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Which tubing should I connect to a flat-bottom 1/4-28 UNF port, when and how?

1/4-28 UNF threads are popular thread types because they can be connected to both 1/16" and 1/8" OD capillaries with appropriate flat-bottom fittings.

1/8" OD capillaries are usually used for flow rates of several hundrets of milliliters per minute. Above this, 3/16" or 1/4" OD tubing is often used.

To integrate devices with a 1/4-28 UNF thread into 3/16" or 1/4" tubing systems, adapters are required.

The table below gives a recommendation for this.

In any case, make sure that the flow-through equipment is designed for the intended flow rates!

Flow rate regime Recommended tubing OD Small part recommendation to connect to 1/4-28 UNF port
Up to 100 ml/min 1/16" *Fitting A58291 and ferrule A58292-1
Up to 500 ml/min 1/8" *Fitting A5829 and ferrule A58294
Up to 3 000 ml/min 3/16"

English adapter A7233 + Fitting A142700

Up to 15 000 ml/min 1/4"

English adapter A7230 + Fitting A142614

*Note: Standard flat-bottom ferrules are made of ETFE, which are used in the low-pressure range (up to approx. 50 bar). Item FZG10 contains a complete set for 1/16" and 1/8" OD capillaries. For highest pressures (up to 172 bar) please use PEEK ferrules instead (set: FZG11).

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