So, let’s run IOMeter, Random Read pattern, again on Maxtor Atlas 15K II HDD and see what our multimeters will show. Since each of them can only measure one parameter (unlike the dual-channel oscilloscope), we connected them to +12V rail.
It is pretty hard to figure something out on the first one, the Mastech M890G: the values on the screen keep jumping up and down hitting maximum 2.9V and dropping to minimum 2.4V. Using the formula above, we can easily translate these values into consumption current: from 0.84A to 1.32A. This is where we can already see that the multimeter is not giving out the correct values: the oscillogram above shows clearly that the difference between maximum and minimum values is much bigger than 1.5 times. As for the average value, we couldn’t make it from the jumping readings on the screen.
Luckily, we have another multimeter, UT70D that can calculate the average value. Moreover, it can also transmit the reading to your PC via RS-232 interface, so the results will be given in the form of a screenshot:
On the left you can see a window of our own software that processes the oscilloscope data. On the right – the window receiving multimeter readings. The large digits stand for the average value, the maximum and minimum are given in smaller font below. We switched the multimeter to calculating the average value mode at the same time our own proprietary software was launched. It stayed in this mode for 60 seconds that we needed to get all the oscilloscope readings.
So, the multimeter reported the following: 1.06A average consumption, 1.13A maximum consumption. The oscilloscope data reads: 1.04A average consumption and 2.71A maximum consumption. As you can see, the multimeter managed to get the average value pretty closely, but failed to catch any of the consumption peaks.
At the same time, we cannot claim that any digital multimeter will register average consumption correctly: we managed to prove experimentally that our particular UT70D model showed very realistic number on our particular hard drive. We don’t know if the reading from other multimeters or the same multimeter on other hard drives will be adequate, as well.
And of course, it makes absolutely no sense to try detecting peak values with a multimeter. In our experiment they were not even close to the real ones. Moreover, if your multimeter shows high numbers it doesn’t mean they are correct. You will only be able to verify this by comparing the readings with the fully-fledged oscilloscope measurements. But if you have a system like that why use a multimeter at all?