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.

Owner | Model | V1 | V2 | V3 |

M0IJP | Q1442 | 5.00 | 4.99 | 5.00 |

G4PEO | EEBLOG BM235 | 4.992 | 4.992 | 4.992 |

G4PEO | AVO8 MK3 | 4.910 | 4.925 | 4.925 |

G4PEO | FLUKE 179 | 4.999 | 5.000 | 5.000 |

G4PEO | UNIT UT61E | 4.988 | 4.988 | 4.988 |

M0HQM | MAPLIN WG020 | 4.99 | 5.00 | |

G3ZIY | 7150 | 5.0005 | 5.0003 | 5.0007 |

G3YSX | ISO TECH IDM505 | 4.999 | 5.000 | 5.000 |

G3YSX | SDM3055 | 4.996 | 5.000 | 5.001 |

G3YSX | Sanwa U50D | 5.0 | 5.0 | |

G6RC | Sinometer | 4.98 | 4.98 | 4.98 |

G0KAD | F77 | 4.98 | 4.98 | 4.98 |

G0KAD | F189 | 5.0003 | 5.0007 | 5.0011 |

G6RC | HP34401A | 4.9994 | 4.9980 | 5.0000 |

M0WID | WG022 | 4.99 | 4.99 | 4.99 |

M0WID | FLUKE75 | 5.00 | 5.00 | 5.00 |

M0WID | AVO | 4.95 | 4.95 | 4.95 |

G4ANN | F189 | 4.9985 | 4.9989 | 4.9991 |

Average 4 | 4.9997 | 4.9995 | 5.0002 | |

Std Dev 4 | 0.0008 | 0.0011 | 0.0008 | |

Average - Std dev | 4.9989 | 4.9984 | 4.9995 | |

Average + Std dev | 5.0005 | 5.0006 | 5.0010 |

| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

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 |

## No comments:

Post a Comment