Safe measurement practice

Safe measurement begins before the instrument is switched on: first list the physical quantities and possible hazards — electrical voltage, high temperature, bright light, sharp or rotating parts.

Then choose an instrument with a suitable range ($U_{\max}$, $I_{\max}$, $t_{\max}$) and check the case, insulation, probes, and fuse. The key assembly rule: make every change in a circuit with the source switched off — “switch off first, then change”. In electrical measurements the ammeter is connected in series and the voltmeter in parallel, starting from the largest range and only then reducing it for precision. It is useful to estimate the expected value in advance, for example $I = U/R$, to confirm that it does not exceed the instrument's $I_{\max}$ or the safe current of the wires. If a measurement involves the $220\ \text{V}$ mains, water near energised instruments, or an open flame, replace it with a computer simulation.

You need to measure the temperature of hot water heated to about $45\ \text{°C}$. List three actions that make this measurement safe and explain why each matters.

In a circuit with a battery $U = 9\ \text{V}$ and a lamp of resistance $R = 10\ \Omega$ you must measure the current. Describe a safe procedure, the rule for connecting the ammeter, and estimate the expected current using $I = U/R$.

A classmate proposes to measure the resistance of a kettle heating element with a multimeter in ohmmeter mode without unplugging it from the socket. Explain why this is dangerous and physically invalid, and propose a safe measurement plan or a simulation substitute.

Risk-free self-check (all hazardous parts only in the model).

1. Build a table “hazard → control measure” for three measurements: hot-water temperature, current in a circuit with a $9\ \text{V}$ battery, and lamp illuminance.
2. In a simulation (for example, PhET “Circuit Construction Kit”), assemble a series circuit: battery, switch, lamp, ammeter. Before switching on, predict the current with $I = U/R$ and select the instrument range.
3. Change the circuit only with the switch open. After each change, record the reading and compare it with the prediction: a difference greater than $10\%$ is a reason to check polarity and range.
4. Perform real measurements only with safe parameters: water not above $50\ \text{°C}$, source not above $9\ \text{V}$, dry hands, and undamaged probes. The $220\ \text{V}$ mains is not used in independent study.

Show where the hazardous zones are in a measurement setup and how to connect the instruments correctly so that a short circuit and electric shock are excluded.

A labelled circuit diagram: source $U$, an open switch $S$, a lamp, the ammeter in series with the lamp, and the voltmeter in parallel with the lamp. Callouts: “open $S$ first”, “check the range $I_{\max}$ and polarity”, “dry hands”, “never connect the ammeter directly across the source”.

In the kitchen you deep-fry potatoes. The oil is heated to $180$ °C. To check the temperature you lower the probe of a kitchen thermometer into the oil, keeping your hand $20$ cm from the rim of the pan and not leaning over it. A drop of water that falls into the oil instantly turns to steam and spatters the oil: at $100$ °C the volume ratio is about $V_{\text{steam}}/V_{\text{water}} \approx 1600$. So the lid is lifted away from you, and your face stays at least $30$ cm away.

This exercises the rule of safe distance and the ban on contact with a hot medium. The thermometer scale is $0$–$300$ °C with $1$ °C divisions; the reading is taken without touching the walls, so you measure the oil, not the metal.

In a PE lesson, after a $100$ m sprint you measure your pulse. In $10$ s you counted $18$ beats, so the frequency is $f = 18/10 = 1.8$ Hz, or $108$ beats/min. Two fingers are placed on the radial artery at the wrist, not on the carotid artery in the neck: pressure on the carotid can cause dizziness. You must stand behind the side line of the track, no closer than $2$ m, because a runner at the finish moves at up to $9$ m/s and brakes in $3$–$4$ s.

This applies the rule of choosing a safe place and a non-invasive method: the measurement must not obstruct moving people and must not compress large blood vessels. The frequency $f = N/t$ is converted to beats/min by multiplying by $60$.

A phone is charged from a $5$ V, $2$ A adapter, so the power is $P = UI = 10$ W. After $20$ minutes of gaming the case has heated to $38$ °C. The temperature is measured with an infrared thermometer from a distance of $5$ cm, without opening the phone or touching the contacts.

Safe measurement requires not opening the lithium battery and not short-circuiting the contacts; at $T > 60$ °C thermal runaway is possible. A non-contact IR thermometer removes the risk of burns and electric shock from a damaged cable.