High-Resolution DC Watt Meter for 12V 10mA Use at -50°C With Data Logging

Hi Guys, thank you for your attention.


I am currently working on a project in Arctic. We have placed some sensors in the soil to gather the temperature information over the year. The sensors and memory units are powered by batteries and solar panels.
Now, we want to monitor and record the power consumption over the year. The problem is, we cannot find a watt meter working in this environment. The temperature is expected to be -50 cencius degree (or even lower) which prevents most commercially avaible watt meters to work. Also we need it to be DC and high-resolution as the power consumption is far less than that of family use. The voltage is ~12V and the current is ~10mA. We also need a storage unit with it.
So, anybody has an idea on this?  It works as long as it can record and save the power consumption in low temperature. We have batteries and solar panels if power source is needed.


Thanks.

You wouldn't use a watt meter, but rather monitor the voltage and current and compute the power. The actual power used would not be simple to compute, I would assume that you have very low currents while the device is asleep, and would draw variable amounts during the sampling phase, and that the duty cycle would be very large. Sampling current during the sleep phase would not give a creditable result, since the sampling process would draw most of the current the system uses. You would need a design that was very low power, since it could  end up drawing more current than the actual circuits that it is monitoring.Assuming that you already have a microcontroller in the system, if it has two spare ADC channel, then that could be used to directly monitor the voltage, and indirectly the current (use  something like a Texas Instruments current sensing IC which generates a voltage proportional to current in a small shunt resistor). Since in the process of designing and commissioning the device (which should be done at your working temperature) you should get a very accurate picture of exactly how much power the device is using anyway, all part of computing the power budget, do you really need to know it more precisely?  The battery voltage and solar cell may be all you really need to monitor, since that gives you the most useful information to my mind.At the temperature you are working at, the battery may be the biggest issue, most don't take kindly to working at such temperatures.Just a few thoughts, covering a small number of your issues, probably raises more questions than it answers,cheers,Richard 

Hi Richard, thanks to your reply.
The battery we are using can properly work in low temperature (though I'm not sure about the model and material).To get the power consumption indirectly from voltage and current works, but I'm not sure whether it is precise enough compared to power meter based solutions. What do you think about this TI model ?http://www.ti.com/product/ina226

Anyways a MCU for this environment is also necessary, which also needs to be considered, too. 

SeanFollowing Richard's train of thought- as another option there is the  MAX4210/MAX4211 High-Side Power and Current Monitor which is an analog IC (from Maxim) that will multiply voltage and current. Of course you will need additional circuitry to take the reading and store it. No doubt there are a few ICs of its ilk, although from what I have seen they tend towards AC and are intended for household power meters.The MAX4210/4211 is rated to only -40C, but I think you may have troubles finding many circuits that will go lower. You may need an internal heating element which of course will drain more power. All this presumes that you have the design capability in house or access to an external design house and the time necessary to develop such a product. 
Good luck.

Thanks Aubrey, MAX4210 seems perfect for the monitoring. I guess even if the chip doesn't crack down below -40, some calibrations are necessary. I'm gonna give it a try.