Accordingly, Pickering no longer purchases mercury wetted reed switches or manufactures relays that use the switches. However, some exceptions may apply, such as replacement parts in some types of specialist equipment. To discuss if this is possible, please email techsales@pickeringrelay.com, or call +44 (0) 1255 428141.

Alternatives to Mercury Wetted Reed Relays

The ban on the use of mercury is completely understandable, meaning a highly capable type of reed relay is now obsolete. However, while above we praised the benefits of the mercury wetted reed relay, was it all roses? No, they did have a few drawbacks:

• A relatively slow switch speed – often more than 2ms.
• Low temperature operation is limited by mercury’s melting point (circa -38˚C)
• Form factor: Relatively large switches and therefore relay bodies.
• Orientation: Older devices had to be kept within about 30 degrees of vertical, though more recent devices can work in any orientation.

Despite these drawbacks, the devices were hugely popular in the 1970s and 80s. They were used extensively in automatic test equipment (ATE) systems, for example, where their shortcomings were not an issue, with the possible exception of (long) switching times as they limited the speed at which the ATE could run. They were also used extensively in medical and industrial control applications.

Also, if you go back three or four decades, there wasn’t really a sound alternative. The quality of reed switches in the 1970s and 80s was very low compared to today’s devices. Specifically, plating methods and materials of the time were poor, which resulted in high contact resistance, and switch manufacturers employed little if any quality control. Pickering, and all other OEMs using the switches, therefore had to vet all incoming parts, test them and use only the best.

Today’s Alternatives

Thankfully, reed switch manufacturing has improved considerably in recent decades. In particular, instrumentation grade, sputtered ruthenium switches are a viable alternative to mercury ones, boasting:

• Low contact resistance and good through-life stability (when used in spec)
• Millions, and sometimes billions, of operations are possible for low loads
• Fast switching times (0.1 to 0.5ms is typical).
• Smaller form factor, making possible high density switching matrices for use in ATE systems, for example.

A major advantage of the mercury wetted switch was of course its ability to switch high power. Today, dry reed relays are available with higher switch ratings than some mercury devices. For instance, in March 2024, Pickering introduced its Series 144, featuring an impressive 80W power rating, while stacking on a compact 0.25-inch pitch. The SIP (single-in-line-package) reed relay offers a switching current of up to 2A, delivering up to 60W, or 1A for up to 80W, with a continuous carry current of up to 3A. Additionally, it boasts high-voltage capability, with a switching voltage of 1kVDC up to 10W and up to 3kV stand-off.

As for the issue of switch bounce, this can be (and has always been) worked around by good engineering practices and allowing a ‘settling time’. To this end, Pickering’s dry reed relay datasheets include an ‘Operate time inc bounce (max)’ figure. In data acquisition and ATE applications, waiting for the contacts to settle minimizing the risk of false readings. And in digital circuits, waiting the operate time prevents false rising and falling edges feeding into logic circuitry.

Lastly, cost. If wetted reed relays were still available as new, off-the-shelf devices they would cost about 100 times more than an equivalent dry reed relay.