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Using Rosemount 644 TT

Using Rosemount 644 TT

Using Rosemount 644 TT

In searching for conversion of 4-20mA to millivolt signal, I encountered a statement in an Emerson document as attached. It says "Conversion of a 4-20 mA source to a measurable millivolt signal constitutes a second power source in the terminal block of the Rosemount 644". I wonder why this is considered as a second power source on 644 terminal? As shown in figures 1 and 2, the transmitter has only 1 input, be it current or voltage.

The other sentence I don't understand is "This technique should not be applied to a 4-20 mA source that is actively connected to a loop control."
Would you kindly help me with these?
I appreciate your kind response

RE: Using Rosemount 644 TT

Second Power Source:
  1. This is only for use with a 644 Fieldbus device
  2. Fieldbus devices are powered from (or through) the Fieldbus segment
  3. The 4-20mA signal requires another power source (not the Fieldbus power source)
  4. Two Power Sources: Fieldbus and 4-20mA signal/input
Actively Connected to Loop Control:
  1. Loop control means the manipulation of an end element (valve, VFD, etc)
  2. It's unsafe to place multiple devices on one loop control (unsafe for the process, typically, and also the drive signal may not be large enough to drive two devices)
  3. Multiple devices can be placed on one loop control, but by using Split Range
  4. The 4-20mA reading would only tell you what the output to your end element is. Not much value in reading it.

This is normally the space where people post something insightful.

RE: Using Rosemount 644 TT

Thank you for your kind response. I got the first part. But I still don't understand the second part. What would be the problem if we connect it to an active device that produces 4-20mA?
As for the first part, this means that Rosemount 644 will always be a two-source device, one from the sourcing device and the other from FF. and therefore will not be intrinsically safe. Is it right?

RE: Using Rosemount 644 TT

>In searching for conversion of 4-20mA to millivolt signal

If all you need to do is convert 4-20mA to a volt or millivolt signal, all you need is a precision resistor. Ohms Law gives you the voltage drop for the current through the resistor.

250 ohms in a 4-20mA signal provides a voltage range of 1.0V to 5.0V; the 5.0 ohms mentioned in the 644 doc provides a range of 20mV to 100mV.

That 644 document was for specifically for converting a 4-20mA signal to a Foundation Fieldbus compatible signal, using the ability of the 644 to convert a millivolt signal on its input to a scaled value useable by a Foundation Fieldbus network.

RE: Using Rosemount 644 TT


The 644 from Emerson Automation Solutions is a temperature transmitter, and this is a sort of technical solution.
First off, when you need to convert 4-20mA to millivolts, you can use a proper converter, and you can find a bunch of them in the market DIN mounted installation.
Second off, you can use any transmitter able to read 4-20mA such as Rosemount 644 and then use a 250 ohms resistor in the output to converting the 4-20mA to 1-5 Vdc. It's typical application, but you're adding a point of error in your application, then the best solution should be applying a proper converter or convert that in the PLC input also using a resistor.

I found two converters online that can be good for you and also more information about Rosemount 644 because the document is to convert 4-20mA to FOUNDATION Fieldbus ( Different digital field network)

RE: Using Rosemount 644 TT

I think the note on page 1 is referring specifically to when they are converting 4-20 mA to Foundation Fieldbus as in figure 2
You have one signal supplied from the 24 Volt power supply
The other signal supplied from Foundation Fieldbus
Both signals would need to have Intrinsic Safe barriers.
But who uses those anymore?

It looks like the 644 would be a useful device for converting the last 1 or 2 instruments from 4-20 to FF, an existing instrument or oddball analyzer for example, I think it will pass over the HART parameters also.

Can Foundation Fieldbus can be Intrinsic safe, I haven't tried.

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