scales properly.
- Read at eye level. Look straight at the scale, not from above or below. If you tilt your head, the pointer can seem to line up with the wrong mark. This error is called parallax.
- For liquids, read the meniscus. When a liquid sits in a measuring cylinder, its surface curves. This curve is called the meniscus. Read the bottom of the curve, at eye level, against the scale.
So to measure 50 mL of water, you pour until the bottom of the meniscus sits exactly on the 50 mL line, with your eye level with that line. Read it from above and you might record 53 mL by mistake. Careful reading is part of making a good observation.
To read a scale fairly you should look at it at ___. When you read the volume of a liquid in a measuring cylinder, you read the bottom of the curved surface, which is called the ___. A measuring cylinder measures volume in ___, while a balance measures mass in ___.
Scientists pick an instrument by thinking about the three ideas together: range, sensitivity and accuracy. The Bureau of Meteorology, Australia's weather agency, is a great example. To report the weather they choose instruments carefully.
- To measure air temperature they use a thermometer with a range that covers a hot Australian summer and a cold alpine night, and enough sensitivity to read to a tenth of a degree.
- To measure rainfall they use a rain gauge marked in millimetres, sensitive enough to record even light drizzle.
- To measure wind speed they use an anemometer that reads in kilometres per hour, with a range up to storm-force winds.
In your own experiments, ask the same questions. Does the instrument cover the values I expect (range)? Can it detect the small changes I care about (sensitivity)? Does it give readings close to the true value (accuracy)? Choosing well is part of working scientifically, and it makes your observations far more useful.
Two students measure the same small rock. Student A uses kitchen scales that read to the nearest 1 g and records 24 g. Student B uses a digital balance that reads to the nearest 0.01 g and records 23.78 g. Predict: which student's instrument is more sensitive, and why does that matter when you want to detect a small change in mass?
How close was your prediction?
At the start of the lesson you listed instruments and chose a tool to measure water in a medicine cup. Now write an improved, complete answer.
Your answer must: (1) name three instruments and the quantity and unit each measures; (2) explain the difference between an analog and a digital instrument; (3) define range, sensitivity and accuracy in your own words. Use the words range, sensitivity and accuracy.
Q1. Name three measuring instruments. For each one, state the quantity it measures and the unit it uses. (3 marks)
Q2. Explain the difference between an analog instrument and a digital instrument. Give one example of each. (4 marks)
Q3. A student must measure how much water, to the nearest millilitre, is in a small beaker. They have a kitchen jug marked in 100 mL steps and a measuring cylinder marked in 1 mL steps. Which should they choose, and why? Use the words sensitivity and accuracy in your answer. (3 marks)
Answers
ā¾MCQ 1
B. Mass is measured with a balance or scales, in grams. A measuring cylinder reads volume, a thermometer reads temperature, and a stopwatch reads time.
MCQ 2
C. Kitchen scales that show the mass as a number on a screen are digital. A printed ruler, a liquid thermometer, and a clock with hands are all analog, because you read a marked scale.
MCQ 3
A. Sensitivity is the smallest change an instrument can detect. The smallest to largest value is the range, and how close a reading is to the true value is the accuracy.
MCQ 4
D. A freezer at -18 Ā°C is below the thermometer's lowest value of 10 Ā°C, so it is outside its range. The other three readings all fall between 10 Ā°C and 110 Ā°C.
MCQ 5
B. You read the bottom of the curved liquid surface, the meniscus, at eye level. Reading from above or below leads to a parallax error and a wrong value.
Short Answer 1
Model answer: A ruler measures length in centimetres or millimetres. A balance measures mass in grams. A thermometer measures temperature in degrees Celsius. (Other correct pairs are a measuring cylinder for volume in millilitres, or a stopwatch for time in seconds.)
Short Answer 2
Model answer: An analog instrument shows the reading on a marked scale that you read yourself, for example a liquid thermometer or a ruler. A digital instrument shows the reading as a number on a screen, for example digital kitchen scales or a stopwatch on a phone.
Short Answer 3
Model answer: They should choose the measuring cylinder marked in 1 mL steps. It is more sensitive, which means it can detect smaller changes, down to a single millilitre, while the jug can only show steps of 100 mL. Because the smaller marks let you line up the meniscus closely with the true level, the reading is more accurate, so it is the right tool for measuring to the nearest millilitre.