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Old March 13th, 2019, 12:43 AM   #1
United States

Engineer_UA is offline
Join Date: Oct 2016
Location: California
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Level instrument device with 120VAC power supply

Hi Gents,

I have an application where I need to provide a 4-wire transmitter with 120VAC power supply. In this setup, I'd provide 120Vac to the "box", which could be a small enclosure that would come with the sensor. This enclosure would provide power to the sensor(powering the loop) and 4-20mA output back to the plant.

I guess I could run the 4-20mA directly from the SCADA RTU to where the sensor is to be located(~300 ft away). Without knowing how the SCADA is configured(sourcing vs sinking) and input impedance, I feel safer with providing an external power supply. I don't have 24VDC available where the sensor is to be installed.

Any information on the vendor would be appreciated. Most sensor that I've used seem to be loop powered directly from PLC/SCADA RTU.

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Old March 13th, 2019, 12:54 AM   #2
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boneless is offline
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Location: OKC
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What kind of sensor are you needing?

Capacative/inductive sensor, magnetic, radar/US or diff. pressure? What is your medium?

I can't think of anything of the top of my head, but couldn't you install your own PS with the sensor?
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Old March 13th, 2019, 07:26 AM   #3
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danw is offline
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Location: midwest, USA
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I'm an instrument guy, not a PLC guy. You should be picking a level instrument that
- has access to make the measurement
- is compatible with the medium
- meets temperature specs (pressure is not usually a problem, but can be - ultrasonic can't handle vacuum and doesn't like anything but atmosphere)
- meets hazardous area rating
- etc, etc

Then see what you do about power.

Siemens (former Milltronics) ultrasonic MultiRanger are AC powered units, mount at grade level, transducer is top mount on a cable.
Siemens LRR460 radar is AC powered

However, the reason 4-20mA is still going so strongly 60 years after its inception is because the power supply at the receiver end power the instrument. The big drive in level radar has been to design 'loop powered' instruments because they're the ones that sell because most sites don't have or don't want AC out wherever, they want to loop power the measurement.

I've only run into a couple DCS analog inputs that required the instrument to be loop powered (one of the AI terminals had DC power on it, for powering the instrument). Opto22 modular I/O conveniently provided a DC power supply terminal if you wanted to use it but the AI was just an AI. I'd venture a guess that 99% of the 4-20mA AI's out there are just AI's that have no DC power on the AI input terminal.

But my 35 years of experience do not include RTUs; just not my field.

If it were me, I got loop powered from the receiver end. If you put a DC power supply at the field end, you risk a ground loop when someone grounds the DC (-). In fact, it's a high probability. Then you'll need to put in a 4-20mA isolator. Yuck.
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Old March 17th, 2019, 09:55 AM   #4
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DwSoFt is offline
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Location: Alberta
Posts: 906
Pressure and temperature are easily loop powered (they need less than 3.6mA to run,) some levels are and some flows are but alot need more than 3.6mA so they need external power. They usually are active so they supply the loop power back to the PLC/ RTU but some are configure able. My experience is that water/wastewater seems to spec the 120vac powered flow and level. Oil and gas specs 24vdc models
Dan Wiebe
Automation Manager
Phoenix Energy Services
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