The simple medical tools of an OPD visit | Explained Premium

Learn about the basic tools used in a doctor's visit: thermometer, stethoscope, weighing scale, and sphygmomanometer, for accurate health assessment.

You’ve developed a minor cold. You heard there’s something going around, maybe a virus? Or maybe it’s just an allergy. You want to play it safe and maybe get the medicines you need before the cold gets worse. So you make an appointment with a clinic for the next day. When you get there, the doctor and/or the nurse wants to get some basic details about your health first. Thus, you encounter the basic tools of a doctor visit: the thermometer, the stethoscope, the weighing scale, and the sphygmomanometer. What do they do?

A thermometer is a device to measure the temperature of an object. Every thermometer has two parts: one part is sensitive to changes in temperature and the other part shows these changes as numbers.

For example, in the once-ubiquitous mercury thermometer, a small volume of mercury in a glass bulb is the temperature sensor and its rise and fall in response to temperature changes happens through a glass capillary marked with numerical values. When the mercury bulb is brought in contact with a warmer object, the liquid expands and rises up through the capillary. The number next to its uppermost level shows the temperature of the object.

This relationship between the temperature and the mercury’s level requires each mercury thermometer to be carefully calibrated such that the capillary’s width, the mercury’s ‘base’ temperature, and the markings on the scale carefully match each other. It’s also important that the material that fills the bulb — mercury in this case — has a linear dependence on temperature changes: that is, if it expands by 1 unit for every 1º C change in temperature, it should do so by 10 units for a 10º C change or by 0.1 units for every 0.1º C change.

Researchers have developed more advanced thermometers today that use digital components to infer the temperature. For example, thermal guns of the sort used to check the temperatures of people entering malls and supermarkets during the COVID-19 pandemic contains a detector that interprets the radiant power of a warm body as changes in the voltage or resistance in a circuit, and uses that to calculate a temperature readout.

Another common type of thermometer is the thermistor: a semiconductor whose resistance is highly sensitive to changes in temperature; thermistor-based thermometers are today a common sight in clinics and hospitals.

The stethoscope is a simple mechanical device. It consists of a diaphragm at one end and an earpiece at the other, with a tube connecting the two. When the diaphragm is placed against the skin, sounds inside the body near that area cause the diaphragm to vibrate, creating pressure waves in the air filling the tube that are transmitted via the earpiece to the listener.