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You’ve got a mild cold. You’ve heard that something is going around, maybe a virus? Or maybe it’s just allergies. You want to be safe and maybe get the medicine you need before your cold gets worse. So you make an appointment at the clinic for the next day. When you get there, the doctor and/or nurse will first want to get some basic information about your health. So, you get the basic tools of a doctor’s visit: thermometer, stethoscope, weighing scale, and sphygmomanometer. What do they do?
What is a thermometer?
A thermometer is an instrument for measuring 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 amount of mercury in a glass bulb is the temperature sensor, and its rise and fall in response to changes in temperature occurs through a glass capillary marked with numerical values. When the mercury bulb is brought into contact with a hot object, the liquid expands and rises through the capillary. The number next to its uppermost level indicates the temperature of the object.
This relationship between temperature and mercury level requires every mercury thermometer to be carefully calibrated so that the width of the capillary, the ‘base’ temperature of the mercury and the marks on the scale match each other carefully. It is also important that the material filling 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 must do so by 10 units for a 10º C change, or by 0.1 units for every 0.1º C change.
Researchers today have developed more advanced thermometers that use digital components to estimate temperature. For example, the thermal guns used to check the temperature of people entering malls and supermarkets during the COVID-19 pandemic have a detector that interprets the radiated power of a hot body as a change in voltage or resistance in a circuit, and uses this 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 a common sight in clinics and hospitals today.
What is a stethoscope?
The stethoscope is a simple mechanical instrument. It consists of a diaphragm at one end and an earpiece at the other end, with a tube connecting the two. When the diaphragm is placed near the skin, sounds inside the body near that area cause the diaphragm to vibrate, producing pressure waves in the air filled in the tube that reach the listener through the earpiece.
The opposite part of the diaphragm holding the eardrum is bell-shaped. The diaphragm is suitable for catching high-frequency sounds while the bell is suitable for low-frequency sounds. Like most other medical instruments, stethoscopes also have electronic versions with additional features. These are sometimes called stethophones.
A particularly important feature is sound amplification, which helps the listener hear the very low sounds that are transmitted in an acoustic stethoscope. Some companies have developed stethophones that can collect sound data, analyze it for signs of certain heart conditions, and transmit the results wirelessly to a smartphone. Some products also allow the stethophone to obtain an electrocardiogram of the heart.
How do weighing scales work?
Possibly the simplest scale (today) is the spring scale, common in clinics, home bathrooms, and grocery stores.
First a quick fact: an object’s mass is an inherent property, the amount of matter in the body, while its weight is the force exerted on that object by gravity. So a body has mass whether or not there is a force acting on it.
Back to the spring scale: this device has a spring. That’s it. When the spring is placed under a plate and an object is placed on top of the plate, the device is calibrated based on how compressed the spring is, measuring the force exerted by the object. Alternatively, the spring can be attached to a hook at the bottom from which an object can be hung, calculating the force exerted by the object based on how stretched the spring is. The principle is the same.
The operator simply needs to ensure that the spring works as expected at regular intervals and that the weighing scale is calibrated at the location where it is to be used, not at another location, to take into account local changes in gravity.
An electronic weighing scale works the same way, except for the part where the compression or stretch of the spring is converted into a weight measurement. An analog device does this by calibrating the spring before it is installed in the scale. An electronic device uses a force transducer for this task. Two common ones are the load cell and the strain gauge. A load cell is a type of circuit that, when voltage is applied, transmits varying amounts of current depending on the weight acting upon it. A strain gauge is a conductor whose resistance changes depending on its length.
What is a sphygmomanometer?
A sphygmomanometer is a device that monitors blood pressure. Its name is of Greek and French origin meaning “pulse pressure monitor”. A common version of the device is manually operated. It consists of an inflatable cuff and a manometer, a device that measures pressure. The cuff is strapped around the person’s arm, positioned at approximately the same height as the heart. The pressure inside the cuff is increased by pumping a bladder of air. The doctor or nurse tracks the cuff’s pressure by attaching it to a manometer, which measures pressure as the level of mercury in a capillary (as in a mercury thermometer). The worker also has a stethoscope placed over the brachial artery, the main artery that brings blood to the upper arm.
When the pressure in the cuff reaches an appropriately high value, the artery will be compressed and blood flow will stop. The worker then gradually reduces the pressure in the cuff by about 2 mm of mercury per heartbeat. When the pressure in the artery drops below the limit that allows blood flow, a hissing sound can be heard through the stethoscope. The manometer reading at this point is the systolic pressure. When the hissing sound stops completely, the manometer reading is the diastolic pressure.
This sound is called the Korotkov sound, after the Soviet physician Nikolai Korotkov, who discovered it in 1905.
Electronic sphygmomanometers automate this process, but with a slight difference. When the pressure in the cuff is neither too high to stop blood flow nor too low to allow blood to flow unhindered, and only partially stops the flow, the cuff pressure will oscillate over a small range in sync with the expansion and contraction of the artery. The device uses this oscillation to calculate blood pressure.
Electronic sphygmomanometers require less skill to operate and can be used at home, but they are less accurate than the mechanical, mercury-based version, which is preferred during diagnostic tests and less critical clinical settings. In particular, electronic devices fail to give reliable readings when the person has a pulse-related abnormality such as pulsus paradoxus or arrhythmia.
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You take data about your weight, blood pressure and body temperature to the doctor, who becomes aware of your general condition – all this in just a few minutes. You also describe your cold symptoms and medical history. She takes out her stethoscope and begins the consultation with a good idea. What’s going on,
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