Measuring Instruments and Workshop Practice
Measuring Instruments and Workshop Practice
This domain tests measurement of length, level, moisture, viscosity and coating thickness, plus the workshop habits that keep measurements trustworthy. Expect questions on instrument choice, zero checks and unit conversion.
Instruments and what they measure
- Steel rule and tape: linear dimensions. Read at eye level to avoid parallax.
- Try square, bevel and spirit level: squareness, angles and level. Check the level by reversing it on the same surface.
- Vernier caliper and micrometer: outside, inside and depth dimensions to 0.02 mm or 0.01 mm.
- Moisture meter: substrate moisture. Wood is often specified at 10–15%; masonry and plaster must be dry before painting.
- Hygrometer and thermometer: air temperature and relative humidity; most solvent-borne paints need above 5 °C and below 85% RH.
- Flow cup: viscosity in seconds. A Ford 4 cup is common; the reading is meaningless without the stated temperature.
- Wet film comb: wet film thickness. Dry film thickness gauge: magnetic, eddy-current or ultrasonic, matched to the substrate.
Workshop practice
Calibrate or zero-check gauges before use, record readings, and take several readings per area rather than one. Keep instruments clean and stored in cases.
Worked example
A wet film comb reads 150 µm. The product has 60% solids by volume. Dry film thickness = 150 × 0.60 = 90 µm. If the specification is 75 µm minimum, the reading passes. The reasoning is that only the solid portion remains after solvent evaporates.
Traps
- Using a magnetic DFT gauge on aluminium or on an uncured film.
- Reading a flow cup without noting temperature.
- Trusting a single DFT reading; take ten readings and average.
- Measuring moisture through a metallic foil or over a skim coat.