Constant-volume gas thermometers


Constant-volume gas thermometer works on the principles of thermodynamics. It us understand this.

  • The best illustration of the zeroth law of thermodynamics is provided by thermometers. Constant-volume gas thermometers are crucial because other thermometers are calibrated using them.

  • A mercury manometer is typically attached to a bulb that is filled with a fixed volume of a diluted gas to create a constant-volume gas thermometer. The manometer is used to gauge pressure variation.

  • According to the Gay-Lussac’s law, an ideal gas's pressure rises in direct proportion to its temperature. Additionally, as temperature drops, so does the pressure similarly drops.

Temperature & Thermometer

  • An instrument that measures temperature is called a thermometer. Food temperatures as well as those of other solids, liquids, and gases, including air, can be determined by it. The three most widely used temperature units are Celsius, Fahrenheit, and Kelvin.

  • The metric system includes the Celsius scale. The metric system of measuring also contains units for length and mass. Nearly all nations in the world use the metric system as their official unit of measurement, which includes Celsius.

  • The Celsius scale is used to measure temperature in most scientific disciplines. Water has a zero-degree Celsius freezing point and a 100-degree Celsius boiling temperature.

  • When recording extremely exact temperatures, physicists and other scientists utilize the Kelvin scale. The temperature for “absolute zero,” or the complete lack of any heat energy, is included in the kelvin scale, which is the only unit of measurement to do so.

  • As a result, scientists determine the temperature of objects in the frigid depths of space, depending on the kelvin scale. At 273 kelvins, water freezes, and at 373 kelvins, it boils.

  • Because the kelvin scale uses such big numbers, a day with 75 degrees Fahrenheit (ca. 24 °C) would be read as 297 kelvins, we do not measure outdoor temperatures in this manner.

Properties of Thermometric Liquid

  • A liquid that may adjust its volume in response to the temperature of the environment is referred to as a thermometric liquid. In other words, thermometers measure temperature by using thermometric liquids.

  • Alcohol and mercury are both typical thermometric liquids. In the usable range, neither freezing nor boiling should occur.

  • The container shouldn't be corroded by it. If the thermometer breaks, it shouldn't be poisonous or otherwise dangerous. Mercury and alcohol are the two liquid thermometers.

  • A liquid thermometer contains a liquid, such as alcohol, to measure temperature. It is used to measure temperature in locations where mercury thermometers are ineffective.

Ideal gas Equation

  • The relationship between a gas's volume V, pressure P and temperature T in the range of low pressures and high temperatures, when the gas's molecules move virtually not depending on one another, is known as the ideal gas law, also known as the perfect gas law.

  • In this situation, the ideal gas law, often known as an equation of state, governs all gases: PV is equal to nRT, where n is the number of moles of the gas and R is the constant for universal gases.


  • The volume of gases that are created or consumed can be determined using the ideal gas law.

  • Vehicle airbags operate according to the ideal gas law. The various gases immediately fill up inside the airbags after they are inserted, inflating them. The interaction between Sodium Azide and Potassium Nitrate fills the airbags with Nitrogen gas.

A constant-volume Gas Thermometer

  • By altering the pressure of the gas inside the gas tube, constant-volume gas thermometers show temperature changes. The volume of the gas inside the gas tube is fixed.

  • The apparatus must consequently be connected to a constant volume thermometer to measure the temperature change. This kind of thermometer has two arms that are enclosed in a flexible tube.

  • Mercury in the arms must be kept at a specific level to keep the mercury level in one of the arms constant. By physically moving the other tube containing mercury up and down, the mercury level inside the tube can be kept constant.

  • The change in pressure seen in one tube and the alteration in mercury levels in both tubes are indicators of any change in the temperature value of the gas as seen in the gas tube. By maintaining the gas inside the gas tube at triple point water temperature, we can calibrate this thermometer.


  • A constant-volume gas thermometer operates according to the Law of Gay-Lussac. According to this law, an ideal gas's pressure rises in direct proportion to its temperature.

  • Additionally, pressure decreases similarly to temperature. Gas thermometers with a constant volume measure the increase in temperature as a function of changing pressure while keeping the volume constant.


The system's temperature change is calculated using a constant-volume gas thermometer. It has a tube with a fixed volume of gas inside, and pressure varies with temperature is one of its thermometric properties.

Gay-Lussac Law

  • A gas law known as Gay-Lussac law asserts that a gas's pressure (when kept at a constant volume and mass) varies directly with its absolute temperature.

  • In other words, a gas's pressure is proportional to its temperature even though its mass is fixed, and its volume is constant.

  • French scientist Joseph Gay-Lussac created this law in 1808. Gay-Lussac's law can be expressed mathematically as follows:





$$\mathrm{P_2=Final\: pressure}$$




A constant-volume gas thermometer is a thermometer used to measure pressure change. In such thermometers, the volume of the gas filled in the bulb attached to the manometer apparatus is fixed. As the only varying properties are temperature and pressure, it follows Gay-Lusaac’s law, where pressure is directly professional to the temperature. This thermometer's advantage over other thermometers is that it will give an accurate result.


Q1. What significance does Gay Lussac's law have?

Ans. This gas law's significance is that it shows how an increase in a gas's temperature causes a corresponding rise in its pressure (keeping the volume constant). In a similar vein, dropping the temperature permits a proportionate drop in strain.

Q2. What is the temperature range for a constant-volume gas thermometer?

Ans. The thermodynamic temperature scale was achieved using a helium constant, volume gas thermometer measures the temperature range of -183 °C to 100 °C.

Q3. What are the types of thermometers?

Ans. There are mainly two types of thermometers: laboratory and clinical thermometers.

Q4. How accurate is a constant volume gas thermometer?

Ans. Over a wide range of values, a constant-volume gas thermometer enables reliable measurement. As we've seen, a constant-volume gas thermometer is filled with perfect gas, which is only possible at high temperatures and low pressures where there is virtually no intermolecular interaction.

Q5. What is the application of Charles' law in real life?

Ans. The practical application of Charles Law is also evident in our kitchen. When bread and cakes are fermented, yeast is employed to create a soft, pliable texture. The expansion of carbon dioxide gas occurs when bread and cakes are baked at high temperatures.

Updated on: 26-Apr-2023

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