I'm not sure if the interconnecting line between the two transmitters would be shown as an electrical signal or as a communication link as found on Table 5.3.2, No. 15 of the ANSI/ISA-5.1.-2009.
A: For the latest ISA flowsheet symbols, you can review the 2009 revision of S5.0.1 (go to http://tinyurl.com/25wu25 for a link to ISA standards), but unfortunately, it doesn't clearly answer your question, just as it doesn't cover many more recent developments. I have been saying for years that updating the standard is much needed.
A: I'd suggest that the remote device be tagged as a PE (pressure element or sensor), and connected by a bubble-line to the PDIT. Communication is HART between the two devices, but calling it a PT tends to confuse. As you've no doubt noted, 5.01:2009 doesn't quite cover it.
Ian H Gibson
A: Since you didn't give any additional information, I assume it is a standard PDT with a loop-powered signal to the control system.
Yes, sounds like the 3051S ERS System, so you'd connect a dedicated com link as an electrical signal between the two instruments.
If I am correct, then the way to show it would be as shown in Figure 2.
A: Based on your descriptions, I would draw it as shown in Figure 3.
A: The connections from the high- and low-pressure detectors to the process device you can draw with fine line, and use bold through the auxiliary devices. Refer to Diaphragm, ANSI/ISA-5.1-2009, Table 5.2.5. The letter designation inside the transmitter bubble is PDT.
If you're also showing the manifold (using three to seven valves, depending your application), refer to Lipták, Process Measurement and Analysis Handbook, page 724, fig 5.2.j.