I have to mate a new system with the old, reading tank level with a New Rosemount and take out the old Hagan, but I need to run an existing 3-27 psi chart. I want to use an I to P, but I am unsure if I can piggyback that 4-20 from the new transmitter. Is this possible?

thanks,
Joe

 

I'm not sure what you mean by piggyback. Do you mean two loads in series: an I/P for the pressure chart AND an analog input load?

It should work. Bellofram's 3-27psi I/P has a nominal impedance of 220 ohms. Others are probably in the same range.

Typical analog inputs are 250 ohms.

A smart transmitter with a 24Vdc loop power supply will drive at least 600 ohms so the sum of the two loads (220 + 250) is still less than the 600 ohms that 24V and a smart transmitter can drive.

 

Let me give a little more info. I am reading tank level on a chart using a Hagan F. There is a new installation (Water Plant) that needs that tank level, and we are using a new 3051 Rosemount for that signal (4-20). So the question is, can I use that New (4-20) Signal to run an I to P, such as a Bellowfram 1001 to replace my existing Hagan, and can I wire it in parallel, as in landing the + and - from the 1001 onto the same terminals as the 3051? Would it Have to be in Series? or would I need some sort of isolator so only the 4-20 is sent to the I to P to drive my chart?

joe

 

No clue what a Hagan F is and 1st 10 hits on Google don't say.

Presuming it is a pneumatic level transmitter that you're getting rid of, you need to power the 3051 loop with a 24Vdc power supply and wire the 3051 4-20mA output to the input of the I/P. The power supply (+) goes to the 3051 (+), the 3051 (-) goes to the I/P (+), the I/P (-) goes to the power supply (-). It's a series loop.

The only caveat is that I/Ps have a 'nominal impedance', meaning that the impedance changes with the load/input. As long as the impedance stays below the 587 ohms that a 3051 can handle with a 24Vdc power supply, you're good. And it probably does.

 

The I/P and the PLC or DCS input must be in series with the 3051 transmitter. The 4-20 milliamp signal is a current signal so must be in series or the signal would be split. Also add the wiring resistance to the I/P and PLC or DCS input, usually 250 ohms. This resistance must be less than the 3051 allows.