History of Temperature Measurement

History about Temperature

• In 1592, Galileo Galilei invented one of the first devices to measure temperature changes: the liquid-in-glass thermometer
• In 1643, Athanasius Kircher created the first mercury thermometer which improved upon earlier designs by offering a more precise & consistent measurement.
• In 1714, Daniel Gabriel Fahrenheit invented both the mercury thermometer and the Fahrenheit scale
• The Fahrenheit scale became widely used for temperature measurements, particularly in English-speaking countries.
• In 1742, Anders Celsius proposed the centigrade scale, later known as the Celsius scale
• The Celsius scale defined 0°C as the freezing point of water and 100°C as the boiling point, offering a more universal temperature measurement system.
• In the 1800s William Thomson, later known as Lord Kelvin, postulated the existence of absolute zero, the theoretical lowest temperature where particles cease to move
• This idea formed the basis for the Kelvin scale of temperature.
• In 1821, Thomas Seebeck discovered the principle behind the thermocouple and the thermoelectric current
• This discovery became the foundation for one of the most widely used temperature sensors in modern technology.
• In 1821, Sir Humphry Davy noted the temperature dependence of metals, further enhancing the understanding of how materials react to heat.
• In 1932, C.H. Meyers built the first Resistance Temperature Detector (RTD), a key device in industrial temperature measurement due to its accuracy and stability.
• In 1948, the centigrade scale was officially renamed the Celsius scale, a shift to standardize terminology in the scientific community.
• In the 20th century, the development of temperature sensors became fully realized with the creation of various advanced technologies and materials
• This has made temperature measurement more accurate and accessible across many fields, from industry to research.

Temperature Measurement

• Temperature, also referred to as thermodynamic temperature, measures how hot or cold something is by assessing the average kinetic energy of the particles in a system.
• While there is no theoretical maximum temperature, there is a minimum known as absolute zero, where all molecular motion ceases.
• Temperature measurement, also known as thermometry, describes the process of measuring a current local temperature for immediate or later evaluation
• Temperature measurement can be classified into thermometers, probes, and non-contact methods

Modes of Heat Transfer

• Conduction is the process where heat or electricity transfers directly through a substance due to a temperature difference or electrical potential between adjacent regions, without any movement of the material itself.
• Convection refers to the movement within a fluid (liquid or gas) caused by the tendency of warmer, less dense material to rise while cooler, denser material sinks, driven by gravity.
• Radiation is the emission of energy in the form of electromagnetic waves or moving subatomic particles, particularly high-energy particles that can cause ionization.

International Practical Temperature Scale

• The International Practical Temperature Scale forms the basis of most present-day temperature measurements
• The scale was established by an international commission in 1948 with a text revision in 1960 and was formally revised in 1990 and remains in use today.

Primary Temperature Points Defined by the IPTS

• Triple Point of Hydrogen 13.81°K or -259.34°C
• Liquid/Vapor Phase of Hydrogen at 25/76 Std. Atmosphere 17.042°K or -256.108°C
• Boiling Point of Hydrogen 20.28°K or -252.87°C
• Boiling Point of Neon 27.102°K or -246.048°C
• Triple Point of Oxygen 54.361°K or -218.789°C
• Boiling Point of Oxygen 90.188°K or -182.962°C
• Triple Point of Water 273.16°K or .01°C
• Boiling Point of Water 373.15°K or 100°C
• Freezing Point of Zinc 692.73°K or 419.58°C
• Freezing Point of Silver 1235.08°K or 961.93°C
• Freezing Point of Gold 1337.58°K or 1064.43°C

Non Electric Temperature Sensors

• Liquid-in-Glass Thermometers
• Many liquid-in-glass thermometers use mercury, which remains liquid within the temperature range of approximately -40 to 700°F (-38.9 to 356.7°C)
• Bimetallic Thermometers
• Bimetallic Thermometers bond two metals with different expansion rates together, creating a bimetallic strip
• These thermometers are commonly found in temperature switches and thermostats, with a temperature range of 100 to 1000°F (-73 to 537°C)
• Filled-bulb thermometers have been used for many years and have a useful range from -125°F to 1200°F
• These thermometers rely on fluid expansion to measure temperature
• When a fluid inside a sealed container is heated, its molecules increase pressure on the container's walls
• There are 4 types of filled bulb temperature sensors in use in industrial applications
• Liquid Filled Systems Temperature Sensors (Class I) use a liquid filling to measure temperature
• In these systems, the liquid's volumetric expansion drives an indicator mechanism to display the temperature
• The steel bulb, stem, and indicator are fully filled with a liquid under pressure ensuring a constant volume
• Vapor Filled Systems Temperature Sensors (Class II) use a combination of a volatile liquid and its vapor to create temperature-dependent fluid expansion
• This measurement method relies on the vapor-pressure characteristics of the fluid, with temperature being measured at the point where the fluid transitions between its liquid and vapor phases
• The expansion of the vapor as it heats up drives the temperature indication
• Gas Filled Systems Temperature Sensors (Class III) detect temperature changes by measuring the pressure variation in response to temperature.
• With the volume held constant, the pressure changes in direct proportion to the absolute temperature
• Gas-filled systems offer a quicker response compared to other types of filled devices and are useful in pneumatic systems, as they convert temperature directly into pressure.
• Mercury Filled Systems Temperature Sensors (Class V) differ from other liquid-filled systems due to the unique properties of mercury
• Mercury offers the widest operational range (-40°C to 650°C) but its toxicity can pose risks to both industrial processes and human health, leading to a decline in its use in filled systems
• Despite this, mercury-filled systems are still valued for their ability to cover a broad temperature range.
• Bistate or phase change sensors are low-cost, non-electric devices made from heat-sensitive crystalline solids
• These change from solid to liquid at a fixed temperature, accompanied by a color change
• The temperature at which the change occurs depends on the specific blend of materials used

