Measuring small furnace draught with an inclined manometer: For accurate measurement of very low negative pressures (draught) in furnaces using an inclined tube manometer, which manometric liquid is most commonly used?

Introduction / Context:
Furnace draughts are typically small pressure differences, often just a few millimetres of water column. Inclined manometers increase the readable displacement by tilting the tube, improving resolution for tiny pressure changes. Selecting an appropriate manometric liquid is essential for sensitivity and safety.

Given Data / Assumptions:

• Draught magnitude is low (order of mm of water gauge).
• Instrument is an inclined glass tube with scale.
• Ambient conditions are standard; the liquid should wet the tube and be safe in boiler rooms.

Concept / Approach:
The pressure difference indicated by a manometer is ΔP = ρ * g * Δh. For a given ΔP, a lower-density liquid yields a larger height change Δh, which is easier to read. Water has much lower density than mercury or carbon tetrachloride, providing larger deflection per unit pressure. It is inexpensive, non-toxic compared with mercury or carbon tetrachloride, and widely available, making it the common choice. Kerosene or glycol may be used for special temperature ranges, but the standard textbook answer for small furnace draught with an inclined tube is water.

Step-by-Step Solution:

Verification / Alternative check:
Boiler operation manuals list inclined water manometers or electronic differential transmitters calibrated in mmWG for draught measurement.

Why Other Options Are Wrong:

• Mercury / carbon tetrachloride: High density reduces deflection; mercury adds toxicity concerns.
• Paraffin / ethylene glycol: Usable but less common and can introduce temperature-viscosity issues.

Common Pitfalls:
Forgetting to correct for temperature (density variation) if high precision is required; bubbles or wetting problems also cause reading errors.

Final Answer:
Water