Liquid in Glass Thermometer Principle

Glass Thermometer

Liquid in Glass Thermometer

Glass Thermometer

Operating principle of Liquid in Glass Thermometer

  • The apparent thermal expansion of the liquid is the principle utilized to determine the temperature of the working fluid.
  • The thermally sensitive element of Liquid in Glass Thermometers (LIG) is a liquid enclosed in a graded glass envelope.
  • A glass bulb is linked to a sealed glass tube in the liquid in a glass thermometer.

Main Features of Liquid in Glass Thermometer

The following are key characteristics of liquid-in-glass thermometers

  • The amount to which these thermometers are placed into the medium being measured determines the accuracy of the readings.
  • Total, partial, and complete immersion are the three types of immersion categorized by the amount of contact between the medium and the sensing device.    
Liquid level of thermometer
  • An error may exist when the thermometer is not submerged to the same depth as during calibration. An emerging stem adjustment is essential when it is not allowed to immerse the thermometer to sufficient depth.
  • The response time of a liquid-in-glass thermometer is based on the thermometer type, the capacity of the bulb, the thickness of the bulb, & net weight of the sensor.
  • To acquire a rapid reaction, the thermometer’s bulb should be built such that it is and the bulb wall is thin.
  • The thermometer bulb should be designed such that it results in compact & thin wall bulbs to get a quick response.
  • The response time of a liquid-in-glass thermometer varies based on
    • The thermometer type,
    • Its bulb volume,
    • Thickness,
    • Overall weight.
  • Their sensitivity is based on the liquid’s reversible thermal expansion properties in comparison with glass.
  • The more sensitive the thermometer is, the greater the thermal expansion of the liquid.
  • Organic liquids such as toluene, ethyl alcohol, and pentane are used to manufacture these liquid-in-glass thermometers.
  • Despite their rapid thermal expansion, they are non-linear and can only be used at high temperatures.
  • The bulb should be small and the bulb wall should be thin to reduce reaction time.
  • The sensitivity is estimated by the liquid’s reversible thermal expansion in relation to the glass.
  • The thermometer becomes more sensitive as the fluid expands.
  • Mercury was the most often utilized liquid due to its fast reaction time, repeatability, linear coefficient of expansion, and wide temperature range.
  • However, because it is dangerous, different working solutions are utilized.
  • There is no chance of evaporation of liquid mercury since its  boiling point is 357 Degrees Celsius

Thermometric liquids

  • The liquids used in such thermometers must have particular properties for suitable use such that they must not freeze at a lower temperature like water which is not recommended.
  • The liquids must also have a sufficiently high boiling point to avoid vaporization at high temperatures.
  • The liquid must expand uniformly with temperature in the measurement range.
  • Otherwise, an irregular division of the thermometer’s scale would be required.
  • Water is also inappropriate for this reason, as it swells uniformly due to the density anomaly.
  • However, thermometric liquids are liquids that have all of the requisite qualities and are thus appropriate for use in liquid-in-glass thermometers.
  • In previous days mercury used was highly toxic or poisonous with a solidification temperature of -39 °C and a boiling temperature of 357 °C.
  • But, Nowadays instead of mercury blue or red-colored ethanol (alcohol) is commonly employed that has a melting point of -115 Degrees Celsius, & a boiling point of 78 Degrees Celsius
  • Every day temperatures between -20 °C and +50 °C may be effectively covered in this temperature range.
  • The temperature of some thermometric fluid is shown in the table below.
Element with range of temperature

Measuring sensitivity of Liquid in Glass Thermometer

  • The sensitivity of liquid-in-glass thermometers rises as the volume of liquid in the thermometer increases.
  • It will rise and expand more as there is more liquid inside the glass tube.
  • As a result, liquid thermometers include a reservoir to increase the volume of liquid in the thermometer.
  • Nevertheless, too much liquid will cause the thermometer to react very slowly to temperature changes since the liquid will take longer to heat and adjust to the outside temperatures to be shown.
  • Mercury is utilized not only because of its wide temperature range from -39 °C to 357 °C theoretically applicable.
  • It barely wets the glass tube
  • As a result, capillarity is reduced, and temperature may be shown and read more precisely.

Applications of Liquid in Glass Thermometer

  • Measure the temperature of the human body
  • Maintain track record of temperature in calibration and standard rooms
  • Measure the temperature of the air in industrial settings during certain operations such as maintenance of a certain temperature.
  • The “Liquid in Glass Thermometer” measures any fluid temperature ranging up to 340 Degrees Celsius if mercury is used as a sensor bulb.
  • Measure the temperature of
  1. Liquids in open tanks, & containers.
  2. Air ducts,
  3. Molten metal baths,
  4. Fluid flow pipes, and
  5. Cooking kettles.

Advantages of Liquid in Glass Thermometer

  • They are less expensive than other temperature-measuring equipment.
  • They are convenient and efficient to use.
  • They do not require a power supply or batteries to charge, unlike electrical thermometers.
  • Usually, they are used in locations where there is an electrical problem.
  • They have excellent repeatability and their calibration is unaffected.

Disadvantages of Liquid in Glass Thermometer

  • They cannot be used in areas where very accurate readings are required.
  • They are unable to give digital or automated results.
  • Temperature readings must be taken immediately after removal since a glass thermometer might be impacted by the ambient temperature.
  • A Liquid in glass thermometer does not allow us to recollect the measured temperature.
  • If the temperature reading is required on a different scale, temperature conversion is required since these thermometers display temperature in either Celsius or Fahrenheit scales.
  • These liquids in glass thermometers are very weak, & delicate compared to normal electrical sensors.
  • They are deemed unsuitable for applications requiring exceptionally high or low temperatures.
  • They must be handled with extra caution since they are prone to breaking.
  • Its application is restricted to locations where only manual reading is sufficient.

Frequently asked Questions:

What is the principle on which a liquid in a glass thermometer works?

Liquid-in-glass thermometers employ the principle of thermal expansion of the material.

Which thermometer works on the principle of liquid expansion when heated?

  • Mercury thermometers operate on the principle of liquid thermal expansion.
  • When the temperature of a mercury thermometer rises, the mercury expands.
  • Temperature measurements are given by the rise in the amount of mercury.

How does a glass thermometer works

A glass thermometer works on the principle of buoyancy, which determines whether objects float or sink. As temperature rises the glass balls sink to the bottom, As the temperature drops the glass balls float to the top.

What liquid is present in a glass thermometer?


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