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The easy answer: Yes.
But in a recent webinar on choosing the best level technology for your application, the more specific answer is this: Yes, AS LONG AS you pay attention to the unit specs and a pretty simple rule of thumb.
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.
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).
On 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):
OK, either configures an LRV or a URV value, but which is which? What’s the difference?
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.
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.
A customer who had had lots of experience with Milltronics and Siemens ultrasonics was installing his first SITRANS LR560 radar level transmitter. They had worked with it in the shop beforehand, going through most of the settings. They even tested it by setting it up to shoot against a file cabinet and used a tape measure the check the indicated distance value.
Everything checked out OK in the shop.
When they installed the transmitter on the top of the bin, they changed the transmitter’s sensor mode parameter from the distance mode they used in the shop for testing to level mode. After aiming, the level value shown in the local display was dead nuts on, but the 4-20mA signal going back to the control room was way off.
The bin was a third full. The 4-20mA showed it about double that. Not only that, the 4-20 was going in the wrong direction. The bin was emptying and the HMI reported an increasing level value. Someone realized that an inverse-acting output was typical of a distance value, so they reconfigured the sensor mode to distance. That got the 4-20mA much closer to a distance value, but it was still not exactly what it should be, and besides, the goal was to read level, not distance, in the control room. What was going on?
That was one of the questions that a customer shared after our recent webinars on pH measurement technology. Here’s a short answer, as shared by Jorgi Day, Sr Analytical Product Specialist for Honeywell, our featured speaker.