dp transmitter

From what I gather, you will be using your DPCell for metering flow? In that case the low side is connected to the downstream side of the Orifice plate during actual operation. The low side is only open during calibration. The manifold is used during maintenance in the field for zeroing the XMTR.

It would have been better if you said whatever it is that Dp stands for...

Since you talk about high and low side, I assume that Dp stands for Differential Pressure.

For zero pressure difference,you can either leave both sides of the transmitter open to the atmosphere, or connect the two together with a tube. Either way ensures no difference in pressure.

For span, you must ensure that the 100mm H₂O pressure applied is measured with respect to the same pressure (atmospheric or other) that the low side 'sees'.

You need to tell us more about your specific application: then we can give you a more specific answer!

1-For zero calibration i will apply pressure(0mmH20) to the high side of the transmitter and leave low side open to atmosphere and adjust 4mA

Not necessarily. You can opt to leave both sides open to atmosphere. That way both sides will have the same pressure (zero difference).

2-For span calibration i will apply pressure(100mmH20)to the high side of the transmitter and leave the low side to atmosphere and adjust 20mA

Usually, yes. Make sure, however, that your reference is reading gauge pressure.

why do we open low side open to atmosphere as a refrence...

If you're measuring level for an open tank, you open the low side. If you're measuring level for a closed tank (under positive or negative pressure), you connect the low side to the top of the tank. The low side compensates for the pressure in the tank so that your level reading is correct.

...and what is the use of isolating and equalizing valves?

You've answered your own question.

Isolation valves isolate or disconnects the transmitter from the process. For what reason? To take it out of service usually.

The equalizing valve serves to equalize the pressures of the low and high sides to check the zero.

I have found connecting the high and low sides of a DP transmitter is essential to a steady, proper zero calibration and is a "best practice" to follow all the time. Applying 100 mmwc to high side is part of a complete calibration, but to know if the transmitter is operating properly, I usually apply (-) 100mmwc to low side, while continuing to apply (+)100mmwc to high side. One can watch the output signal return to a zero signal, and linearity is easily checked also by stopping at 25%, 50% 75% or any other points desired. Low sides of some transmitters can be damaged without effect to high side, once initial zero was corrected to the damaged low side. You never know what the last person who calibrated before you did to complete the calibration! Hope this helps.

connecting the high and low sides of a DP transmitter is essential to a steady, proper zero calibration and is a "best practice" to follow all the time

agree

I usually apply (-) 100mmwc to low side, while continuing to apply (+)100mmwc to high side

disagree...if you apply -100mmwc to the low side and +100mmwc to the high side at the same time, you'd get 200mmwc pressure difference (high minus low).

If you had said that you applied +100mmwc to the high side, and then removed it, and then applied -100mmwc to the low side (as a way of checking perhaps), I would have agreed. The pressure difference would still be 100mmwc for both instances.

I agree with you 100%! No need for the extra 100 on the low side. The force balance system of the xmtr along with the pilot (or relay) takes care of the rest. Thats why it's called a "Force Balanced System" at least on pneumatic types.

<....why do we open low side open to atosphere as a refrence....>

Because it is a differential pressure transmitter, and atmospheric pressure changes over time due to local weather characteristics; a differential pressure transmitter is used instead of an absolute pressure transmitter to cater for these changes. The atmospheric changes are only slight, though they are very significant in comparison with a full scale range of 100mmH₂O.

<....what is the use of isolating and equalizing valves....>

These enable calibration and testing to be carried out with a goal of obtaining the correct calibration of the instrument in view of the above.