The Amazing 4 to 20 Signal (or why “old” doesn’t always mean “obsolete”)

I’m old enough to remember how a rotary phone worked and brilliant enough to know that it is never coming back.

The electronics in my cell phone shrinks with every passing year AND it gets more powerful. I still stare in utter amazement at the resolution and color gamut of my 52″ HDTV. Why spend $200 for Red Sox tickets when I can watch them lose up close and personal in the privacy of my den?

Yet for all the force of the digital tsunami that has been washing over the analog beach head, some analog technology continues to hold its own. I still like putting film in my camera. There are lots of audiophiles that still prefer to put a vinyl record on a turntable—so much so that LP sales are going up. And the 4-20 analog signal invented about 60 years ago? That process instruments’ output is still the standard. Modbus, Fieldbus, Profibus, Profinet, ISA 100, HART, Ethernet-IP… you name it, every week it seems that one of these digital protocols is the new gold standard.

But analog 4-20 is still king. It’s one thing to be nostalgic about film and vinyl. It’s quite another to discover that there is a communication standard that:

• Needs two wires to transmit information AND power.

• Can go two miles without amplification

• Can be read with a $20 voltmeter

• Is inherently resistant to noise

• Is intrinsically safe

• Automatically tells you when something is wrong.

Try pulling off this inexpensive engineering marvel with RS485. While digital protocols continue to duke it out in the press, every output device including controllers and direct output sensors continue to pump out 4-20 mA signals.

The glory of this analog signal is rooted in a very basic law of physics: What goes in must come out. Unlike voltage which can the life sucked out of it traveling over wires, current never changes. Electrons that go into one end of of a wire have to come out the other end. You don’t need an EE degree to figure this out. That’s why I love our industry. We don’t care what’s fashionable. We only care about what works. Don’t get me wrong. I didn’t buy Aquametrix to continue churning out pH probes or controllers using the same designs created a decade ago. I bought the company to create the most innovative products in the market.

The 2400 controller which will constitute the first big product innovation since I purchased Aquametrix, will have a combination of features not found in any other analyzer. And that includes Modbus and Ethernet in addition to 4-20 mA. But I am always mindful of one overriding principle: Whether it’s magnetic hard drives, steam generators, film, AM radio, violin, turntables or 4-20 mA loops—a good idea is a good idea whether it comes straight from a university laboratory or is 100 years old.

I have to go now. The power just went out in my neighborhood and I have to find my transistor radio. Mark