megger calibration
- Thread starter andy
- Start date
B
breezer
try it and see.
B
breezer
i am sorry you think so, but it depends on where you take /send it to, hence try it and see
If its your money you might be wasting, then it is worth a quick pre-calibration check to guarantee a formal 'pass'.
Unless it says otherwise the test voltage it produces should be 500VDC, you can check that with another meter on DC volts. If you buy some high wattage 1 megohm resistors from RS or similar you can place these in series/parallel combinations across the terminals to get 500k (2 in ||) (1 meg, one on its own!) and 2meg (2 in --) .
For this investment of under a fiver you can now check the calibration against these known "transfer standards" ( the official name for a secondary standard that is not an official standard itself) any time you like between initial calibrations.
(If at any time you find the two resistors don't read the same as each other, it is time to replace them.!!)
Of course if 'official' calibration is free then there is no point, but to have such a confidence test 'in the bag' allows a defective instrument to be spotted quickly. -For the price of an official cal, you can buy many hundreds of 5% or better resistors that will stand 500V !
Regards M.
Unless it says otherwise the test voltage it produces should be 500VDC, you can check that with another meter on DC volts. If you buy some high wattage 1 megohm resistors from RS or similar you can place these in series/parallel combinations across the terminals to get 500k (2 in ||) (1 meg, one on its own!) and 2meg (2 in --) .
For this investment of under a fiver you can now check the calibration against these known "transfer standards" ( the official name for a secondary standard that is not an official standard itself) any time you like between initial calibrations.
(If at any time you find the two resistors don't read the same as each other, it is time to replace them.!!)
Of course if 'official' calibration is free then there is no point, but to have such a confidence test 'in the bag' allows a defective instrument to be spotted quickly. -For the price of an official cal, you can buy many hundreds of 5% or better resistors that will stand 500V !
Regards M.
B
breezer
what make of multi meter, and when was it last calibrated?
B
breezer
so if it is a cheapo one (i have no objections to them, they do have their uses) how do you know how acurate it is? it may be way off at one end of the scale (it may not)
like cleaning a rolls royce with a tea towel
you can not check something using something else which itself has not been checked for acuracy and expect to get a good result
like cleaning a rolls royce with a tea towel
you can not check something using something else which itself has not been checked for acuracy and expect to get a good result
Well 625V is 25% high, which makes the test a bit harder than it should be.
Really, it depends on the specs, both of the megger and the multi-meter, probably worth checking both handbooks. Even cheapo meters are usually between 5 and 10%, and some jolly expensive meggers only guarantee +/-30%, so price is not everything.
It certainly doesn't sound totally unbelievable, especially if the megger is a bit frisky, and the meter a bit optimistic - what does the meter make of the mains for example ?
This is the disadvantage of not using a third party calibration, you have to decide for yourself what to trust !
Take one of the resistors into work, and see what their megger reckons it is, then repeat the test with yours !
M.
Really, it depends on the specs, both of the megger and the multi-meter, probably worth checking both handbooks. Even cheapo meters are usually between 5 and 10%, and some jolly expensive meggers only guarantee +/-30%, so price is not everything.
It certainly doesn't sound totally unbelievable, especially if the megger is a bit frisky, and the meter a bit optimistic - what does the meter make of the mains for example ?
This is the disadvantage of not using a third party calibration, you have to decide for yourself what to trust !
Take one of the resistors into work, and see what their megger reckons it is, then repeat the test with yours !
M.
mapj1 said:
Don't go overboard with the "high wattage" spec when you buy - 500V through 1Mohm is ¼W....
You can build yourself a little test rig - DIN box with a couple of RCDs, some resistors switchable for different values and a voltage indicating device. I believe that if you start out with the VID and your test kit calibrated, and you keep regular records of checking your test kit against the test rig (every 3 months? 1month?) then you do not need annual recertification as long as your results are consistent.
Or buy one
See here: http://www.screwfix.com/talk/thread.jspa?threadID=11502&tstart=480
and some of the replies interspersed in here: http://www.screwfix.com/talk/thread.jspa?threadID=12522&tstart=135
True, BAS, but remember that the voltage rating of some smaller sizes may not be enough, even if the wattage rating is adequate. I'd stay away from th cheapest of the cheap, and anything in 0201!
Something like the /VR37 series, at about 20p each from Farnell go nicely under ceramic choc blocs. Also some low value wirewounds for testing the earth loop meter might be a sensible investment.
regards M.
Something like the /VR37 series, at about 20p each from Farnell go nicely under ceramic choc blocs. Also some low value wirewounds for testing the earth loop meter might be a sensible investment.
regards M.
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