McLeod Vacuum Gauges Basic

 

McLeod Vacuum Gauges

It works by keeping a definite volume of gas and compress the same with a fixed pressure. The new compressed pressurized volume is then read as a measure of the initial absolute pressure. It generally used to measure vacuum pressure.

At constant temperature,

Boyle’s Law applies: P1V1 = P2V2

Here P1 the initial pressure of the corresponding gas, having been calibrated based on

P1 = P2V2/V1.

For calibrating other detectors, it is preferring as a standard.

90° Rotation Type McLeod Gauge

We have to place a known volume of mercury into the gauge reservoir before measurement. After that, the gauge is connected to the vacuum system for detection and rotate 90 degrees., by keeping a fixed volume of gas, which is compressed by the column of mercury. As per Boyle's Law to read in units of initial pressure the scale has been calibrated to the level at which the mercury rises in the capillary.

It has no moving parts, so the measurement does not affect by friction, inertia, or hysteresis. The only limitation of the unit is due to capillary effects assuming constant temperature and proper quantity of filling mercury. It is hard to obtain a flat mercury surface free from capillary effects below 10,000 mmHg, accuracy is affected.

Piston-Type McLeod Gauge

The unit is fixed and a piston is used to keep the gas. The elevated gas going up when the mercury reaches point D, when the piston with a micrometer adjustment is moved up; thus, the initial volume (V1) is the volume between points A and D. The process pressurizes to fill the gauge with mercury. The mercury level is at point A when the siphon breaks. The mercury level in the reservoir is below point D to allow elevated gases to be free when the instrument connected to the vacuum system. The mercury fills the large bulb to point C as the piston slides up. The compression ratio is increasing until the mercury touch point B above the small bulb if the vacuum is higher by moving the piston upward. Readings are taken on the micron side of a dual scale.

Capillary effects limit the reading accuracy. It generally used as a laboratory instrument that measures the samples and not in the continuous process line. Where a rugged case required in industrial applications, it is also used there. We have to condensate and keep the condensable vapors before going to gauge to avoid inaccuracy.

List of Prominent Manufacturer:

• HyVac Products (www.hyvac.com/products/McLeod)

For manometers, please follow the link: Instrumentation Basics: Manometer Basics (instrumentbasics.blogspot.com)

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