How does Boys' gas calorimeter measure the calorific value of a gaseous fuel?

In one line

A known volume of gas is burnt at a steady rate, and its heat is absorbed by water flowing through cooling tubes; from the water's mass and temperature rise we get the calorific value per m³.

Explained simply

Picture a gas burner with a coil of water pipes wrapped around the flame chamber. Cold water flows in, hot water flows out. If you measure how much water passed and how much hotter it became while a known volume of gas burned, you know the heat per cubic metre of gas. The steam formed condenses on the cold tubes, so this method also gives HCV, and the collected condensate lets us find LCV.

How it works

1. The gas passes through a gas meter (measures volume V, corrected to STP) and a pressure governor, and burns in a Bunsen-type burner.
2. The burner sits inside a chimney surrounded by copper coils through which water flows at a steady rate.
3. Thermometers record inlet (t₁) and outlet (t₂) water temperatures.
4. The hot combustion gases rise and come down over the coils, giving all their heat to the water.
5. Steam condenses and is collected as condensate (mass m) from the bottom.
6. Run until steady temperatures are reached, then record V of gas burnt, W of water passed and m.

Key formula

\( \text{HCV} = \dfrac{W\,(t_2 - t_1)}{V} \)

\( \text{LCV} = \text{HCV} - \dfrac{m}{V} \times 587 \)

• W = mass of cooling water passed (kg), t₂ − t₁ = rise in water temperature (°C), specific heat of water taken as 1 kcal/kg °C.
• V = volume of gas burnt at STP (m³).
• m = mass of steam condensed (kg) during the same time; 587 kcal/kg is the latent heat of steam.
• Result in kcal/m³.

Quick example

0.1 m³ of gas at STP is burnt; 35 kg water rises by 12 °C; condensate = 0.025 kg.

HCV = 35 × 12 / 0.1 = 4200 kcal/m³. LCV = 4200 − (0.025/0.1) × 587 = 4200 − 146.75 = 4053.25 kcal/m³.

Common mistakes

• Using 0.09H × 587 for a gas when condensate mass is given; use m/V × 587.
• Not converting the gas volume to STP.
• Calling the bomb calorimeter suitable for gases; Boys' calorimeter is used for gaseous and volatile liquid fuels.

How MU asks it

Less frequent than the bomb calorimeter, but may appear as "Explain the determination of calorific value of gaseous fuel" (5 marks) or as a short numerical (3 marks). Write a labelled diagram, steps, both formulas with units, and show the LCV correction clearly.