If I used a real time panel display I'd still have to note, record or calculate the time, speed, distance and proper pitch data AND I'd have to record the voltage and current numbers as well, rather than just looking at them post dive.
There's some of the confusion explained, the Sentry gives you what the Eagle tree does not, Watt-hours consumed, which is the actual quantity of power used. It is voltage, current and time all summed up in one value, so you don't need to keep track of those individual values either mentally or with a data logger. It makes the math really easy once you know how many Watt-hours your battery has. Amp-hours is not a measure of total power, because it has no reference to a voltage level, which changes over the discharge of a battery.
If your battery holds 600Wh and you are diving thirds, you turn when you reach 200Wh on the Sentry. Knowing the real-time current is also irrelevant because it will change with voltage at a fixed speed. That is why the Sentry displays Watts, since the amount of power to maintain the same speed does not change, when your voltage does decrease over a charge cycle. If you try to base your speed off of current, your assumed speed would actually decrease as the voltage decreased, due to the drop in power (Watts).
A data recorder will not record actual speed, distance or prop pitch, so you would still have to record those manually or mentally.
Watts are related to speed, with the only variable being drag. When in a cave, you can run between know distances at a fixed power to determine your speed vs power relation for the amount of drag you have. Sounds complicated, but it's not. If you set your scooter so that it is running at 200 watts and then travel 300ft, you note the time it takes to go that distance and determine speed. If it takes you two minutes to travel that 300 ft, you are going 150ft/min. and you can now calculate your range at that power level. If you generally maintain that 200W power setting over the course of your dive until you reach your third, (200Wh) your run time will be one hour (200W * 1 hr = 200Wh) and range will be 9000ft. (150 ft/min * 60 minutes = 9000ft), assuming flow in the cave stays constant, yeah right. But it also lets you recheck your initial speed estimate any time you reach a know distance. If you reach the 1500ft marker and it only took you 9 minutes, then you are going a bit faster than 150 ft/min. This same planning can be done before the dive.
But I'm not sure that's relevant either. I always want to leave a 1/3rd reserve in the battery whether it's in a cave or in the water and planing on the fly isn't my preferred approach. I want to plan the dive and then use the display to confirm that the battery is performing as predicted, not make decisions on the fly.
Again, EXACTLY what I'm trying to say. You don't need 10,000 data points to tell you your battery capacity. The Sentry will tell you how many Watt-hours your battery has, if you make one dive as a burn test.
Once you have run at a few different power levels to determine the speeds associated with them, you'll have a very easy mental reference as to what power will give you what speed during planning or on the fly. Now that you know what power gives you what speed, you can estimate your range for the amount of power you have available to use. This is a new and more accurate way of determining your turn around point. Before you would plan to turn at distance into the cave based on an estimate of how much power it would take to get there. Now you can turn at the actual power limit you impose on yourself. If flow is high or you are moving faster than planned (using higher power) and you still dive to your planned distance, you will use more than the allotted third. Turning at a known power consumption point will always be more accurate than turning at a planned distance, based on variables you can't always control. It's not planning on the fly, it is turning on a hard value (Watt hours), just like turn pressure, not an planning estimate, like distance or runtime. The displayed voltage is simply a backup to the Watt-hour indication.
As a potential customer, I'd be thrilled if your unit had a data along function. Given that it doesn't, I'd be inclined to use a data logger with it. As an engineer, will your unit let me run a data logger in series with it?
The internal transmitter for the Sentry is connected in-line between the battery and motor, and anything can be connected in-line with it.