Rosemount Annubar Gas Flowmeter is installed on flow line and HART signal is connected to HMI. Currently we need to connect an external digital display from this annubar to an office (cable length from Annubar to display is almost 1000 FT). As recommended by Rosemount, current loop is made by connecting with TEST and +ve terminal of Annubar. Digital display is 220VAC powered, and Annubar is self powered from PLC. The loop connected to Digital display showing 0.7VDC and mA current drops when digital display is connected (showing less reading). How to rectify?
> As recommended by Rosemount, current loop is made by connecting
> with TEST and +ve terminal of Annubar.
I don't think that is correct. The Test terminal is just across a diode for use by a low impedance meter (<30 Ohms).
I was confused by all your loads.
A display, An office or is it the digital display you are referring to?
The HMI usually covered by the PLC input module
1,000 ft cable
The Rosemount 3051 should be able to power about 580 Ohms using a 24 Volt supply just over 2 x 250 Ohm loads.
Since you have the signal already in the PLC, you might be able to re-transmit the signal from an analog output. another way is to use a signal isolator.
If you are just a little shy on loop Voltage, it's possible you don't need all of the usual 250 Ohms usually specified for HART. some other load load may do double duty or perhaps you can increase the loop Voltage slightly.
2000 feet (1000 out and back) of wire drops something. 2000 ft of 18g is 26ohms; 20g is 42ohms; 22g 66 ohms. 50 ohms drops 1 volt at 20mA. So 20g drops a little less than a volt, 18g a little more than a volt.
The 3051 has a 10.5V lift-off spec (minimum voltage to power the transmitter). The AI probably drops 5V. The copper wire drops ~1V. A 24V supply should drive the 3051, the 250 ohm AI, and 250 ohms of digital display input resistance.
I agree with Roy about the description of the DP xmtr wiring using the test terminal - that is not correct wiring. I've never experimented with that particular errant mistake, but if it actually works (by accident) it could well fail to provide the ability to drive the rated load at 24Vdc. I think the DP xmtr wiring should be corrected first, using the DP's (+) and (-) terminals. Doing so might very well correct the current loop loading condition exhibited by adding the digital display to the loop.
Loop powered digital displays with an LCD displays (black digits on a gray background) drop as little as 1V (Precision Digital PD685) up to about 3V. Depending on what your digital display input resistance is, a 1V loop powered display might work with the flow loop as it is.