Why do we need to include 250 ohms resistor in the loop while using HART protocol?

It is just conversion of  current signal into voltage signal according to OHMS LAW, V=IR By putting the current values4mA -20mA(4,8,12,16,20mA) Multiply by250 Ohm.You will get1v,2v,3v,4v,5v respectively.Now this voltage signal cab be used for Display1V as  zero2V for25%3V for50%4V for75% and5V as100%, I think this will clear benefit of conversion.

The resistor's position in the loop is between the Transmitter and the DC power supply. As we connect the communicator it should be tap between the resistor and the transmitter or directly parallel to the resistor. Why? Because the DC power supply has a LOW PASS CAPACITOR FILTER that basically squelches analog signals like ripples. Thus, if there is no resistance in between the PS and the TX that analog signal coming from the HART devices will be filtered out.

The dc power supply we use in the loop has low pass capacitor filters, and in absence of a resistor, the filter is in parallel with the Transmitter, thereby bypassing the digital information, and adding a resistor just provides impedance between the Transmitter and filter circuit

While doing bench calibration if you connect it directly across a24Volt power supply it will be quite happy because the4-20mA current signal won’t be disturbed, but you will not be able to communicate with it using HART Communicator. You need some impedance so the HART communicator can modulate the lines.

The250 ohms resistance in the loop serves two purposes:-1) The HART Communicator communicates to the transmitter by FSK method. The capacitor in the power supply for the transmitter distorts this HART signal. Hence the resistor serves as an aid to get the signal back, for the communicator.2) DCS, PLC needs voltage (1 to5V) signal to understand the signal from the transmitter. The resistor again serves to convert this current signal into voltage signal for the DCS/PLC.

to get a1v to5volts dc signal to be used in transforming analog signal to digital signal. Our signal is4 to20mA so if we put250ohms resistor we can get1 to5 volts signal which is needed in ADC analog digital converter 

only4- mA HART Protocol Need Ohm Resistor in loop  to convert4- mA  in to1-5 V which Required for DCS or PLC to Work Which are field devices..

To limit the flow of current in a circuit

because the signal and alimentation are in the same cable, alim=24v, signal =4 -20mA, V=RI,250x4mA=1V and250x20mA=5V