Valco Compression Fittings -
Comparison of Compression Fitting Designs

The potential for dead volume and mixing is a consideration in other aspects of fitting design as well, and varies considerably among manufacturers. For example, the common gas distribution reducing union in Figure 1 illustrates two problems for instrumentation: a large connecting volume, and various steps and restrictions which cause mixing. While there are many uses for these fittings upstream of the analytical system (such as bulk gas distribution), they cause problems when used downstream in critical applications.

Figure 1: Common commercial reducing union

Additional difficulties may be encountered if this type of fitting is loosened and retightened repeatedly. The male threaded part can become flared to the point where it is impossible to get the nut on, and the tube end often flares out in the fitting detail so that it’s difficult to remove the tube.

The Valco internal union (Figure 2) has a larger mass surrounding the ferrule, so that even with repeated remakes or overtightening, it’s impossible to flare the fitting as in the external design. When a union is selected with a bore to match the ID of the connecting tubing, mixing and dead volume are virtually eliminated.

Figure 2: Valco zero dead volume reducing union

For connection of fused silica tubing of the same or differing sizes, the through-bore union shown in Figure 3 is recommended. This fitting permits the use of our one-piece fused silica adapters to effect a true zero dead volume connection. The ferrule features an integrated pilot which adapts to the ID of the unions, resulting in an inert, zero volume connection.

Figure 3: Valco zero dead volume through-bore union

Every Valco and Cheminert fitting is manufactured to exacting specifications. Fitting concentricity – the relationship of the center of one fitting to another – is held to within 10% of the bore size (0.05 mm in a typical 1/16” union with 0.5 mm bore), which is better than that of commonly used tubing. This results in fittings which contribute no “extra column effects” or loss of efficiency to the chromatographic system.

More about Valco fittings:
  • No tubing deformation
  • Interchangeability
  • Reliably clean
  • Precision machining, finishing, and tolerances
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Read about Cheminert fittings:
  • High pressure
  • Low pressure