Archives for February 2019

Selecting the correct vacuum gauge or gauges is critical to the success of a heat treatment process. It is important to know how they work and what options are available so that the correct choice can be made. There are several important considerations when using a vacuum gauge. They include the method of operation, the gas composition (inert or reactive, corrosive), the gas sensitivity (calibration factor), and the process being performed in your system.

Given the wide range of pressures encountered when running processes in vacuum furnaces (a staggering 9 orders of magnitude), no one gauge is adequate over the entire range of possible vacuum levels. As with vacuum pumps, multiple gauges are necessary to properly cover the entire operating range with the needed precision and accuracy. Given that it is critical to monitor the vacuum pressure at various points in the process and perhaps multiple locations throughout the vacuum system, the correct selection of each gauge ensures that we achieve optimal results.

Dry pumps are becoming increasingly popular as an alternative to oil sealed rotary vane pumps for many medium and high vacuum applications (e.g., in low-pressure vacuum carburizing where fine granular soot is carried from the process into the pump). Designers and users of vacuum furnaces must have a good understanding of how claw and screw pumps operate. This includes the principles of operation, pump design, sealing, operating characteristics, features, purging, and ancillary devices.

A dry screw pump (Fig. 1) is used alone when high vacuum is not required and a slower drawdown is allowable, or with a Roots blower (aka booster) when higher performance is required. For the highest system performance, both a Roots booster pump and a secondary high-vacuum (e.g., diffusion) pump are utilized with the primary pump, which provides the fastest pumping speed and highest ultimate vacuum. When used in series with either a booster pump or with both a booster and secondary high-vacuum pump, the primary pump is referred to as a “backing” pump.