Archive for category 4-20mA
Written by: Dan Weise
Recently, I was helping a customer troubleshoot the analog input on his Siemens MultiRanger 200.
For common troubleshooting, using voltage values to confirm a zero, mid point and span is all that’s needed. I find it easier to put a voltmeter across the analog input and read the voltage drop than to wire an amp meter into the circuit to read directly, but that assumes that the analog input’s resistor value is a known. For the most commonly used input resistance (250 ohms), the equivalent voltage drop is 1.0 to 5.0V.
Being the guy who actually reads the user manuals, I looked in the Siemens manual to find the input resistance, and it’s not there.
A process plant’s technician was mystified about how to get a typical gauge pressure transmitter to read in the vacuum range. “All our gauges are 0 to 30 inches mercury, and that’s what we need to transmitter output to be. But the transmitter you sent us just stays around 4mA when we pull a vacuum.”
We walked out to the reactor vessel to look at the installation. The transmitter’s Low side port was open, its high side port was plumbed into a tee along with a conventional bourdon tube pressure gauge reading gauge pressure vacuum.
I could see why he was confused. The mechanical gauge goes from 0 to 30. I asked what range he used to configure the pressure transmitter. His answer, “0 to 30 inches mercury, same as the mechanical gauge.”
So, what was happening?
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 »
Before I talk about the value of a universal 4-20mA analog output on a level controller, let me explain why anyone would care. It’s all about ground loops.
Since the early days of electronic instrumentation, way back when, even before cell phones or PCs, instrument people struggled with ground loops that create an offset error, drive the signal off scale, or burn up an analog circuit.