What is thermometer

Thermometer is an instrument that helps us measure human body temperature. In hospitals you might have seen doctors and nurses using this instrument to measure patient temperatures. Thermometers are also used for measuring air, water, food and other temperatures. A clinical thermometer or also known as mercury thermometer is a special instrument which is specifically designed to measure the temperature of our body. There are three types of clinical thermometers mercury, digital and infrared.

A mercury instrument has been the mainstay of taking temperatures for years. Mercury thermometer is a narrow glass tube which has the same thickness throughout its length. On the inside section of this tube, there is a narrow bulb, which contains mercury. Inside the instrument there is also an additional thin tube which originates from the inner bulb which is called capillary tube. For reading the temperature there is a scale printed inside the glass.

With rise in temperature mercury expands in the tube and travels up the measuring dial inside the instrument. The capillary tube prevents the flowing of the mercury back into the tube, which gives more time for us to read the temperature. In order to get a correct reading of the temperature there are some divisions between two consecutive numbers.

In Europe all body temperature measuring instruments usually show the temperature units in Celsius, while in the United States thermometers are designed to measure the temperature in Fahrenheit.

How to use a mercury thermometer?

What you want to do before using the instrument is what clinicians call, shake it down. By shaking the instrument before use, it will mechanically move the mercury from the upper end of the unit down toward the mercury bulb end. It is necessary to shake the thermometer below 97 degrees Fahrenheit or 35 degrees Celsius. Mercury thermometer is nice and versatile instrument, which can be used both under the tongue as well as under the arm.

After shaking the instrument down, you must put the instrument under your tongue and keep your mouth shut and still for about 2 minutes, this is called an equilibrium reading.  When measuring the temperature under the arm, it can take a little longer usually around 3 to 5 minutes. Normal temperature of the human body when measured orally is about 98.6 and 97.6-degrees Fahrenheit when measured under the arm.  If a mercury instrument ever breaks, you must make sure to not touch the mercury with your hand, and you must use gloves to collect and properly dispose the mercury.

Digital thermometer

A digital thermometer is an electronic device for measuring human body temperature. This digital instrument consists of a heat sensitive device called a thermistor, which is embedded into the tip of the instrument that goes under a person’s lounge. The difference between mercury and digital thermometer is that digital thermometers are more versatile, safe and they deliver equilibrium readings in 30 seconds or less.

How does a digital thermometer work?

Digital thermometers are more complex instruments than conventional mercury thermometers, they use a semiconductor element called thermistor located inside the instrument to measure temperature. The resistance of the thermistor constantly changes as the temperature to which it is exposed starts to vary. With NTC (negative temperature coefficient) type of the thermistor, the thermistor’s resistance decreases when the temperature to which is exposed increases. Conversely, when the temperature coming from the tip of the thermometer decreases, the resistance increases. A current is constantly being sent through the thermistor by a sensor element located inside the digital instrument.

The amount of the current that is allowed to flow is based on the resistance of the thermistor. The sensor element constantly compares the thermistor’s current readings to the current values that have been stored in its memory. The series of periodic initial readings that are compared are sent to a microprocessor. A microprocessor is an electronic brain that is imbedded inside the digital instrument. It is used in such devices as calculators, computers and electronic games. The microprocessor takes constant periodic temperature readings from the sensor and plugs them into a complex mathematical formula. The calculations made by the microprocessor are made by using the sensor reading to project a body temperature-rise curve.

The microprocessor is able to deliver or predict the equilibrium human body temperature readings in under 30 seconds or less from using this curve. The power source of a digital instrument comes from a tiny battery stored inside the instrument. To extend the life of the tiny battery, the inner microprocessor automatically shuts off the instrument if it’ not used for a certain period of time.

Infrared thermometer

Infrared thermometers provide non-contact temperature measurement making them particularly useful for avoiding the spread of germs and disease from one patient to another.  One of the disadvantages of the infrared instrument is that they measure surface temperature only. One of the biggest advantages of the infrared instrument is the speed of the reading. Infrared is a special type of an energy which is emitted from the human body and its part of the electromagnetic spectrum, which works in along with what is known as emissivity or the measurement of the efficiency where a surface emits thermal energy.

How do infrared thermometer sensors work?

The field of view of a sensor is determined by the angle in which the sensor is sensitive to thermal radiation. This means that the sensor inside the infrared instrument will detect all objects in the field of view.  The sensor returns the average of all objects in the field of view. It’s important that the measured object completely fills the field of view, if it’s not the case then the temperature sensor can detect objects that are not supposed to measured, resulting in an incorrect measurement.

Common key features

  • Quickly detects elevated body temperature
  • Fast response on a backlit LCD screen
  • Measures body temperature from 89.6 to 108.5 F without contact
  •  Adjustable visual and audible alarm when temperature exceeds programmed limit
  • Optimum measurement distance 1.9 to 5.9 inches
  • Does not break easily
  • Eliminates need for replacement supplies
  • Reduces potential spread of disease from using contact devices
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