Hi,
if the problem already exists, consider the following:
Almost every HART device is also a device that uses one of its measurements to shape a 4-20mA Signal (usually the current taken by the electronic). If the device has a local display and some buttons, you can set the measuring you want to read as "Primary Variable", which makes it the companion of the 4-20mA Signal. You need to know the measuring upper and lower range, which might be also adjustable via the display/buttons. If there is no local display, you need a configuration software. For free you can get PACTware and some DTM (Youtube has some details how to use this software)
Additionally to PACTware, you will need a HART Modem. There are some available for less than 150€, just ask Google. If the configuration is done you can measure the current and you will have an idea about the flow. Remember the range settings: 0% is 4mA, 100% is 20mA. Just some simple math...
I think this is much much easier than to understand how HART works and how to perform a dialog with a HART device.
If 150€ is too much money, you can try to adapt a bell202 modem. But don't fry the device 
Good luck!