Many years ago I was doing a job for a major oil company. I was very young and I didn’t design the electrical aspects, but it was my job to maintain the system. We had built an automated tank system designed to drain 500 gallons from a large fuel storage tank, allow it to sit and settle for several hours, and then drain the water from the bottom and return the fuel to the storage tank. The float switches were critical components.
We had built a system for them that included reed-type float switches. These float switches in turn operated relays which controlled the system. This was in the days before PLCs, so it was a relay logic system.
We had no end of trouble with these switches. We worked with the float switch manufacturer and the oil company engineers, but all the solutions that they had did little to solve the issue: burned out contacts. We had back-up float switches for alarm/safety and so we resigned ourselves to replacing the switches every few months.
I progressed in my skills and have spent many years since then designing industrial systems for handling fuel. I have always stayed away from reed switches unless the job allowed the expense of an intrinsically safe barrier/control. Then one day I happened to be speaking to a reed-switch engineer and mentioned my lack of trust for reed switches. He said, well, just put a 100 to 200-ohm resistor in series and they’ll last forever.
He explained that a relay is a lot like an electrical motor. It draws a high load in the first few milliseconds to fill the magnetic field. A resistor slows the flow of the power enough to take the peak off the surge. This had never occurred to me, but I instantly saw the logic.
When I recovered from the shock of learning that there was a very simple and very cheap solution for the problem, I asked why float and flow switch manufacturers didn’t -- at the minimum -- provide a notice in the box the item is shipped in to advise the customer of this.
His answer? “I guess we ought to tell them about it.”
I don’t think they ever did, but one of my best selling products, a differential pressure gauge, now includes a reed switch -- and a resistor. In selling these for more than 10 years, we have only sold a handful of replacement switches, usually to people who simply exceeded the load rating of the switch.
Tell us your experiences with Monkey-designed products. Send stories to Rob Spiegel for Made by Monkeys.