17-08-2021, 10:18 AM
Yes, the maths is a minor pain, but as a scientific calculator is constantly at my side, it's not a huge problem. To be honest, as I mostly do audio, I rarely need to know the AD+DC value - as such, I'm quite happy that AC coupling is the default.
I do agree that the marketing term "true RMS" can be misleading. Of course, it just means that RMS conversion is taking place in some form, as opposed to average conversion. But it doesn't imply AC or AC+DC, even though one could argue that it should be the latter - although I personally prefer the former...
For example, one of my favourite eBay bargains - the Fluke 8920A - uses a thermal converter, so is about as "true RMS" as it gets. Conveniently, it has an AC/AC+DC switch on the front panel, which is handy, but potentially confusing for some. But it covers all bases.
The other problem is bandwidth. Those true RMS Chinese meters have avoided the cost of an analogue RMS converter by sampling the signal and doing the RMS conversion in DSP. Much as I understand why, I still think it's amazing that we're at the point in the evolution of electronics where many thousands of transistors being on or off is cheaper than a couple of dozen transistors working in a linear fashion! Anyway, the sample rate is quite low, resulting in a -3dB point of 3kHz, which isn't so great for audio, but still perfectly good for more 50Hz PSU work. But a definite trap for the unwary.
A Fluke 110 for free is quite a bargain! It might be a basic meter, but it's well-made and should last well. We've got a batch of 115s at work - they're nice meters.
I do agree that the marketing term "true RMS" can be misleading. Of course, it just means that RMS conversion is taking place in some form, as opposed to average conversion. But it doesn't imply AC or AC+DC, even though one could argue that it should be the latter - although I personally prefer the former...
For example, one of my favourite eBay bargains - the Fluke 8920A - uses a thermal converter, so is about as "true RMS" as it gets. Conveniently, it has an AC/AC+DC switch on the front panel, which is handy, but potentially confusing for some. But it covers all bases.
The other problem is bandwidth. Those true RMS Chinese meters have avoided the cost of an analogue RMS converter by sampling the signal and doing the RMS conversion in DSP. Much as I understand why, I still think it's amazing that we're at the point in the evolution of electronics where many thousands of transistors being on or off is cheaper than a couple of dozen transistors working in a linear fashion! Anyway, the sample rate is quite low, resulting in a -3dB point of 3kHz, which isn't so great for audio, but still perfectly good for more 50Hz PSU work. But a definite trap for the unwary.
A Fluke 110 for free is quite a bargain! It might be a basic meter, but it's well-made and should last well. We've got a batch of 115s at work - they're nice meters.