Posts Tagged 4-20 mA
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 »
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?
A customer needed to re-range a Siemens SITRANS P DS III transmitter without applying pressure. He’d read my earlier blog post about using the pushbuttons to program the DS III (Pressure Transmitter Pushbuttons to the Rescue!), and where to find the Z-fold instruction sheet.
All was going to plan, until he started looking for the settings for lower range value (LRV) and upper range value (URV). And then, in an all-too-familiar scenario, he hit a roadblock. The instructions don’t actually mention URV and LRV, the terms he’s familiar with, by name or acronym. So what’s a guy to do?