Aneroid Capsules

Features

• Minimised distortion and low heat input
• Joints are homogeneous, leak tight and very strong
• Flux and solder costs are eliminated
• Very high welding speeds and outputs can be achieved
• Thin foils can be welded to much thicker sections
• Internal corrosion, post joining cleaning and heat treating are eliminated
• Higher linearity and low hysteresis give superior product
• Components can operate at a higher temperature than those which are soldered
• Welding skill is removed from the operator
• For evacuated capsules no subsequent evacuation process is required.

Problem

Manufacturers of absolute pressure aneroid capsules, used to measure barometric pressure in such instruments as aircraft altimeters, weather balloons and ejector seat interlocks, needed to find a fabrication technique which could weld two or more diaphragms at the edges and create an entrapped vacuum within the capsule.

With conventional joining methods such as TIG, microplasma, soft soldering and atmosphere brazing, the capsules were welded first and subsequently evacuated, sealed and leak checked. There was often a high reject rate because all four of these processes put a considerable amount of heat into the component. This tended to upset the heat treated properties of the diaphragm materials, which in turn gave poor reproducibility of the capsule’s deflection characteristics.

It was not possible to TIG or laser weld capsules made from beryllium copper, for instance, because of their high thermal conductivity and tenacious oxide layer. Soft soldering and brazing had both the disadvantage of poor reproducibility and entrapment of fluxes. Brazing also suffering from high heat input into the component.

Solution

Electron beam welding, performed in a vacuum with very high energy density in the weld zone, not only eliminated all these problems for the open capsules (which have a tube connector to a variable pressure media) but also enabled a trapped vacuum to be produced for absolute aneroid capsule production.

Process

Electron beam welding can be used to join many types of metals and alloys with the exception of those containing elements with a high vapour pressure such
as tin, lead, zinc, etc.

Materials currently being welded include:

• Stainless steel
• Monel
• Beryllium copper
• Tantalum
• Nispan C
• Maraging steels
• Inconel
  

  Barrier diaphragms, rated diaphragms, open capsules and nesting capsules

Most capsules welded have diaphragms which fall into the thickness range of 0.02mm to 1.0mm. Welding speeds are typically between 760 - 1500 mm/minute, and total cycle times vary between 5 and 60 seconds depending on the individual capsule.

With rapid transfer systems production rates up to 1000 parts per hour can be achieved for the less sophisticated thermostat switch capsule.