Types of Thermometer and Working Principles

 

          TYPES OF THERMOMETERS AND 

WORKING PRINCIPLES

INTRODUCTION

A thermometer is a device that measures temperature or a temperature gradient (the degree of hotness or coldness of an object). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb of a mercury-in-glass thermometer or the pyrometric sensor in an infrared thermometer) in which some change occurs with a change in temperature; and (2) some means of converting this change into a numerical value (e.g. the visible scale that is marked on a mercury-in-glass thermometer or the digital readout on an infrared model). Thermometers are widely used in technology and industry to monitor processes, in meteorology, in medicine, and in scientific research.

Any substance that somehow changes with alterations in its temperature can be used as the basic component in a thermometer. Gas thermometers work best at very low temperatures. Liquid thermometers were once the most common type in use. They were simple, inexpensive, long-lasting, and able to measure a wide temperature span. The liquid was almost always mercury or coloured alcohol, sealed in a glass tube with a gas like nitrogen or argon making up the rest of the volume of the tube.

In the early 21st century, mercury thermometers were supplanted by electronic digital thermometers, which were more accurate and did not contain toxic mercury. Digital thermometers use a thermistor, a resistor with a resistance that varies with temperature. To measure body temperature, infrared thermometers that focus infrared light onto a detector that measures the amount of light received and convert the electrical signal produced by the detector into a temperature were also used.

HISTORY

Air expands and contracts with temperature. So does water. So do metals like mercury. Even before Galileo - who understood that science began with meticulous measurement - savants had worked out that it ought to be possible to use air and water to invent a "ruler" that would mark the grades from cold to hot and back again. They experimented with thermoscopes involving a column of air in a tube with one end in a container of coloured water. In 1610 Galileo tried it with wine instead, and so is credited with the first alcohol thermometer.

The first sealed thermometer was designed in 1641 for the grand duke of Tuscany: it used alcohol, and it had degree marks. But the man credited with using the freezing point of water as the "zero" or starting point was a Londoner, Robert Hooke, in 1664. An astronomer called Roemer in Copenhagen chose ice and the boiling point of water as his two reference points, and started keeping weather records, but there were still uncertainties about how to devise an accurate scale that would be reliable everywhere.

In 1724, a German instrument maker called Gabriel Fahrenheit settled on mercury as the most suitable liquid for measuring temperature. He calibrated his first thermometer using a mixture of ice and water with sea salt as his zero. But salt water has a much lower freezing point than ordinary water, so for his purposes he chose his freezing point as 30, and the temperature inside the mouth of a healthy human as 96. With those three points, he established the boiling point of water at 212 and later adjusted his freezing point of water to 32. That way, he could count 180 degrees between boiling and freezing, at sea level.

But 180 was still an awkward number. So two decades later, Linnaeus - the Swede who invented the taxonomic system naturalists now use for naming species - and a Swedish astronomer called Anders Celsius separately worked out a scale of just one hundred degrees between freezing and boiling points. Because there were 100 steps between the two states, it was called a "centigrade" scale.

A little more than a century later - in 1848 - Lord Kelvin started contemplating the theory of heat and of course a much greater range of temperature. He used the centigrade scale, but started from absolute zero, the point at which all molecular motion stops, the lowest conceivable temperature in the universe. This turned out to be -273.16C. There is an absolute temperature version of Fahrenheit, called the Rankine scale, but hardly anybody uses it. In 1948, an international conference on weights and measures adopted the Celsius scale as the standard measure, but old habits die hard, and Fahrenheit is still widely used in Britain and the US.

 

What Are the Different Types of Thermometers? What You Can Use to Check Your Temperature

 

Following are the different types of thermometers that we use on a daily basis:

• Clinical thermometer /medical thermometer
• Laboratory thermometer
• Digital thermometer
• Infrared ear thermometer
• Mercury thermometer
• Alcohol in glass thermometer
• Constant pressure gas thermometer
• Constant volume thermometer
• Platinum resistance thermometer
• Thermocouple thermometer
• Pyrometer thermometer

Clinical Thermometer

                  

Clinical thermometers are meant for clinical purposes. It is developed for measuring the human body temperature. It is a long narrow glass tube with a bulb containing mercury at the end. The normal human body temperature is 37˚C; which can fluctuate between the ranges 35˚C to 42˚C. Hence, the clinical thermometers have the range 35˚C to 42˚C. The level of mercury tells our body temperature in ˚C. Since mercury is a toxic element, thus these thermometers have been replaced by digital thermometers nowadays.

 

Digital Thermometer

These thermometers are used for measuring the temperature by the means of an electronic circuit. The information captured is sent to a microchip that processes it and gets displayed numerically on the digital screen. They are easy to use, inexpensive, and accurate. Digital thermometers can be considered to be advanced thermometers that are used for measuring body temperature.

 

How to use Digital Thermometer and Clinical Thermometer?

