multifunction tester calibration and checkbox

Thread Starter

ninjaman

Joined May 18, 2013
318
hello,
i have a multifunction tester for testing domestic electrical installations.
i also have a checkbox that can be used to check the accuracy of the tester.
both are out of calibration by a good few years.
i dont want to have to spend a lot of money on calibrating both machines.
could you help me understand the percentage bits.
the continuity tester has an accuracy of +/- 1.5%rdg +3dgt, it is autoranging from 20/200/2000 ohms.
the checkbox for this function has three values to measure against, the recorded values in the box are 0.497, 0.996 and 2.002 ohms
i am not sure if i have to get 1.5% of the measured values of the checkbox and compare those to the readings of the tester?
so if i am right, the least i could measure is 0.489 ohms. this is -1.5% of 0.497. my tester gave me 0.45 ohms

is this method correct? the percentage values are the same for insulation resistance as well, when i test using the insulation resistance tester i get values that are with accuracy. could there be something wrong with the continuity tester?
i have not yet tested the rcd or earth loop tester
any help would be great,
all the best
simon
 

AnalogKid

Joined Aug 1, 2013
8,377
Manufacturer / model of the tester? Photos?

Are you sure about values of the three test resistors? For something that can measure up to 1.999K, they seem very small.

ak
 

Hymie

Joined Mar 30, 2018
797
As a continuity tester, the low resistance values are not out of place.
With an additional error of +3dgts; a reading of 0.486 ohm would be within spec – but not 0.45.
 

Hymie

Joined Mar 30, 2018
797
Further to my above post, the measurement error should be applied to the values read by the continuity meter (and not the actual resistance applied).

Therefore a reading of 0.450 ohm could result by presenting the meter with a resistance value between 0.44025 – 0.45975 ohm (+0.00675 made up from the 1.5% error and +0.003 from the 3 digit error).
Either way, it would appear that the meter and/or the checkbox are out of spec.
 

roadey_carl

Joined Jun 5, 2009
134
The calibration boxes are fixed value resistors usually 5% tolerance. Make sure both the meter and calibration box are at the same ambient temperature. Most multi-function testers don't have anything you can physically adjust inside to calibrate them anymore so if it's falling out of spec it will be because of oxidation on connectors, leads or internal relays. Paying someone to "calibrate" it is a huge waste of money as it's just plugged into a machine which known values are applied to the meter to make sure it's working within manufacturers specification.... if it's not, they don't repair it anyway. It should be called "meter checking" rather than "calibrating". Just make sure the leads are Zero'd properly and all the contacts are good.
 
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