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Square root in TX or controller

Hi 

Can someone explain the difference (if there is in the functionality of the loop or anything else) between configuring the DP square root in the transmitter or in the controller? I saw a flowmeter in the field displaying mA instead of flow, does that mean the square root is not configured in the transmitter? Appreciate some explanation on this point please. 

Thank you  

yacine
  • Emily Saopraseuth
    Hi Yassin –
     
    There is no functionality difference between performing the square root in the transmitter or the controller.  It comes down to user preference. It would be advisable to choose one or the other as a plantwide standard to avoid square root being done in both places.
     
    If the square root is done in the transmitter, Rosemount pressure transmitters have a scaled variable function that can output flow by create a relationship/conversion between the pressure units and user-defined flow units.  More information can be found the Rosemount 3051S Reference Manual in section 2.9.7 (https://www.emerson.com/documents/automation/manual-rosemount-3051s-series-pressure-transmitter-hart-protocol-en-75976.pdf).
     
    Thanks!
    Emily
     
    Emily Saopraseuth | T +1 952 204 6375 | M +1 952 232 8454 | 
     
  • Jeffrey Mach

    In reply to Emily Saopraseuth:

    Emily--
    In theory there is no difference, but practically speaking, the analog transmitter output and PLC/readout input have finite accuracy and are subject to noise. If the process is variable, it would be more useful to use the square root scaled output, since at 10% of max flow, your pressure drop will only be 1% of that at max. The square root transfer function applied in a dP transmitter acts as a form of amplifier for the lower range measurements and makes the signal proportional to flow. For example: @ dP = Min, Signal = 4 mA; @dP Max, Signal = 20 mA for both linear and root transfer functions, but @ dp = 10% of flow creating Max dP, Signal = 5.6 mA for root transfer function, but 4.16 mA for linear transfer function.

    Yassin--
    If the readout says "mA" that only says the signal-level output of the transmitter, and that may or may not have a root function applied. You really need to connect a communicator to the unit to really understand what's going on (look for "transfer function"). An orifice-meter with a differential pressure sensor may use a root-function on the transmitter, because flow is proportional to dP^2, but there are other flow sensors whose response is linear with flow, where the root function is not as widely used (e.x. Coriolis) . I mention this because your note says "flowmeter" without describing what kind.
  • leirbag
    There is no difference, but, from my point of view, the better way is to keep this function in the DCS controller. The reason is to avoid the mistakes of the installed dP manipulation in the field. Anyone can change this transmitter without having to worry but set the pressure range.

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