There's a whole post on Quora regarding the same topic. It leads to whether or not the 644 can convert the analogue signal. Surely it is an extract from the other link below, so you can check either of them...
https://www.quora.com/Can-the-Rosemount-644-convert-analog-4-20mA-signal-into-volts...
Here I found many topics regarding pH measurement. This link is for common mistakes, but it is linked to many devices, which I believe was your question. I saw another article in the site on how to change voltage signals into Ampers, but I cannot find it...
It can measure, source, configure, and document different process variables. And more importantly, it already is a HART communicator...
You can get more info in the features: http://en-us.fluke.com/products/multifunction-calibrators/fluke-754-process-calibration-tool.html#features or in the...
If you need to understand more about pressure transmitters, I would recommend to check out a couple of videos, articles, etc.
I think other's experience may make it easier for you to understand than theory (Bernoulli's equation)...
