Set up a pressure transducer with HW-685 current to voltage converter

It may not be appropriate for me to post this topic here since the hardware that I need help with was not purchased here. Furthermore, although I selected Arduino as a tag, one of the components of my application is actually a Particle Photon (not an available tag), but that discrepancy should have no bearing on the issue I am confronting. Assuming it's OK to continue, here is a description of my application and the problem I am having getting it to work:

I have a new pressure transducer that is designed to measure water levels up to 1 meter. When it is connected to an HW-685 current to voltage converter, I do not get valid readings of water level. 

At the start of my setup process I connected a 4-20 mA signal generator to the voltage converter and successfuly calibrated the latter. Custom firmware running on an attached Particle Photon accurately reports that the water level increases from 0 inches to about 40 as I increase the current from 4 to 20 mA. That verifies that the converter and the firmware are all working precisely as intended.

But when I connect the sensor the level is reported to be 40 (+/-) inches even though the sensor isn't under water; obviously I expected it to read 0. I subsequently discovered that if I completely disconnect the sensor, the reported value remains unchanged! Put another way, the presence or complete absense of a sensor has no bearing whatsoever on the readings. Just for the hell of it, when the sensor was connected, I put it in about a foot of water but no change at all. 

The wiring is precisely as shown here except the power for the converter is coming from the VUSB pin on the Photon, not the 12 volt power supply:

https://mm.digikey.com/Volume0/opasdata/d220001/medias/docus/313/KIT0139_Web.pdf?_gl=1*9lpqz2*_up*MQ..*_gs*MQ..&gclid=Cj0KCQjwrojHBhDdARIsAJdEJ_elZ6LYAN6BIaTbzaDau3BU7QjSBnvo4vXHRwDJ5WqiGVO29fIpkKIaAokKEALw_wcB&gclsrc=aw.ds&gbraid=0AAAAADrbLlgVsWs06G75yCw3VEhjFwWWZ

Note that I just received a second sensor - one that i designed to measure up to 5 meters of water - and connecting it in place of the other one does not change behavior in any way.

Here is a photo of the test environment:

The sensor is connected on the left side of the photo. Note that while the signal generator is in the photo, it is not currently connected to anything but power.

Can anyone here help me figure out why this is not working?