Due to some recent questions about test equipment, thought I would do a little post on DMM specifications and how they relate to the real world.

"Accuracy" is really a relative concept. You have to read and understand what the manufacturer of a piece of test equipment is saying when he quotes specifications. The figure that counts is uncertainty, which is a calculated value.

As an example, let's do an uncertainty calculation on a recently recommended piece of equipment, the Protek 608 DMM, measuring 1kohm of resistance. Let's say you wish to match resistors to 0.1%, a not unreasonable figure of 1 ohm. The specification of this machine is 0.2% of SCALE, or .2% of 5k, or 10 ohms, plus 5 digits,or 5 ohms That's going to give us a total spec of +/- 15 ohms. Statistically, we are going to assume a rectangular distribution of this figure, which says that there is 100% probability that it will fall within these bounds. So we are going to divide that number by the square root of 3, or 1.732, or 8.66 ohms. This reduces the spec to 1 sigma, or a 60% probability it lies whithin this figure (+/-).

Now for resolution. The resolution of the 5k scale is 5,000 counts, or 1 ohm. We don't know if the meter is rounding up or down for any measurement, so we are going to assume for calculations that we can read it to half a digit, or .5 ohm. Taking a distribution of this number you get 0.288 ohms.

Now to add all this up, we are going to use a method called root sum of squares, (RSS) or the square root of the sum of the squared value of the uncertainty contributions. The sum of squares is 75.078, the RSS is 8.66 ohms. Now, there is only a 60% probability that ANY ONE MEASUREMENT is going to end up in this 17.3 ohm range (+/- 8.66 ohms). Not good odds. So, we are raise this to a 95% probability by multiplying the number by 2. Thus, we get +/- 17.33 ohms as the uncertainty of any one measurement.

What does all this mean? It means that the protec is NOT going to match resistors to 0.1%. Any measurement can fall within the band of +/- 17.33 ohms AND STILL BE WITHIN THE MANUFACTURER'S SPEC FOR THE METER. And all you can really count on is the spec. Inexpensive meters are spec'ed like this because they really are this bad. Internal components in the meter self heat, giving you different readings each time you make a measurement.

This is why I advocate the purchace of good used benchtop meters from Ebay, or really high quality handhelds from Fluke or HP. Probability theory does not lie, and you absolutly get what you pay for. Do you need my $9700 Agilent 3458 to make useful measurements? No. But a cheap chinese handheld is not going to perform to the level that we all profess to want to work to.

BTW, I don't work for anyone who manufactures or sells instruments.

"Accuracy" is really a relative concept. You have to read and understand what the manufacturer of a piece of test equipment is saying when he quotes specifications. The figure that counts is uncertainty, which is a calculated value.

As an example, let's do an uncertainty calculation on a recently recommended piece of equipment, the Protek 608 DMM, measuring 1kohm of resistance. Let's say you wish to match resistors to 0.1%, a not unreasonable figure of 1 ohm. The specification of this machine is 0.2% of SCALE, or .2% of 5k, or 10 ohms, plus 5 digits,or 5 ohms That's going to give us a total spec of +/- 15 ohms. Statistically, we are going to assume a rectangular distribution of this figure, which says that there is 100% probability that it will fall within these bounds. So we are going to divide that number by the square root of 3, or 1.732, or 8.66 ohms. This reduces the spec to 1 sigma, or a 60% probability it lies whithin this figure (+/-).

Now for resolution. The resolution of the 5k scale is 5,000 counts, or 1 ohm. We don't know if the meter is rounding up or down for any measurement, so we are going to assume for calculations that we can read it to half a digit, or .5 ohm. Taking a distribution of this number you get 0.288 ohms.

Now to add all this up, we are going to use a method called root sum of squares, (RSS) or the square root of the sum of the squared value of the uncertainty contributions. The sum of squares is 75.078, the RSS is 8.66 ohms. Now, there is only a 60% probability that ANY ONE MEASUREMENT is going to end up in this 17.3 ohm range (+/- 8.66 ohms). Not good odds. So, we are raise this to a 95% probability by multiplying the number by 2. Thus, we get +/- 17.33 ohms as the uncertainty of any one measurement.

What does all this mean? It means that the protec is NOT going to match resistors to 0.1%. Any measurement can fall within the band of +/- 17.33 ohms AND STILL BE WITHIN THE MANUFACTURER'S SPEC FOR THE METER. And all you can really count on is the spec. Inexpensive meters are spec'ed like this because they really are this bad. Internal components in the meter self heat, giving you different readings each time you make a measurement.

This is why I advocate the purchace of good used benchtop meters from Ebay, or really high quality handhelds from Fluke or HP. Probability theory does not lie, and you absolutly get what you pay for. Do you need my $9700 Agilent 3458 to make useful measurements? No. But a cheap chinese handheld is not going to perform to the level that we all profess to want to work to.

BTW, I don't work for anyone who manufactures or sells instruments.