Hi octal,
could you say how fast current consumption is changing? 10 nA .. 100 mA (200, 300 mA) is very wide, and it may cause to change the range of DMM. It could be not fast really. So you can loose some importand data point. I don't know what to offer instead. But I'd pay attention to the issue.
The IoT devices I work on have 2 scenarios:
1- just wake up for sending data (using LoRaWAN) and eventually receive data (LoRaWAN downloads) and then go to deep sleep again: this process can take from 15s to 30s (depending on some params)
2- wake up, and execute some stuff (start a pump, or activate a motor, ... do some measurements and then shutdown again).
On those processes, I don't mind if I lose a point or two, I do read current each 500ms only. The idea is more to measure the current consumption in each scenario and estimate the true consumption from a theoretical model that has been fitted to experimental measurements.
The Second scenarion is more complicated: My device does only wakeup to measure a data on its ADC and emit data and then shutdown. In this case, the device is powerd only from an embedded battery and I must evaluate each bit of consumed energy.
For this second scenario, and from the answers I got on this thread, I checked various DMMs specifications and looks like autoranging in Amp mode is really slow on most of them. I think that there are no savings to do to achieve this work, I must find a way to buy a a true power meter, or a true intelligent power supply with battery emulation technology in it. It will let me evaluate true power consumption at various stages of battery life. But this costs $$$ (the lowest price I saw was about more than $3000 if I remember).
The DMM4040 proposed to me has been used only 4 months and is like new. I know the person who used it and I know how much care it takes when using such tools. So there is no fear from that. And he told me he can sell it to me arround 900€. The only point is that at this price, finally, it's better to look for a more modern DMM like the DM6500 suggested by people here on previous posts.
Thank to everyone ... Yet I'm still impressed by the fact that no one reviewed this DM4040 (nor its Fluke rebadge) on the web despite the fact it's old.