EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: drummerdimitri on March 20, 2018, 10:22:27 am
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I'd like to calibrate some cheap old Radioshack DMMs since they have drifted from my Fluke 289's measurments and would like to get the best voltage reference that I can calibrate them against.
Will only consider purchasing on Aliexpress but there seems to be lots of models with slightly different AD584 chips.
Which one should I go for?
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If you trust your fluke 289, you could use that as a reference.
Assuming the other meters are 2000 count, you could measure a 1,5V battery simultaneously with the fluke
and a cheapo, adjust the cheapo to match the reading of the fluke.
For resistance (usually not adjustable) you could use generic resistors and measure them with the fluke, write down the values,
and remeasure whit the cheapo, on lower resistance values contact resistance will play a role so be aware of that.
How many trimpots are in these cheapo meters?
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If you trust your fluke 289, you could use that as a reference.
Assuming the other meters are 2000 count, you could measure a 1,5V battery simultaneously with the fluke
and a cheapo, adjust the cheapo to match the reading of the fluke.
For resistance (usually not adjustable) you could use generic resistors and measure them with the fluke, write down the values,
and remeasure whit the cheapo, on lower resistance values contact resistance will play a role so be aware of that.
How many trimpots are in these cheapo meters?
Problem is I've had my fluke for about 6 years now and never had it calibrated so I am not sure how accurate it is reading.
The cheapos have two trim pots one for ac v and one for dc v.
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The best I use for voltage reference in calibrating el cheapo DMMs is by using a L7805 Voltage regulator powered via a 9V battery, and I measure its output voltage using my Fluke 87V and the Sanwa CD800a. Then by adjusting the trimpot inside the DMMs, I set its reading to the reading/average between those two multimeters.
For resistance calibration, I use 3 different precision resistors(1k, 10k and 20k), each rated with 0.001% and 0.005% tolerance respectively. Got them all from Vishay(I don't remember their part number, I just know they are those with oil inside for temperature stabilization and etc)