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BK 5491B DC Accuracy

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Circuitous:
I’ll follow this up with a more detailed review and possible teardown of a BK 5491B DMM (50,000 count).
I want to make sure that I understand the specified accuracy of this meter on DC voltage.
This specification is slightly different from Dave’s video:
5491B DC Accuracy = +/- ( 0.02% of reading + 0.008% of range)

So, if I measure a 10.000 volt source, using the 50 volt range, the reading should be:
+/- (10.000 x 0.0002 + 50.000 x 0.00008) = 0.002 + 0.004 =    +/- 0.006 volts

When measuring a 10.000 volt source, the reading should be between 9.994 and 10.006, right?

What I’m getting is a very, very repeatable:  10.018 volts.  When I reverse the leads, I get -9.982 volts.  This seems way out of spec, I originally questioned the accuracy of the voltage source, but not any more (I’ll post more on that later).  When I short the leads, the meter reads 0.000, could the internal voltage reference be way off?

I appreciate any thoughts on this.

amspire:

--- Quote from: cw on February 14, 2012, 02:19:33 pm ---I’ll follow this up with a more detailed review and possible teardown of a BK 5491B DMM (50,000 count).
I want to make sure that I understand the specified accuracy of this meter on DC voltage.
This specification is slightly different from Dave’s video:
5491B DC Accuracy = +/- ( 0.02% of reading + 0.008% of range)

So, if I measure a 10.000 volt source, using the 50 volt range, the reading should be:
+/- (10.000 x 0.0002 + 50.000 x 0.00008) = 0.002 + 0.004 =    +/- 0.006 volts

--- End quote ---

Isn't the specification for accuracy +/- ( 0.02% of reading + 0.08% of range)  which means +/- 0.042 volts for the 50V range?

I think you are within the specs.

But there is also the issue of whether something is going on with the measurement. There is a +0.018 offset at both the negative and positive ends.

Have you tried swapping the two probes probes, or even use some freshly stripped plain copper insulated wire? Also how is the reference powered - by batteries or from a mains power supply? If it is mains pwered, try measuring a 9V battery in both directions.

Do you need to use the REL button to get 0.000 volts when the leads are shorted, or are you getting 0.000V with the REL annunciator off?

Richard

Bored@Work:

--- Quote from: cw on February 14, 2012, 02:19:33 pm ---DC Accuracy = +/- ( 0.02% of reading + 0.008% of range)

...

So, if I measure a 10.000 volt source, using the 50 volt range, the reading should be:
+/- (10.000 x 0.0002 + 50.000 x 0.00008) = 0.002 + 0.004 =    +/- 0.006 volts

--- End quote ---

You are using the equation the wrong way "real value -> displayed value", while the equation is for "displayed value -> real value".  But the good news is, the error in doing this is minimal for your particular values (the range offset dominates). Therefore,

--- Quote ---
When measuring a 10.000 volt source, the reading should be between 9.994 and 10.006, right?

--- End quote ---
is acceptable, although the correct statement would be "when reading a value of 10.000 V the real voltage is between 9.994 V and 10.006 V."

--- Quote ---What I’m getting is a very, very repeatable:  10.018 volts.

--- End quote ---

If the meter would be within calibration this would mean the real value is somewhere between 10.012 V to 10.024 V.

--- Quote ---When I reverse the leads, I get -9.982 volts.

--- End quote ---

If the meter would be within calibration this would mean the real value is somewhere between -9.988 V to -9.974 V.

Now comes the funny part. If you calculate the average of the voltages you measured (ignoring sign), you end up with 10.000 V. Did you do the measurement in relative mode?

Circuitous:
Good questions.  I rechecked the accuracy in the datasheet, online manual, and the printed manual are:  0.02% of reading + 0.008% of range.  I can get out the calibration sheet tonight and check that as well.

I only used one set of probes, I’ll try another set (different maker) and see how that looks.

The meter zero’s when I connect the probes, without using REL.  I did try it with REL and got the exact same readings.  I will try something like giving it a 10.000 V input, then using REL and measuring another voltage, comparing that to what I would have expected to get if I hadn’t used REL.  Perhaps that will remove the 0.018V offset.

--- Quote ---the equation is for "displayed value -> real value".
--- End quote ---
Good point, I’ll try to correct my phrasing in the future.  I just don’t always think of it that way.

I haven’t had a chance to call BK and see if their \$100 calibration will correct the meter, or just tell me how far out of spec it is.
Does anyone know if the calibration for this meter is kept in software, or is just a pot?

I'll run a few more tests and post some more data... might not be able to do that tonight.

amspire:

--- Quote from: cw on February 14, 2012, 08:57:36 pm ---Good questions.  I rechecked the accuracy in the datasheet, online manual, and the printed manual are:  0.02% of reading + 0.008% of range.  I can get out the calibration sheet tonight and check that as well.

--- End quote ---
I was looking at this spec sheet:

http://www.tequipment.net/pdf/BK/5491B_Datasheet.pdf

It says that 50V DC voltage is 0.08% of range while the 50mA current range is 0.008% of range. You are not looking at the current accuracy table instead of the volts table?

Richard.