Electronic Thermometers/Sensors

• Thermocouples
• Thermocouples consists of two wires made from different metals that are joined at one end, known as the hot end.
• The other end, called the cold junction, is where the open circuit voltage is measured
• This voltage, known as the Seebeck voltage, is the electromotive force generated due to the temperature difference between the hot and cold junctions

Principles of Operation in Thermoelectric Effects

• Peltier Effect occurs when a current passes through a thermocouple with its junctions at the same temperature
• Heat is generated at one junction (the "hot" junction) and absorbed at the other (the "cold" junction)
• This effect means thermocouples can not only sense temperature differences but also used to create a temperature difference when an electrical current is applied, making them useful for cooling or heating applications
• Thompson Effect refers to the absorption or release of heat when an electric current passes through a conductor that is unevenly heated
• This effect occurs when there is a temperature gradient along the conductor, causing heat to either be absorbed or released at different points
• Seebeck Effect, when two dissimilar metal wires are joined at one end, they produce a voltage at the other end that is approximately proportional to the temperature difference between the junctions
• Essentially, the junction of two different metals acts like a temperature-sensitive battery

Thermocouple Junctions

• In a grounded junction, the thermocouple wires are physically attached or welded to the inside of the probe wall
• This allows for good heat transfer from the outside and provides faster response times compared to ungrounded junctions
• An ungrounded junction has the thermocouple junction detached from the probe wall
• This design is often used for measurements in corrosive environments since there is no electrical connection between the thermocouple and the outer probe
• In an exposed junction, the thermocouple protrudes from the tip of the sheath, offering the best response time but it is limited to non-corrosive and non-pressurized environments.

Thermopile

• Thermopile detectors consist of several thermocouples connected in either series or parallel
• These detectors are passive radiation-sensing devices that generate a voltage in response to incident infrared radiation
• As a Sensor, thermopile detectors convert thermal energy from infrared radiation into an electromotive force (EMF), which is proportional to the amount of infrared energy absorbed
• The generated voltage can then be used to measure the intensity of the radiation
• As a Generator, in addition to being used as sensors, thermopiles can also function as small power generators, converting heat into electrical energy

Three Laws that Apply to Thermocouples

• The Law of Intermediate Metals states that adding different metals to a thermocouple circuit does not affect the voltage generated, as long as the added junctions are at the same temperature as the original junctions in the circuit
• The Law of Homogeneous Materials states that a thermocouple circuit made from a single type of wire (even if it has varying thicknesses or temperature differences along the wire) cannot generate an electromotive force (EMF)
• This means that a thermocouple must involve at least two different materials to produce a voltage
• The Law of Intermediate Temperature allows a thermocouple calibrated with one reference temperature to be used with another reference temperature
• This can also permit the addition of extra wires with similar thermoelectric characteristics to the circuit without altering the total EMF generated

Types of Thermocouple

• Common thermocouple types include: K, T, J, N, E, S, R, B
• RTD or Resistance Temperature Detector is a temperature sensor that measures temperature based on the principle that the electrical resistance of a metal changes with temperature
• For most metals, the resistance increases in a linear fashion with a rise in temperature, making the relationship predictable and proportional
• Thermistors are like RTDs in that they are resistive temperature sensors that change their resistance in response to temperature variations
• However, thermistors exhibit a much larger change in resistance per degree change in temperature, making them highly sensitive for measurements over narrow temperature ranges
• Pyrometers, also known as radiation thermometers, were invented by Josiah Wedgwood
• Pyrometers are non-contact temperature sensors that measure the temperature of an object by detecting the amount of thermal electromagnetic radiation emitted from the object
• These are commonly used for measuring high temperatures, particularly those above 1500°C, where contact-based methods may be unsuitable or cause damage to the object being measured
• Two main types of pyrometers are Radiation Pyrometers & Optical Pyrometers
• Radiation pyrometers, also known as an infrared (IR) thermometer, detects radiant energy emitted from an object
• All objects emit infrared energy, and the amount of this energy is proportional to the object's temperature
• Radiation pyrometers measure the intensity of the emitted infrared energy and generate a signal that is proportional to the temperature of the object
• Optical pyrometers operate on the principle of using the human eye to compare the brightness of the hot object to that of a calibrated lamp filament inside the instrument
• The device compares the brightness of the radiation emitted by the object to a reference lamp, whose brightness can be adjusted