A McLeod gauge, also known as a McLeod manometer, is a type of low-pressure measuring device that is used to measure very low pressures, down to pressures near vacuum. It consists of a metal bellows that is connected to a low-pressure chamber and a high-pressure chamber through a calibrated leak. As the pressure in the low-pressure chamber decreases, the bellows expands, and the pressure in the high-pressure chamber compresses the gas in the bellows, causing it to collapse. The amount of compression of the bellows is proportional to the pressure difference between the two chambers.

The McLeod gauge is commonly used in laboratory and industrial settings where very low pressures need to be measured, such as in vacuum systems, cryogenics, and vacuum pumping systems. It is also used in scientific research and in the semiconductor industry to measure ultra-high vacuum (UHV) pressures.

One of the advantages of the McLeod gauge is its ability to measure low pressures with high accuracy and repeatability. However, it can be relatively complex to use and maintain, and it is limited in its pressure range compared to other types of manometers. It is also more expensive than other types of manometers, but its accuracy and repeatability make it well suited for precise low-pressure measurements in scientific and industrial applications.