Archive for category installation

LUT400 universal 4-20mA analog output gets rid of the ground loop

The Problem
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.

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Wet rhymes with Set (or how to remember LRV/URV configuration prompts)

Lots of people like the pushbuttons on industrial pressure transmitters because the basic settings that every transmitter needs can be set up without a HART communicator. This includes things like the tag name, engineering units, LRV (Lower Range Value, the zero, or what 4.0mA represents), URV (Upper Range Value, the span, or what 20.0mA represents) and damping (an average or filter factor that dampens noise).

Honeywell ST700 ST800 configuration screenOn the new Honeywell ST700/ST800 series smart transmitters, the tag name and engineering units are easy to configure and self explanatory, but I seem to stumble when setting up the LRV and URV because I’m faced with a non-descript choice. There’s two sets of options (under Transmitter Setup, not Calibration):

Enter LRV
Enter URV
Set LRV
Set URV

OK, either configures an LRV or a URV value, but which is which?  What’s the difference?

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Siemens SIMATIC PDM programming tricks

When you’re making programming changes to a field device, you don’t always have time to wait. Here’s a hidden feature that helps you speed up the process between Siemens SIMATIC PDM and HART field instruments.

Normally, when you’re using PDM software, it takes a minute or so to upload or download changes to and from your HART devices. Seems like an eternity when all you need to do is change a range.

So, I’m going to let you in on a feature you might not have seen before.

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How do I get an RTD signal to two different devices?

RTDs are great temperature sensors – accurate and easy to install.  But they are not friendly when it comes to trying to get a single RTD to go to two places, like when an RTD temperature measurement has to go to both a controller and a recorder.  People call and ask, “How do I split an RTD signal?”  The short answer is, “You can’t.”

An RTD cannot be wired in parallel or in series to a second device.  Any RTD input supplies a known, regulated ‘excitation’ current to the RTD.  Mixing RTD inputs would mix currents and that’s a Big No-No.

There’s also a lead wire compensation circuit for 3- or 4-wire RTDs that would create problems if a single RTD were connected to two different RTD inputs.  There’s just no feasible means of making two RTD analog inputs play nice together.

But all is not lost. There are several ways to achieve your goal.

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