what is inside a data logger ?

[ebastler]

it's just an start up. we should start at a point any way , even a zero point . you can consider it as a student assignment but finally we want to make it a real product.
i appreciate your concern and demand  wish you give me your advice to help me develop a real product.

Hmm, at least that is an interesting choice for a startup's first product!

Capt. Bullshot has already provided a great list of questions to guide your design, in reply #10 above. Please start with these to make sure you are addressing the key requirements (or are even aware of them!). A couple of specific comments from my side:

I would stress the power supply aspect in particular. The required operating lifetime of the meter, battery capacity (which may be lower than expected at cold temperatures), active/suspend duty cycle, and power consumption of your processor and sensor will determin much of your design.

One aspect I would add to Capt. Bullshot's list is how to make your meter tamper-proof. Not all future users will be perfectly honest; some may have an interest in reducing the metered water consumption. And I would expect them to get quite creative about that!   So you need to try and figure out all the ways how somebody could manipulate the meter, the sensor, or the connection between them, and develop ways to prevent or at least detect these manipulations.

You had asked about SD card interfacing in your initial post. With a view to reliability and also tamper-proof implementation, I would recommend against a (removable) SD card for storing the data. I think you want an enclosure that remains permanently sealed, with non-removable storage inside, and an interface (RF or optical?) to download the data.

Finally, a non-technical point: Have you figured out who you want to sell this to? Your customer would not be the end users (water consumer), but the water suppliers, right? Maybe smaller companies or not-for-profit initiatives in less developed areas? Have you actually started to reach out to such water suppliers? I strongly recommend that you do that as early as possible, certainly well before you hve what you think is your final product; and reach out to more than one of them. They may have needs and expectations which neither of us on the forum are aware of, and meeting their needs will be critical to your startup!

Best of luck with your project -
Juergen