Why is it necessary to use a nonvolatile liquid in a barometer or manometer?

Atmospheric pressure is an essential concept in both weather forecasting and science, referring to the force exerted by the weight of air in the atmosphere at a given point. It is commonly measured using instruments such as barometers and manometers, which employ a column of liquid to detect pressure changes. A barometer measures the atmospheric pressure at a specific location and can predict weather changes, while a manometer is used to measure the pressure in a closed system, often in scientific experiments and industrial processes.

Crucial to both devices is the liquid column, as changes in atmospheric or system pressure are indicated by fluctuations in the height of the liquid. Consistent measurements are essential for accurate readings, which is why the selection of an appropriate liquid is pivotal.

In the context of barometers and manometers, the significance of using a nonvolatile liquid cannot be overstated. A nonvolatile liquid, by definition, has a low tendency to evaporate. This property is critical because the central function of these measuring devices is to correlate changes in liquid height directly with pressure changes.

A key characteristic of a nonvolatile liquid is its stability, which ensures that the liquid's volume remains consistent over time. As evaporative processes are minimal, fluctuations in the height of the liquid column can be accurately attributed to actual changes in pressure rather than variations in the liquid's volume. This precision is essential for dependable readings, making nonvolatile liquids like mercury the liquid of choice in traditional barometers and manometers.

Understanding the effects of volatility on pressure readings is crucial for comprehending why volatile liquids are unsuitable for use in barometers and manometers. Volatility refers to how readily a liquid evaporates at a given temperature. When a liquid is volatile, it can easily turn into vapor, even at lower temperatures. In the closed environment of a barometer or manometer, the use of a volatile liquid would lead to erroneous readings due to the evaporation of the liquid itself.

With volatile liquids, evaporation that occurs inside the tube could contaminate the vacuum or air space above the liquid. This introduces additional variables, such as changes in the concentration of the evaporated substance, that can alter the pressure inside the tube independently of the external atmospheric or system pressures. Such unwanted evaporation would cause a decrease in liquid level that does not reflect the true pressure changes, resulting in unreliable and inaccurate measurements.