Products > Test Equipment
Easy way to test the calibration of a DMM (Fluke 45)?
Fried Chicken:
--- Quote from: KungFuJosh on April 03, 2024, 02:42:16 pm ---
--- Quote from: CalibrationGuy on April 03, 2024, 09:07:40 am ---Plug the meter into the socket and measure 120vac, but observe all cautions when working with line voltage.
--- End quote ---
I don't know anybody with wall voltage that's reliably on point, or stable throughout the day. I have to use a variac when I do circuit testing.
--- End quote ---
I noticed this, and it occurred to me that a wall socket will not be accurate, as it will be different behind every transformer, and depending on the copper you have, any resistances along the way.
That said, it can act as a reliable signal source. ERCOT publishes the current grid conditions, including the frequency to three decimal places. This should be the same wherever you measure from afaik, but AC is witchcraft.
mawyatt:
The AC mains are an interesting source.
Here is US the grid frequency can drop during the day with all the heavy loads, but picks up at night and early morning so the average mains frequency over 24 hours is exact.
The voltage can vary and don't think there is any attempt to make the average over 24 hours an exact amount, just attempt regulation at the substations to a fixed amount using tap changes on the large transformers. With the usual large inductive (motor) loads, sometime capacitors are added to improve (tune out the inductive reactance) the Power Factor.
Also the waveform can be distorted from the non-linear loads such as AC to DC conversion, so assuming a pure Sinewave is questionable.
Best,
shabaz:
I tried a total of five REF50XX samples (4.5V and 2.048V versions), all 2014 era.
Sample 1: 4.50228 V
Sample 2: 4.50254 V
Sample 3: 2.04845 V
Sample 4: 2.04867 V
Sample 5: 2.04852 V
The screenshot shows the graph/stats for about 7 minutes (started about 15 seconds after power-up) for the last sample.
The setup was the worst possible (see earlier photo), and the IC socket is squeezing on the chip. The chip factory trimming and general/mechanical stability are good enough for 10 mV levels of accuracy at a typical room temperature, with no calibration nor burn-in. It's an easy circuit to assemble properly, given the crude set-up performed adequately.
shabaz:
Trying quoting for the first time:
--- Quote from: J-R on April 03, 2024, 08:26:43 am ---It's true that not everyone needs the same level of performance from their test equipment, but these random unknown references really are bad no matter how you look at it. The technical shortcomings have already been well discussed (and perhaps somewhat ignored), but a different point is that you are frequently contributing to people who would be totally fine scamming you wherever possible.
--- End quote ---
That works both ways. There's no innocence on either side, with Westerners openly scamming by unlocking functionality that they have not paid for on Eastern test equipment. I don't choose to automatically mistrust every seller based on the occasional bad experience here or there.
--- Quote from: J-R on April 03, 2024, 08:26:43 am ---Instead, send your money to individuals who are not really doing it for the money but for the love of the hobby. Win/win for everyone.
--- End quote ---
Not everyone thinks the same. It is possible to "buy local" as well as to help others across the world, who might actually need the money.
--- Quote from: J-R on April 03, 2024, 08:26:43 am ---And again my main point about why these cheap references are "dangerous" is that they have specific voltages printed on them and are sold as "references" so buyers apply additional weight to them. What ratio of buyers do we think have adjusted their DMM based on the "reference" compared to adjusting the "reference" based on their DMM?
--- End quote ---
Let's guess at an extreme 90% of buyers adjusting their DMMs based on that board. Even if they did, so what? They would have a DMM that was possibly inaccurate by tens of mV at the most? What are the implications:
(a) For a hobbyist, if it affects their projects, it will be a great learning curve on what to trust. If something is worth doing, it's worth doing wrong.
(b) On the other hand, if you're a professional: a qualified engineer may well rely on prototype or 'uncalibrated' equipment when developing products; there's nothing abnormal in that. But equally, that engineer is in every way liable for trusting a $10 device over a calibration if they ignored a process, or were supposed to be working to a standard, or if a body of other professionals would not have done the same for that specific task.
Fungus:
--- Quote from: Fried Chicken on April 03, 2024, 04:37:52 pm ---That said, it can act as a reliable signal source. ERCOT publishes the current grid conditions, including the frequency to three decimal places. This should be the same wherever you measure from afaik, but AC is witchcraft.
--- End quote ---
They make clocks that use the mains as an oscillator and they're very accurate.
Apparently the power companies make great efforts to keep the frequencies stable and even compensate them over the course of the day to keep the total number of cycles per 24 hours correct.
(I'm not sure why they'd do that, was it just so that people could have accurate clocks?)
I don't know what happens in a power cut. Do they have an internal backup oscillator?
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version