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2 Loop Powered indicator in series with TT 644H

In my project we are using 751 loop powered indicator and 644H temperature transmitter.

we have to connect two nos of loop indicator in series with the temperature transmitter.

model code for the field communicator: 751AM7E5BC

model code for the TT :: 644HAE5J6M5CNQ4K1011

i have considered the wiring scheme same as shown below:

i have the following queries

1. is the above scheme acceptable?

2. what is the length constraint for the transmitter and the loop powered indicator?

3. what will be the  minimum voltage supply required to operate these instruments?

4. will the TT can be taken in HART mode?

• HART
• Temperature

watson

• 17 Aug 2015 3:56 AM
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7 Replies

• leirbag

  

  • 17 Aug 2015 2:33 PM

  Hi watson, I cannot see your image, but I presume you have placed in series a DC source, the 2 loop indicators and the temperature transmitter, it's not big deal. The minimum power supply for transmitter is 12 VDC and the maximum voltage drop for loop indicator is 1 VDC, that means you need a minimum 14 VDC power supply, without voltage drop on the wires. If you have a 24 VDC power supply remain a 10 VDC voltage drop for the wires. Estimate a maximum 22 mA for the current and calculate the maximum length using the the ohmic resistance formula or a wire resistance site calculator. HART communication is available.

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• Rachelle McWright

  

  • 17 Aug 2015 2:41 PM

  In reply to leirbag:

  Image added.

  Best Regards,

  Rachelle McWright: Business Development Manager, Dynamic Simulation: U.S. Gulf Coast

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• leirbag

  

  • 17 Aug 2015 5:11 PM

  In reply to Rachelle McWright:

  Thanks Rachelle. The image is OK, the wiring diagram is OK.
  
  Copper resistivity is 1.7 x 10-8 Ω m. There are 4 cables with 2 wires. Let's say we are using the same type. The resistance of a 10 meter copper wire with a cross sectional area of 0.8 mm2 can be calculated as
  
  R = (1.7 x 10-8 Ω m) (10 m) / ((0.8 mm2)(10-6 m2/mm2))
  
  = 0.21 Ω X 0.022 A X 2 =0.00924 V per 10 m of cable with 2 wires. For 1 km of cable you obtain apx. 1VDC drop. Of course it's not necessary to "use" all the spare voltage up to 10VDC.

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• Randy Paschke

  

  • 17 Aug 2015 6:26 PM

  In reply to leirbag:

  In addition to the wiring resistance, you'll need to consider the DCS loop resistance. For HART communication, it's recommended to have a minimum of 250 ohms between the power supply and the transmitter, so generally this becomes the dominant resistance voltage drop in the loop. There is also a formula for Minimum Supply Voltage available on page 19 of the 644 Product Data Sheet at http://www2.emersonprocess.com/siteadmincenter/pm%20rosemount%20documents/00813-0100-4728.pdf. You would add 2 volts to account for the 751s.

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• leirbag

  

  • 18 Aug 2015 5:25 AM

  In reply to Randy Paschke:

  DCS input channel already ensure 250 ohms minimum condition. That means plus 5VDC voltage drop for the loop. 24 VDC power supply it's enough.

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• Natalie Strehlke

  

  • 18 Aug 2015 5:04 PM

  In reply to leirbag:

  Regarding question #4: The Rosemount 644H Temperature Transmitter you indicated has a 4 – 20mA with HART output signal. The analog signal is always present and HART is superimposed on the analog signal but HART is really only present when polling the transmitter for information via HART otherwise it sits quietly in the background. The HART signal can also be brought into the DCS via a HART input card if desired to provide additional information about the transmitter, including diagnostics.

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• Cheng Vue

  

  • 18 Aug 2015 9:13 PM

  Hi Watson-
  
  Please see my comments below.
  
  1. is the above scheme acceptable?
  
  The wiring diagram you have above should work as long as you adjust for the additional voltage drops of the 644 and two 751's
  
  2. what is the length constraint for the transmitter and the loop powered indicator?
  
  To fully answer your questions, I would need more information but there shouldn't be any specific line constraints. But as a general rule, the longer the line, the high chance of signal loss across the line. Leirbag also answered the question of voltage drop from the wires.
  
  3. what will be the minimum voltage supply required to operate these instruments?
  
  You will need a minimum of 14 V at the terminal of the transmitter (12V min for the 644 and 1V min per 751). You would also need to take into account the load resistor for HART communication.
  
  4. will the TT can be taken in HART mode?
  
  Yes, the 751 should be HART transpatent and is based off of the 4-20 mA output.

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