Hi,
It's not a sine wave, and the voltage is lower than in most places but it's always been like that, so that part of it is not the fault of the scope.
Mains has a sine (-ish) waveform shape with some distortion, this is expected.
However, it measures 150v max and -144v min, which cant be right. That would mean a net DC current flows in every thing we plug in that has a transformer in it as the DC current ratchets up when there is a non symmetrical wave like that feeding it.
Think about measurement setup and possible errors involved. Assuming you are using 50V/div setting (5V/div 1x input + 10x probe attenuation) with 8 vertical divisions and 8 bit ADC, vertical sensitivity is 400V/256 = 1.5625V. Error is in the order of 4 LSB. On top of that, scope has some inaccuracies in scale and offset, this easily explains possible error. Also, AVG waveform value should be better criteria than (MAX + MIN)/2.
IOW: scope is not well suited to accurately measure small DC offset on large AC waveform. HDS242 has built-in DMM, you can use that in DC mode instead.
Ok thanks and i believe you are correct i did not think this through.
It just struck me as strange because i never saw that with a CRT scope.
Yes i know i could use the meter and get better results, but i wanted to use the scope itself.
And yes, the waveform of our line was never perfect i mentioned that and i measured it with a CRT too and it's distorted. I thought that was strange too but then it was always like that in this neighborhood.
The other thing is i dont really need a better scope and since i payed just around $100 USD for this thing i guess i cant complain (ha ha).
I was bugged about the bandwidth being only around 24MHz but one of the most important things i had to measure with it so far required a horizontal time base setting of just 1 second per division (big chuckle) which means even a 1MHz scope would have done the job, and actually i did use a 1MHz digital scope a while back to measure the same thing and it worked ok. The scope pics look very similar. [EDIT] Actually that was a 1MSa/s scope with 100kHz bandwidth ha ha.
The PC software is very limited though, only allows you to upload the waveform to the computer for viewing and storage. No real time scope display on the PC. The portability is just great though if i get another scope i am going to make sure it is either battery operated or can be USB operated using a USB power pack used for charging phones. Cant beat that kind of isolation from the line either. I see a lot of scopes that run on USB power now too.