0
\$\begingroup\$

This question already has an answer here:

I have tried searching for an answer and looked online, but I'm just getting more confused.

I have a DMM that measures temperature with a thermocouple lead, with a resolution of 1 degree C (shows 020) for 20 degrees C and manual states accuracy of ±1%+3 digits. I understand the 1% of the reading, but I cannot figure out the 3 digits part.

For 20 degrees C, 1% is 0.2. Does this mean I should add 3 to the 0.2 meaning ± 3.2C or add 0.3 to the 0.2 meaning ±0.5C? Or is it something I am missing?

Also temp reading always flickers between 2 readings. Does this mean temp is in between the two?

Thanks

\$\endgroup\$

marked as duplicate by Edgar Brown, Finbarr, Kevin Reid, Elliot Alderson, Voltage Spike Feb 22 at 17:41

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • \$\begingroup\$ You have omitted the make, model and link to user manual. The 1% will relate to the full-scale value - not the reading. Please edit your question to give the missing details. \$\endgroup\$ – Transistor Feb 17 at 17:10
  • 1
    \$\begingroup\$ make and model should not matter should they surely its a standard for all DMM's \$\endgroup\$ – Cliff Hitchen Feb 17 at 17:41
1
\$\begingroup\$

Let's take a typical midrange handheld multimeter manual so we know what the heck we are talking about:

The relevant section is here:

enter image description here

Accuracy is stated as +/-3% of reading + 3 digits. That means if your temperature is indicated at 20°C, the multimeter is between 18°C and 28°C, a perfect thermocouple junction would be between 16.4°C and 23.6°C.

In practice that might be a bit optimistic, especially for Fahrenheit, as it should include linearity errors as well as reference errors and cold-junction compensation errors, and we would not expect those to improve for °F. It will also likely be in greater error if there are large temperature gradients in the vicinity or if temperature is changing rapidly with time.

Typically the reference and linearity errors are more of an issue at very high temperatures. And, of course, the sensor errors, which are not included in the multimeter accuracy. Conservation of energy and thermodynamics limit the sensor errors near room temperature, of course.

Turning our attention to the sensor, limits of error for a Chromel-Alumel thermocouple are as follows:

Type K Accuracy (whichever is greater-- over the range 0 to 1250°C):

Standard: +/- 2.2C or +/- .75%

Special Limits of Error: +/- 1.1C or 0.4%

That's for standards-compliant properly manufactured thermocouple alloys.

You can improve the accuracy, especially of measurements near room temperature, by immersing your probe deeply into a ice-water slurry, taking a 0°C reading and correcting your actual reading for any error you see. For example, if your meter reads -1°C in the slurry, add 1°C to your reading. And, of course, do it all in a stable and temperature controlled environment as free of drafts as possible. That will correct for cold-junction compensation and any small thermal EMF errors which dominate near room temperature.

\$\endgroup\$
  • \$\begingroup\$ great answer so the 3 digits is added / subtracted from the lowest figure (on right) in this case 20 +/- 3 (23 or 17) and the 1% +/- 0.6 giving your figures thanks for explaining \$\endgroup\$ – Cliff Hitchen Feb 17 at 17:40
0
\$\begingroup\$

1% is the gain error tolerance.
+/-3 degrees C the offset error tolerance.

Average the readings you see can improve the offset by 0.5 deg.

\$\endgroup\$
-3
\$\begingroup\$

Lets Understand this by simple example

your temperature accuracy is 1% +3 digits

now if your meter is showing 020C than it has accuracy of 0.20C and +3 Digits means Your temperature is between 19.7 To 20.3

Alternatively, how close the measurement is to the actual or reference value of the signal being measured.

Accuracy may be represented as a percentage as well as digits.

Example: an accuracy of ±2%, +2 digits means 100.0 V reading on a multimeter can be from 97.8 V to 102.2 V.

\$\endgroup\$
  • \$\begingroup\$ you say 19.7 to 20.3 but how do you arrive at these figures \$\endgroup\$ – Cliff Hitchen Feb 17 at 5:54
  • \$\begingroup\$ 0.2 from the 1% but why 0.1 for 3 digits? \$\endgroup\$ – Cliff Hitchen Feb 17 at 6:02
  • \$\begingroup\$ 20.0 +0.3 To -0.3 (For 3 digit) \$\endgroup\$ – Jay Vadoliya Feb 17 at 6:09
  • \$\begingroup\$ sorry I still dont get it your adjusting it by 0.3 for 3 digit but what about the 1% (0.2) \$\endgroup\$ – Cliff Hitchen Feb 17 at 6:23
  • \$\begingroup\$ You can find explanation HERE \$\endgroup\$ – Jay Vadoliya Feb 17 at 7:39

Not the answer you're looking for? Browse other questions tagged or ask your own question.