I recently purchased 2 little voltmeters they look like the sort that would go in a control or instrument panel they are connected with just 2 wires which provide the operating supply ( they light up green and red) however the green one states it will work between 20and 500 volts and the red one between 60 and 480 volts. When they are both on the green one indicates normally around 241 volts the red one shows 235 volts why the discrepancy I know it's not much but makes you wonder if one of them is lying. Secondly I've noticed that the green one tracks voltage changes faster than the red one and that a few times the green one jumps down to 238 then up to 241 multiple times while the red one stays the same and I think can see a slight flicker in my filament lamps when this is happening incidentally both meters are connected to the same plug a 2 pin 5 amp one
Sounds like some digital meters I had a look at on a famous internet auction site...
6V difference in readings could mean than each is only 3V off - which on a 500V scale is just 0.6% of f.s.d. which probably isn't too bad (or equally they could both be off by a much larger amount). If they're digital they'll sample the voltage at intervals and the analogue to digital converter (ADC) will work in certain sized steps (usually the full range divided by some power of 2 - e.g. dividing the range into 256, 1024, 2048, or 4096 etc steps) - the greater the number of steps the more expensive the ADC - so you get what you pay for. Supplying the meters with a variable voltage (say through an old fashioned variac) might be educational. Likewise the sample rate could be different between the two meters which might explain why one is more likely to respond to any short duration glitches. There could also be some 'smoothing' going on in there.
Yesterday I went and did two jobs in Nottingham which along with lots of speed cameras has roadside LED displays that display your actual speed colour coded to tell you quickly if you are behaving correctly.
I bought a new sat nav recently and the GPS speedometer it displays matched the calibrated roadside displays, but the “analogue” speedometer on the dashboard with its swinging needle significantly over states the vehicle speed probably by about 10%.
As I was coming home on the motorways I did wonder how many people driving at around sixty and not keeping up with the flow of traffic actually think they are driving bang on the seventy speed limit.
Mind you it foes keep people well within the limit unlike the guy I worked with on Thursday who has just completed a speed awareness course, he lives in Wales and drives a transit van, not a good combination as I think there is only two roads in Wales on which you can drive at seventy in a Transit van.
We did have a discussion based on his recent updated knowledge from the deed awareness course about my Nissan NV200 van being described as a “car derived van” on its logbook, which should permit me to drive it faster than he can drive his Transit on many roads, a subject that Bod has commented on before.
a “car derived van” on its logbook, which should permit me to drive it faster than he can drive his Transit on many roads, a subject that Bod has commented on before.
There is a second criteria to the "car derived van" in as much relating to the maximum weight of the van, I recall this being < 2000kg. Highway Code will reveal!
This comes as a surprise to many pick up truck owners, that whilst having over 150 bhp are restricted to 50 mph on single and 60 mph on dual carriageway......
And when that dual carriageway is the one from the end of the M50 above Ross on Wye to Coldra on the M4 At Newport or out across the Heads of the Valleys into Wales the ten miles an hour speed difference allows the faster vehicle driver to stop for a short break and still get there at the same time, but refreshed.
I bought a new sat nav recently and the GPS speedometer it displays matched the calibrated roadside displays, but the “analogue” speedometer on the dashboard with its swinging needle significantly over states the vehicle speed probably by about 10%.
That is intentional. It is rather like the permitted tolerance around nominal voltage: +10%/-6% (App 2, para 15, which is the best I can do to return to the subject of the thread!)
Most police forces allow nominal +10% so on the basis of speedo inaccuracy, you will not go far wrong on a motorway if you stick to 80 mph. Worst case is speed awareness course.
Worst case scenario on Thursday wound have been the bucket that flew off the back of the builders truck as we went around the ring road going through my window screen.
Point is that any measuring device has a degree of error. There are some incredibly accurate clocks, for example even pendulum ones can be accurate to within a second per year.
I would expect that my best Rabone Chesterman steel rule is pretty close to a foot long (at 20 deg C) but even so. I doubt that I could measure anything within 20 thou, so that's a 1:6000 error.
So my expensive Megger MFT starts to look pretty poor at +/- 1%.
Conclusion is, Kelly, that I think that your meters are behaving within spec.
Another factor is the response of the two meters to harmonics or the actual shape of the wave form which should be sinusiodal but probably isn't.
A meter which is using the electromagnetic effects will probably be quite accurate in giving a RMS voltage but one that samples the peak and assumes that this is root 2 of the RMS value will be greatly upset by a third harmonic which pushes the peak higher and reduces the voltage to each side of peak.
Maybe the two meters differ in how they handle harmonics.
I hadn't even considered harmonics I'm in a residential area so I don't know if it's a real problem I think the 5 or so volts difference in the meters readings is just down to natural errors. I did look at my mains waveform and it' had the peaks flattened out quite a bit must be all those switching power supplies in all the gadgets in various houses
