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CARC DVM Evening

 On 26th April CARC met for an evening on Digital Voltmeters. This was part of an occasional series that we are running on test and measurement. Mike G0KAD have a short talk on DVM and how they measure voltage using the dual ramp technique. A number of members bought along their voltmeter from their shack, mostly digital and a few analogue. Stewart G3YSX provided a number of "standards" that he built a while ago based on band gap reference ICs that provided a 5V output. We measured these voltage sources and recorded the results.

The measured results are tabulated below. However the first problem is to determine how good the references are. I  approached this by assuming that although out of calibration the four five-digit DVMs were likely to the most accurate. The average voltage reading for these DVMs is shown in the Average 4 (decimal places) row. I then had Excel calculate the average and the standard deviation. For two sources the standard deviation was 800uV and the third one (V2) had a standard deviation of 1,1mV. The standard deviation is a statistical method of calculating the range of values in which 2/3 of measurements are expected. Thus in the case of V1 there is a 66% probability that V1 was between 4.9989 and 5.0005v. Whilst I did not calculate it below there is a 95% chance a value being within two standard deviations of the mean, i.e. in the case of V1 between the values 4.9981v and 5.0013v.

In the table I have highlighted in green measured values that lie within one standard deviation of the particular  source.

OwnerModelV1V2V3
M0IJPQ14425.004.995.00
G4PEOEEBLOG BM2354.9924.9924.992
G4PEOAVO8 MK34.9104.9254.925
G4PEOFLUKE 1794.9995.0005.000
G4PEOUNIT UT61E4.9884.9884.988
M0HQMMAPLIN WG0204.995.00
G3ZIY71505.00055.00035.0007
G3YSXISO TECH IDM5054.9995.0005.000
G3YSXSDM30554.9965.0005.001
G3YSXSanwa U50D5.05.0
G6RCSinometer4.984.984.98
G0KADF774.984.984.98
G0KADF1895.00035.00075.0011
G6RCHP34401A4.99944.99805.0000
M0WIDWG0224.994.994.99
M0WIDFLUKE755.005.005.00
M0WIDAVO 4.954.954.95
G4ANNF1894.99854.99894.9991
Average 44.99974.99955.0002
Std Dev 40.00080.00110.0008
Average - Std dev4.99894.99844.9995
Average + Std dev5.00055.00065.0010


However it is difficult to draw significant conclusions from the above beyond noting that for 
most practical benchwork all the meters tested were good enough, and only specialist 
applications need anything better that a £20 eBay/Amazon/Bangood model.

To understand things better I looked at the measurements of source V1 and looked at the 
errors in a little more detail. I used a reference voltage of 4.9997 as determined above. I then sorted the table on the square of the percentage error.


Owner

Model

V1

Error (%)

Error mv

G6RC

HP34401A

4.9994

-0.01

0

M0IJP

Q1442

5.00

0.01

0

G3YSX

Sanwa U50D

5.0

0.01

0

M0WID

FLUKE75

5.00

0.01

0

G0KAD

F189

5.0003

0.01

1

G4PEO

FLUKE 179

4.999

-0.01

-1

G3YSX

ISO TECH IDM505

4.999

-0.01

-1

G3ZIY

7150

5.0005

0.02

1

G4ANN

F189

4.9985

-0.02

-1

G3YSX

SDM3055

4.996

-0.07

-4

G4PEO

EEBLOG BM235

4.992

-0.15

-8

M0HQM

MAPLIN WG020

4.99

-0.19

-10

M0WID

WG022

4.99

-0.19

-10

G4PEO

UNIT UT61E

4.988

-0.23

-12

G6RC

Sinometer

4.98

-0.39

-20

G0KAD

F77

4.98

-0.39

-20

M0WID

AVO 

4.95

-0.99

-50

G4PEO

AVO8 MK3

4.910

-1.79

-90


It is not surprising that the AVOs were at the bottom of the table. They are old and a 
long time out of calibration, They are sensitive to their environment, their pre-measurement calibration 
their exact orientation and to how well the mirror scale is used.

Some of the meters at the top of the table are there because they did not "try too hard" that is the results 
were reported to a lesser degree of precision. 

There is a certain measure of you get what you may for in the results. All the meters except one AVO
were better that 1% accuracy and the majority better than 0,2%/19mV in 5V.

Stewart/G3YSX

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