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Clinical Thermometer Range and How it is Used?
The normal temperature of human body is 37°C (thirty seven degree Celsius). When a person gets fever, his body temperature rises (and he feels hot). The thermometer used for measuring the temperature of human body is called clinical thermometer. A clinical thermometer is used to measure our body temperature by a doctor (or at home). A clinical thermometer is shown in Figure.
A clinical thermometer consists of a long glass tube having a thin and uniform bore. There is a glass bulb at one end of the glass tube. This glass bulb contains mercury. Outside the glass bulb, a small, shining thread of mercury can be seen in the thermometer tube (see Figure). The other end of glass tube is sealed (after removing air). A temperature scale is marked on the glass tube of the clinical thermometer. The clinical thermometer has a temperature scale marked from 35°C to 42°C (see Figure).
The clinical thermometer is a mercury thermometer which has been specially designed to measure the temperature of human body. A clinical thermometer has two special features which make it different from the laboratory thermometer.
(i) A clinical thermometer has a very short range of temperature from 35°C to 42°C (see Figure 4). This means that a clinical thermometer can measure temperatures only from 35°C to 42°C. The short range of a clinical thermometer is due to the fact that the temperature of human body normally does not go below 35°C or above 42°C.
(ii) A clinical thermometer has a kink (or constriction) in its glass tube just above the bulb containing mercury (see Figure). The kink is to prevent the back flow of mercury into the thermometer bulb when the thermometer bulb is removed from the mouth of a patient. The kink prevents the mercury level in the thermometer tube from falling on its own. So, even when the thermometer bulb is removed from the mouth of a patient, the mercury thread will keep standing at the maximum level reached. Due to this we can read the correct body temperature of the patient even after removing the thermometer bulb from his mouth. (After noting the body temperature, the level of mercury can be brought down by giving a few jerks to the thermometer tube).
How to Use Clinical Thermometer to Measure Body Temperature
We can use a clinical thermometer to measure the body temperature of a patient as follows:
Activity 3
1. Wash the thermometer properly (preferably with an antiseptic solution). If antiseptic solution not available, wash it with clean water.
2. Hold the thermometer tube firmly in your hand and give it a few jerks so that the mercury thread in the thermometer tube falls below the reading of 35°C. (Never hold a thermometer from its bulb).
3. Put the bulb of the thermometer under the tongue of the patient and hold the thermometer tube gently. Keep the thermometer bulb in the mouth of the patient for about one minute. Then take out the thermometer from patient’s mouth.
4. To read the temperature, hold the thermometer horizontally in your hand and rotate it slowly. A position will come when you will see a magnified image of the mercury thread in its tube. We read the temperature on thermometer tube in level with the top of the mercury thread. This will give the body temperature of the patient.
Precautions in Using a Clinical Thermometer
- The clinical thermometer should be washed before and after use (preferably with an antiseptic solution).
- Before using the clinical thermometer, we should ensure that the mercury level in its tube is below 35°C mark.
- Read the clinical thermometer by keeping the level of mercury along the line of sight.
- The clinical thermometer should never be held by the bulb while reading it (otherwise the bulb will break).
- Handle the clinical thermometer with care. The thermometer can break if it hits against some hard object.
If we look at the scale marked on the clinical thermometer tube, we will find that the temperature difference indicated by the two big marks (say, 37°C and 38°C) is 1 degree (see Figure). We will also see that there are 10 small divisions between the 1 degree temperature. Now, since 10 small divisions represent 1 degree C temperature, therefore, 1 small division represents \(\frac{1}{10}\) degree C temperature or 0.1. degree C temperature. Thus, with a clinical thermometer, we can measure the human body temperature accurately up to one-tenth of a degree (or 0.1°C).
A clinical thermometer has usually two temperature scales marked on its glass tube on the two sides of the mercury thread : Celsius scale of temperature and Fahrenheit scale of temperature (We have not shown the Fahrenheit scale of temperature in Figure to keep the diagram simple). The Fahrenheit scale of temperature marked on the clinical thermometer has a range of 94°F to 108°F (°F means degree Fahrenheit). The normal temperature of human body on Fahrenheit scale is 98.6°F. Fahrenheit scale of temperature was used in earlier days. India has now adopted the Celsius scale, so we should read the Celsius scale temperatures marked on the clinical thermometer.
The normal temperature of human body is 37°C. By saying that the normal temperature of human body is 37°C, we do not mean that each and every healthy person should have a body temperature of exactly 37°C. Actually, what we call normal temperature of human body (37°C) is the average body temperature of a large number of healthy persons. So, the normal body temperature of every healthy person may not be exactly 37°C. It could be slightly higher or slightly lower than 37°C. So, as long as a person is healthy, he should not worry even if his body temperature as shown by the clinical thermometer is slightly higher or lower than the standard body temperature of 37°C.
How Does a Clinical Thermometer Work
When we put the thermometer bulb in the mouth of a patient, then some of the body heat of patient is transferred to the mercury in the thermometer bulb. This heat makes the liquid mercury of the thermometer bulb to expand (increase in volume) due to which the mercury thread is pushed to a higher level in the thermometer tube. Higher the fever of a patient, greater will be his body heat and hence higher will be the level of mercury or higher will be the temperature shown by the thermometer.
A clinical thermometer cannot be used to measure high temperatures because it has been designed to measure only human body temperature which varies over a short range. If a clinical thermometer is used to measure high temperatures, it will get damaged. For example, we should not measure the temperature of hot milk by using a clinical thermometer. This is because the temperature of hot milk is very high due to which it will cause a large expansion of mercury. And since there is no extra space in the glass tube of clinical thermometer to accommodate large expansion of mercury, the thermometer tube will break. We should also avoid keeping the clinical thermometer in the hot sunshine or near other hot objects (like a flame). It may break under these conditions.
Comparison Between Clinical Thermometer and Laboratory Thermometer
A similarity between clinical thermometer and laboratory thermometer is that both are mercury-in-glass thermometers. The differences between a clinical thermometer and a laboratory thermometer are as follows :
- The clinical thermometer has a very short temperature range (35°C to 42°C) whereas a laboratory thermometer has a large temperature range (usually from, -10°C to 110°C).
- The clinical thermometer has a kink (or constriction) in its tube to prevent the back flow of mercury into the bulb whereas a laboratory thermometer has no kink.
- The clinical thermometer measures temperature more accurately (up to 0.1°C) than a laboratory thermometer (which usually measures up to 1°C).
Digital Thermometers
Most of the common thermometers are mercury thermometers which use a liquid metal called mercury for their working. Mercury is a toxic substance (poisonous substance) and hence it is very difficult to dispose of safely if a thermometer breaks. So, there is a lot of concern over the use of mercury in thermometers. These days, digital thermometers are available which do not use mercury (see Figure).
Maximum-and-Minimum Thermometer
The maximum temperature of the day usually reaches in the afternoon whereas the minimum temperature usually occurs in the early morning. In weather forecasting and other meteorological work, it is necessary to know the highest and the lowest temperatures reached in the surroundings during a whole day. There is a special thermometer called maximum-and-minimum thermometer which automatically records the maximum and minimum temperatures of the day. The maximum and minimum temperatures of the previous day reported in weather reports in TV and newspapers are measured by the maximum-and minimum thermometer.