kma said:
Hi. I want to ask a quick question, how accurate are multimeters? Like is every current detected on a multimeter a legitimate current or can it sometimes detect false currents, and can it miss any currents that are there? And how accurate are the values on there when it comes to what the current is?
I hope this question makes sense
Yes, it's a great question. Previous answers are great too. Like most technical subjects it can be rather complicated.
One of the things EEs have to get good at is understanding their test equipment. This is often more complicated than it appears.
This web site lists lots of DMMs for sale. They neither cost the same or work the same, in features, accuracy, reliability, useability, etc.
If you haven't yet, read a bit about the AC measurements; True RMS (TRMS), Average AC, measurement bandwidth and peak to average specifications. Read the whole data sheet* for
an expensive meter and see if you understand each entry. You don't need to make a career out of this, but you should know that those specs exist and sometimes matter.
Step 1 is the specifications for the meter. Manufacturers like Fluke, Keysight, Keithley... have pretty good specs available, but they are expensive. These are meters for people that need good tools. The cheap DMMs you can buy on eBay and Amazon for $20 or less, don't (or I wouldn't trust them if they did). You get what you pay for. The cheap ones are good too, I have both types in my lab, I use them for different things.
Step 2, no matter how much you paid for your instrument, is to work with it in your experiments to somehow verify that it's working OK. Small errors are nearly impossible to detect, but sometimes the errors aren't small because the instrument isn't appropriate for the task, has EMC issues, maybe broken, or set up incorrectly.
This sort of lab work is sometimes just as important as choosing IC's or transistors and follows a similar, often iterative, process: define what you need; read documentation; design, setup and VERIFY your test; collect data, including calibrations or sanity checks; think about if it makes sense (i.e. matches models and such). Other times you just want to know if the power is on and ±20% with minimal effort is good enough.
However, the truth is that practicing engineers simply don't have the time or money to always do all this stuff. We are usually operating on experience and faith, hoping we get good data but also keeping in mind the instruments might be wrong enough that we need to care. Knowing the circumstances where you need to be careful is a bit of an art.
*Tip: if you are reading a whole data sheet, don't skip the footnotes in tiny fonts. They are often the most important part.
