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
This type of panel mount meter may either be intended to provide accurate measurement of the voltage, in which case the accuracy might be of the order of 0.1%, or provide an indication that voltage is present and about the right value, so the accuracy will be considerably lower.
There will almost certainly be a method of calibration to allow the indication to be more or less correct in the installation at the expected voltage. This may either be accessible by removing a cover, or be factory preset and sealed.
On the subject of Sat Nav speed measurement, I believe they measure the time taken to travel a particular distance. But the distance is the horizontal distance, so may be affected by hills and bends. On a hill the speedometer will measure the distance along the hypotenuse of the triangle (i.e. up the hill), but the Sat Nav will measure the horizontal distance, which will be shorter. In my experience, despite this the sat nav is almost always more accurate than the speedometer.
There is a forum member who borrowed a highly accurate GPS speedometer designed to accurately determine the performance of a car on a track rather than for avoiding a speeding ticket on a road. So we may have someone with a detailed knowledge and experience of recording vehicle speeds with GPS amongst us.
He also has more knowledge and experience of measurement instruments than most people.
yMy first experience of GPS was in the first Gulf War when accuracy was +/- 50m. (Map of ship along side showed it on dry land. ? )
It's all very well suggesting that GPS speed calculators are horizontal only, but they are perfectly well aware of the topology. So the software is, or should be, perfectly cable of making the appropriate adjustments.
In addition, I understand that GPS can measure altitude.
Actually these days even in residential areas, harmonics can make the waveform all kinds of nasty. Put a scope on the mains a while back (via an isolation transformer which i would have thought would've smoothed over the worst, and the waveform was distinctly 'flattish' at the peaks and had some odd angles on the sides (ie sharp changes in slope). I suspect the reason is the time of day, fairly low load which would have been mostly electronic loads, from home PC's, modern tv's etc through to LED bulbs.
. . . I understand that GPS can measure altitude. . . .
It does, regardless of the system you are using - NAVSTAR (America), GLONASS (Russia), Galileo (Europe) and BeiDou (China). GPS Speed is calculated as a time between two points, which are assumed to be a straight line. This will give an error if you are not travelling in a straight line.
NAVSTAR.was the original system and was developed by the US Airforce. It broadcast two signals, one coded for use by the US military, which was accurate, and a second one for use by everyone else, which incorporated a random error, which is where you probably got your error from. President Reagan signed an order in 2000 that removed the random error, however it could be reapplied if required for “defence purposes”.
As has been mentioned a few times on this thread and others I think we must all appreciate that digital meters have a downside. There is a tendency to believe the resolution to cause accuracy whereas the old anologue meters we knew we were guessing the reading on the scale using line of sight with paralax errors. Whilst digital meters give more positive results i.e. an actual reading we should not take that reading as gospel. On DC they are quite good but a reading of 1.0000V is 1.0000 my-meter-volts and NOT 1.0000 absolute volts. If I use that meter to compare different voltages but over a close range then it might well be quite accurate as a comparitor, say 0.5V to 1.5V for instance or 0 to 2v or whatever as a comparison. AC is something different though and even clever meters sampling can be fooled to some extent. In the bad old days of my youth just about all the meters were anologue and if dropped or banged there was a price to be paid in future readings, You interpret the needle flicker to some extent and that probably gives you more of an inkling as to what is going on. A digital meter with a bargraph display perhaps is a midway to some extent.
I might have a digital watch that shows it is 12:36 and 18.23716 seconds but then again I set it two and a half minutes slow and it loses 5 to 10 seconds each day.
If we treat 90% of digital meters as just good guesses like the old anologue ones then we are on the road to enlightenment methinks.
I doing it again, it being using the timeline on Google maps to check where I was and when whilst preparing invoices.
Most of the time the GPS on my phone is pinpoint accurate and correctly identifies the house I was in, occasionally it goes completely haywire.
Recently I did a job above Cardiff and according to the Google maps time line I jumped on a train, then went into Cardiff city centre then back out again, that’s either a complete fabrication or one hell of a senior moment.
However as I have no recollection of it and I finished the job during that time, I just deleted that from my timeline.
Hi ebee and mhrestorations I totally agree I did think that being digital and brand new still in there little plastic bags my meters would of been spot on what first concerned me was the fact that they both showed different voltages this being in spite of the fact they were connected to the same plug if the difference had been say 1 volt between them then I wouldn't have worried but it was six volts which as I said made me think one of them was being les than truthful. But after all the replies I've had I get that it's just a mix of how the meters were set up and reading errors caused by sample rate and frequency as well as harmonics on the mains so if one reads say 241 and the other 236 then il accept that the voltage is around 240 even more so if other meters show around the same figure that will do. Ide forgotten that the voltage flickers a bit I haven't used an analogue meter in so long it had gone from my mind. Regarding mains waveform that's how mine looked flattened out tops or crests I did wonder if it was a funny on my scope so I coupled up a signal generator and that gave a perfect sine wave so it really was the mains crests being crushed when I say crests of course it applies equally to both halves of the sine wave what do you call a negative going crest??
