Measuring industrial quantities
Industry measures many quantities: length and diameter, angle, mass, force, temperature, pressure, flow of liquid or gas, speed (rpm) and electric values. Each has an SI unit: metre (m), kilogram (kg), kelvin or degree Celsius, pascal or bar, litre per minute, volt, ampere.
Why measure? To make the part to the right size, to check quality, to keep a process safe (a boiler that is too hot), and to save material and energy.
Three words to know: accuracy (how close to the true value), precision (how close repeated readings are to each other) and least count (the smallest value the instrument can show). A shop usually compares the part with a standard at 20 °C, because metal expands when warm.
Using measuring instruments: rule, vernier caliper, micrometer
Steel rule: marks every 1 mm. Fast, but rough: about 0.5 mm.
Vernier caliper: a main scale and a sliding vernier scale. Least count = 1 main division − 1 vernier division. If 50 vernier divisions match 49 mm, least count = 1 − 49/50 = 0.02 mm. It measures outside size, inside size and depth. Reading = main scale value just before the zero of the vernier + (matching vernier line × least count).
Micrometer: a screw with pitch 0.5 mm and a thimble with 50 divisions. Least count = 0.5 ÷ 50 = 0.01 mm. Reading = sleeve reading + (thimble line × 0.01 mm). Turn only the ratchet knob so the force is always the same.
Using measuring instruments: gauges, dial indicators and sensors
Limit gauges (go/no-go): a part is good if its size lies between two limits. For a shaft, the GO ring is made to the upper (largest) limit, so a good shaft must pass through it; the NO-GO ring is made to the lower (smallest) limit, so a good shaft must not pass through it. They give a quick yes or no without a number.
Gauge blocks and dial indicators: gauge blocks are very flat steel blocks of exact thickness used as standards. A dial indicator shows tiny movement of a probe, so we can check flatness or runout.
Sensors: a thermocouple gives a voltage for temperature, a pressure transducer for pressure, a flow meter for flow. A common industrial signal is a current of 4 to 20 mA: 4 mA means the bottom of the range and 20 mA the top. The computer converts it back to a number.
Good practice: clean the part and the jaws, check the zero reading first (zero error), measure at the same place several times, never force the instrument, and calibrate it against a standard from time to time.
Try it: ruler against caliper
Measure the thickness of a pencil with a school ruler. Then try a drawing compass and the ruler, or a cheap plastic caliper if you have one. Write the numbers. Which reading do you trust more? In the 3D, set the slider to an odd diameter like 15.37 mm and compare all three instruments. Predict first, then look.
Key formulas and definitions
- Least count of vernier caliper = 1 main scale division − 1 vernier scale division (commonly 0.02 mm).
- Least count of micrometer = pitch ÷ number of thimble divisions = 0.5 mm ÷ 50 = 0.01 mm.
- Vernier reading = main scale reading + (matching vernier line × least count).
- Micrometer reading = sleeve reading + (thimble line × 0.01 mm).
- Corrected reading = observed reading − zero error.
- Signal 4–20 mA: fraction of range = (I − 4) ÷ 16.
Worked examples
1. A vernier caliper has least count 0.02 mm. The main scale reads 24 mm and the 17th vernier line matches. Find the reading.
24 + 17 × 0.02 = 24 + 0.34 = 24.34 mm.
2. A micrometer sleeve shows 7.5 mm and the thimble line 23 is at the reference line. Find the reading.
7.5 + 23 × 0.01 = 7.5 + 0.23 = 7.73 mm.
3. A caliper shows 0.04 mm when the jaws are closed (positive zero error). It reads 12.36 mm on a rod. Find the true diameter.
Corrected = 12.36 − 0.04 = 12.32 mm.
4. A shaft must be 20 mm with limits 19.98 to 20.02 mm. A shaft measures 20.03 mm. Will it pass the GO gauge?
The GO gauge is at the upper limit 20.02 mm. 20.03 mm is bigger than that, so it will not pass. Reject the part.
5. A temperature transmitter covers 0 to 200 °C and gives 4 to 20 mA. It shows 12 mA. What is the temperature?
Fraction = (12 − 4) ÷ 16 = 0.5. Temperature = 0.5 × 200 = 100 °C.
6. Two readings of the same rod are 15.38, 15.38 and 15.37 mm but the true size is 15.20 mm. Is the instrument precise, accurate, or both?
The readings are very close to each other, so it is precise. They are far from the true value, so it is not accurate (there is probably an error such as zero error).
Common mistakes
- Not checking the zero reading before measuring. A zero error moves every reading.
- Squeezing the micrometer too hard instead of using the ratchet. This bends the metal and gives a wrong reading.
- Mixing up accuracy and precision. Precise means repeatable; accurate means correct.
- Reading the vernier at an angle (parallax) or taking the wrong matching line.