Question:
How can I Calculate the Tolerance on a Measurement
Answer:
We will explain the terminology and show a few examples to help you understand this topic
Fluke meters from the Industrial product range express accuracy as a percentage of reading, please see this web page to understand the difference between percentage of range and percentage of reading.
https://www.fluke.com/en/learn/blog/digital-multimeters/accuracy-precision
The quoted specification that you find in the manual or datasheet is from the manufacturing process. It covers every meter of that model that has ever been made, when measuring the applied value they are all expected to read between the calculated limits. It is valid over 18°C to 28°C and a further correction factor is applied per °C outside that range.
If you want to determine the accuracy for a particular instrument, it would require calibration as an individual device.
To explain it in a practical way, we are going to use the display value of a 6000 count multimeter.
Think of the display as just a number, without a decimal point or unit identifier (V, A, Hz etc), on a particular Fluke meter this number is a “count” from 0000 to 6000.
A count of 2 would be the least significant figure which equals 0002, this value is fixed whatever the input and is often expressed using the term "digits".
The % of reading is only significant at the high end of a range, at the low end this percentage can be below the resolution of that range, where it is at the low end that the “count” (digits) becomes important.
Consider an accuracy of ±([1% of reading] + [2 counts]) for this instrument on its voltage function.
For an input of 5mV:
The instrument will auto range such that the 0000 to 6000 count represents 0.0mv to 600.0mV. This means the smallest step (resolution) is 0.1mV
1% of 5mV is 0.05mV = 0000, (because 0.05 mv is below the resolution) and the count is 0002
giving an accuracy of ±([0000] + [0002]) = ± 0002 which represents ±0.2mV
this calculates to a measurement tolerance of : 4.8mV to 5.2mV
For an input of 200mV:
The instrument will auto range such that the 0000 to 6000 count represents 0.0mV to 600.0mV
1% of 200.0 is 2.0 = 0020, the count is 0002
giving an accuracy of ±([0020] + [0002]) = ± 0022 which represents ±2.2mV
this calculates to a measurement tolerance of : 197.8mV to 202.2mV
For an input of 50V:
The instrument will auto range such that the 0000 to 6000 count represents 0.00V to 60.00V
1% of 50.00 is 0.50 = 0050, the count is 0002
giving an accuracy of ±([0050] + [0002]) = ± 0052 which represents ±0.52V
this calculates to a tolerance of : 49.48V to 50.52V
For an input of 450V:
The instrument will auto range such that the 0000 to 6000 count represents 0.0V to 600.0V
1% of 450.0 is 4.5 = 0045, the count is 0002
giving an accuracy of ±([0045] + [0002]) = ± 0047 which represents ±4.7V
this calculates to a tolerance of : 445.3V to 454.7V