What liquid is typically used to fill a "U" tube manometer?

A "U" tube manometer is commonly filled with mercury due to its high density and low vapor pressure, which makes it an ideal choice for accurately measuring pressure differences. Mercury's high density allows for a relatively small column height to measure significant pressure differences, making it particularly useful in applications where space is limited. Additionally, mercury does not wet the glass, ensuring that the readings remain clear and accurate without any meniscus interference that can occur with water or oils.

In environments where precision is critical, using mercury helps to reduce the effects of temperature changes that could affect measurements, as mercury maintains a consistent behavior across a range of temperatures. While water or oils could be used in some less critical applications, they are generally not suitable for high-accuracy pressure measurements due to their lower density and potential for vaporization or contamination.

The use of mercury is especially advantageous in applications demanding high sensitivity and precision, such as laboratory settings or industrial process control, where even slight variations in pressure are critical.