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# Differential Transmitter

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 wfsymon (Industrial) 18 Oct 11 7:01
 Dear all,the basic need of the flow measurement is the DP transmitter, am working with that,and Y all the DP transmitters having the option of the output as Square root and Linear ...Y we r having in the transmitter but maximum not using it.can anyone explain me the LINEAR & SQUARE ROOT output.and then why we are displaying in terms of FLOW in the local display of the meter.
 KiwiMace (Mechanical) 18 Oct 11 13:48
 That is some crazy english, but nevertheless there are hundreds of languages I don't know, so I'm going to attempt an answer, but in future please try to use complete words and not text speak if you want an answer.DP transmitters are primarily used to measure flow and level, and sometimes just... differential pressure.Changes in level in a constant sectional area tank, as most are, provide a linear change in differential pressure.  To provide a linear indication of 0 - 100% you can feed the signal directly.  Straight up DP is the same.Flow in a pipeline changes in proportion to the square of the differential pressure, as measured across some restriction in the pipe like an orifice plate.  To convert this parabolic output into a nice linear 0 - 100%, you need to take the square root of the native output of the transmitter. So, because the transmitter can be used for level and flow, the transmitter has options for linear or square root output.I would assume your display is showing a flow because it is configured to do so.  RTFM on that one.
 wfsymon (Industrial) 1 Nov 11 2:18
 Dear kiwimace,thanks for the valuable reply,but for general flow calculation,is the temperature and pressure inputs are required or not.for example,steam flow from the DEAERATOR (a unit in the thermal power plant as a boiler accessory)whereas the steam temperature is 100 - 110 deg.celcius only.If I enable the SQUARE ROOT option in the transmitter will give correct reading or else enable LINEAR in transmitter after connecting in the respected controller if am enabling the flow block in the controller will give correct reading.can you help me on this.
 danw2 (Industrial) 1 Nov 11 19:42
 Volumetric flow is the square root of the percentage of flow rate.   The square root is extracted only once.When the transmitter extracts the square root, then the controller interprets the signal on a linear scale.When the transmitter transmits DP as a linear DP value, then the controller extracts the square root.A volumetric (Sq rt of DP) measurement is considered "accurate" when the flow is at the flow condition design temperature and design pressure that were used to size the orifice plate and determine the DP at a certain flow rate.The volumetric measurement is subject to uncertainties when the flow is at a temperature and/or pressure other than the design conditions.Volumetric flow can be corrected to inferred mass flow by temperature and pressure compensation.  That's what multivariable DP flow meters do.

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