• Turn on the thermometer and click till the number zero appears on the display.
• Place the tip of the thermometer under the armpit.
• Wait for a few seconds before you hear the alarm from the thermometer.
• Remove the thermometer and check for the temperature on the display.
• After use, clean the tip of the thermometer with cotton and alcohol.

Laboratory Thermometer   

Since clinical thermometers can’t be used to measure temperature other than the human body, we need a special type of thermometers for other purposes. A laboratory thermometer, which is colloquially known as the lab thermometer, is used for measuring temperatures other than the human body temperature. It ranges from -10˚C to 110˚C. Laboratory thermometers are designed for lab purposes such as checking boiling point, freezing point, or temperature of other substances. You can use a laboratory thermometer for checking the temperature of a solvent but not a clinical one.

 

Infrared Ear Thermometers

 

The temperature in the ear is also known as the tympanic membrane temperature or eardrum. This is one of the ways to take your body temperature using the inside of your ear. Children like this way of taking temperature because it is taken quickly. The normal ear temperature is 99.5 degrees F (37.5 degrees C) in adults. They should not be over-squeezed or used on ears with a lot of wax.

Infrared thermometers capture body heat in the form of infrared energy given off by a heat source. This type of thermometer is based on the fact that the laws that govern the radioactive emission of bodies allow the precise calculation of the temperature of the radiating object from its emission spectrum, without requiring direct contact with it.

How to uses an Infrared ear thermometer?
To use the thermometer in the ear, also known as a tympanic thermometer, you must:

·        Place the tip of the thermometer inside the ear and point it towards the nose;

·        Press the “power” button on the thermometer until you hear a signal;

·        Read the temperature value, which appears instantly;

·        Remove the thermometer from the ear and clean the tip with cotton or alcohol gauze.

The infrared ear thermometer is very fast and easy to read, so it requires you to regularly buy plastic protective caps, which makes using the thermometer more expensive.

Thermometers such as mercury thermometers, alcohol in glass thermometer, constant pressure gas thermometers, and platinum resistance thermometers are designed on the basis of technology.

 

Mercury Thermometer

These thermometers are not used in some sectors since they can easily break. Mercury is a poisonous liquid that can cause health hazards. Mercury thermometers are used to provide accurate temperature readings. It used to measure the temperature under the arm, orally and rectally.

Alcohol in glass thermometers

 

Made from sealed glass, it displays the temperature by the level at which mercury or alcohol reaches a graduated scale. These liquids dilate and contract due to changes in temperature. Generally, the measurement scale you use is Celsius, although it can also be expressed in degrees Fahrenheit. These thermometers currently contain colored alcohol due to the danger of contact with mercury.

Constant Pressure Gas Thermometer

 

A constant pressure gas thermometer can function at constant pressure and volume conditions. Constant pressure gas thermometer offers highly accurate measurement and is used to adjust other thermometers. 

In this thermometer, volume increases directly with the increase in temperature. It measures temperature in the range from o k 5oo k.

 

Constant Volume Gas Thermometer

 

In a constant volume gas thermometer, the pressure increases when the temperature increases. Constant volume gas thermometer measures temperature in the range 0k to 500 k. 

In this thermometer, pressure increases directly with the increase in temperature. It measures temperature in the range o k to 5oo k.

Platinum Resistance Thermometer

Platinum Resistance Thermometers uses a platinum wire attached to an electrical resistance to measure the temperature. Generally, these thermometers are used to measure outside temperatures. Platinum Resistance Thermometers are slow and offer accurate readings.

The resistance of the material in the thermometer increases when the temperature increases. Platinum resistance thermometers measure temperature in the range of 500 k to 2300 k.

Thermocouple Thermometer

                           

These devices are used for the quick measurement of temperature. It finds its application in laboratories. Electrical resistance is used in the device to generate a voltage that varies according to the connection temperature. It also measures temperature in the range of 500 k to 2300 k. 

 

Pyrometer Thermometer

 

These devices are used for the quick measurement of temperature. It finds its application in laboratories. Electrical resistance is used in the device to generate a voltage that varies according to the connection temperature. It also measures temperature in the range of 500 k to 2300 k. 

 

Which Type of Thermometer is Most Accurate?

According to Mayo Clinic, you will get the most accurate results if you use an oral thermometer. Make sure you’re keeping the thermometer under your tongue with your mouth firmly closed.

 

It’s also important to keep the thermometer in place for at least 30 seconds if you’re using a digital thermometer. You should avoid eating or drinking anything at least 20 minutes before you take your temperature for the best results.

 

 

How do you know if a thermometer is accurate?

To check if a thermometer is accurate, we need to insert the stem of the thermometer in the ice water such that the stem is at least an inch deep without touching the glass. Wait for a minute or less so that the thermometer registers the reading of the ice water. If the thermometer reading is 32℉ or 0℃, then the thermometer is said to be accurate.