I am now the proud owner of a shiny new true RMS multi meter I've seen revues of it on YouTube and it gets very good reviews on all aspects of its accuracy and construction and of course safety not sure if I'm aloud to say which one it is maybe someone will tell me if its ok. Anyhow it reads the same as 2 of my other meters which are also true RMS although I didn't know they where. Any how I've found out that the 2 LED meters I bought that I posted about some time ago are both telling lies although one of them is occasionally telling the truth. It's interesting that the voltage is never still but is usually around 240 and also if I look at around 23:30 it drops off by 3 to 4 volts which must be when local economy 7 kicks in
What is more important is how the supplier declares the accuracy which is normally a % of full scale plus so many digits Kelly.
Effectively a meter on a scale reading say up to 999.9 volts will declare say, plus minus 0.3% (ie 3 volts) and plus minus 5 digits. So if you're connected to a true 240.0 V, it could read between 236.5 and 243.5 V and still be within specification................
It is important not to confuse resolution (there are 1000 marks on my metre ruler) with precision (the gap between any 2 adjacent marks is 1mm +/- 0.1mm =10%_, the over-all length may be 1m +/- 1mm= 0.1% )
Digital meters are no different to analogue things like rulers in terms of having an inherent uncertainty, but the numbers sometimes give a false illusion of accuracy.
Even a poor meter can tell 220 V from 240, which in many cases is enough.
What is the guaranteed accuracy spec on the 'lying' meters?
If you really want accuracy some of the 24 bit A/D converters on the market are quite good, on DC of course. Try Analogue Devices website. Reading True RMS AC is a bit more difficult, and the RMS calculation needs a bit of computer power. 24 bits give a resolution of about 1 part in 1.5 Million and an accuracy of about 2 bits is available. The best digital meters can have at least 12 decimal places of resolution and very high accuracy of perhaps 10 - 11 decimal digits but are a bit £££ for general use. It is interesting that Frequency is probably the most accurate physical phenomenon we can measure to extreme accuracy, with voltage not too far behind. Things like temperature are some way behind, but there are not so many requirements for that to ultra accuracy either.
Hi I don't know the specs on the lying meters they came with no instructions or spec sheet just the 2 meters in there own little plastic bags I suppose that should of been a warning to me but I was caught up in the moment. The meter I have now is an Astro AI 6000AR Says its 6000 counts not sure what that means. It Messerschmitt down to one decimal place and when it reads say 240.7 another meter flickers between 240 and 241 another one reads a solid 241 so I'm confident that what I'm reading is right certainly right enough for me. I can't afford to get it calibrated that's way to expensive I struggled raising the £25 the meter cost!
6000 counts is the maximum reading that is 5999 (0 being a reading as well). It seems that the 3 meters you have match very well and read the same as far as they can. There is always a +- 1 uncertainty with any digital device even if totally accurate, so your readings are essentially the same. It reads 240.something using 2400 of the 6000 maximum counts available, so again perfectly fine, because if you had 599.9V it would use the range maximum of 6000. Again ja bit confusing but just the way digital things work.
Thanks David that explains it unfortunately I've fallen into the trap of believing just because it's digital it must be right of course now I know that's far from true we live and learn. Ive re read my previouse post I see spell check has struck again wish ide noticed it before can only apologise
...unfortunately I've fallen into the trap of believing just because it's digital it must be right ...
Kelly,
Take consolation from the fact that you are far from alone. I used to occasionally help with a training course where we had the trainees checking the operation of a ship's generation plant and they would invariably gather round the digital read-outs, never looking at the analogue meters, let alone comparing the readings (and were always amazed later when we pointed out to them what they had missed!!!)
