12V to 3V voltage regulator for sensor implementation, cost & power efficiency
You can use a linear voltage regulator, or a switching DC-DC converter.


Linear: low efficiency, low noise.


0.2W at 3V is 67mA. A linear regulator will draw the same current, ie 67mA from the battery, and dissipate 0.6W as heat. The heat won't make the regulator burn, as the power is very small, but it still wastes a large part of the battery energy.

Your battery is 3.4 Ah, and it's lead acid, so it is better not to discharge it too deep. Let's go with 50% discharge. So half of 3.4 Ah is 1.7 Ah, divided by 67mA gives 25 hours battery life. Not that good.


Switching DC-DC converter: high efficiency, higher noise.


Let's use a crummy switching converter, with 75% efficiency. 0.2W on the output means 0.2/75% = 0.27W from the battery, which on 12V is 22 mA.

This is 3x less current, so you get 3x longer battery life.

The deciding factor should be how much power the data logger uses. If it uses a lot more than the heating element, then optimizing this is a bit moot... however if the data logger is very low power, then it's worth it.

Since this is a flow meter, the accuracy will depend on how accurate the heating power is, which depends on the accuracy of your regulator, so you need to keep this in mind.

I mentioned noise, because switching converters do generate a bit of high frequency noise. If the data logger is properly designed it won't have any problems with this, but it is still a good idea to test the setup before going into the woods.