Archive for category Process terminology

Why has 4-20 mA gained so much acceptance and survived so long?

Last month, I presented a webinar for our customers. At the end of Control 101, when we opened the session up for questions, a customer asked about making the choice between 4-20 mA and digital signal outputs.  It got me thinking about this list I put together a while back — one that I pull out whenever customers ask me “Is 4-20 mA still valid?”

The answer: Yes, it is. And here are the 18 reasons I came up with… so far. Read the rest of this entry »

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Industrial Wireless 101: Which wireless antenna do I need?

In our continuing quest to understand industrial wireless, here’s the next bite. We already talked about the three industrial wireless bands and what they’re used for. Now, we’ll look at wireless antenna.

The antenna’s primary purpose is to focus or direct the signal that it is sending or receiving. The antenna strength, called gain, (measured in decibels) concentrates the transmitter or radio signal in a given direction, and reduces it in unwanted directions. The higher the antenna’s decibel value, the more focused the signal.

I’ve found that the word gain sometimes confuses people: they think gain refers to an antenna ADDING power to a radio. That’s actually what an amplifier does. Antenna gain is about concentrating or dispersing radio frequency energy and directing it where it needs to go.  By packing that available energy into various patterns, as the different antenna types do, a radio signal can be spread out to a broad field or concentrated into a small tight pattern to go a farther distance.

Just like the three industrial wireless RF bands, there are three main types of antennas. And while lots of people have tried to explain them, with technical charts and inkblot-style diagrams, this simple description really hit home to me:

  1. Omni-directional antennas radiate their signal in all directions, like a floor lamp radiates light equally around itself.
  2. Semi-directional antennas radiate in a specific direction across a large area, like a spotlight shining on stage.
  3. Highly-directional antennas focus their signal on a very specific target, like a laser pointer focusing on a specific portion of a photo.

So, let’s take it a step further.

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Industrial Wireless 101: Why do we need three frequency bands?

Regardless of what information you need to send, or where you need to send it, if your data is going “over the air”, you’ll need to choose a frequency in one of three ranges that do not compete with the FCC licensed bands for radio transmissions.

These three ISM (industrial, scientific, medical) bands are 900 MHz, 2.4 GHz, and 5 GHz. They’ve often been described as the industrial equivalent to Citizens’ Band radios, specified so they don’t interfere with broadcast radio signals.

History with technology would lead you to believe that the more Hertz you have, the better your radios will perform. But what you need to understand is that there’s a tradeoff. Each band has its strengths and weaknesses, and there’s a best use for each.

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What does NAMUR NE 43 do for me?

People have asked me about setting fault alarms in level transmitter analog signals at 2mA or 3mA levels. What they typically don’t understand is that a two-wire transmitter uses the electrical current below 3.6 mA for its own power and operation. So, a 2.0 mA or 3.0 mA fault indication just isn’t possible. At these low currents, there wouldn’t be enough power to generate the fault indication signal and to keep the transmitter functioning properly. Read the rest of this entry »

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I’m not getting the readings I expect to see.

“This unit’s not giving us accurate readings.” When I hear that statement about process instrumentation, I have to step back and ask, “Are you sure it’s an accuracy issue?”

The most basic problem is that people tend to group all unexpected reading errors under the heading of “Accuracy”.

So, here’s a quick primer on measurement terminology.

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