# What is 3% means of a NTC resistor?

#### tony_lth

Hi,
In the datasheet of a TDK NTC resistor, its spec is as following:
-40deg ~ +125 deg, measure accuracy is 3%, etc.
So what is the 3% means here? Because its measure range is (125-(-40))=165 deg, does 3% mean 165*3%=5deg error?
Thanks.
Best,
Tony Liu

Hi,

#### FvM

##### Super Moderator
Staff member
No NTC datasheet has percent measuring accuracy spec, the have resistance and possibly B coefficient spec. It can be converted in temperature accuracy which varies over temperature range. Due to B tolerance, least error is around reference temperatur, e.g. 25 °C.

### tony_lth

points: 2

#### crutschow

Likely the accuracy is +/- 3% of the nominal resistance value at 25C.

### tony_lth

points: 2

#### tony_lth

Hi. Gurus,
I used the following TDK NTC.
In room temeprature at 27 deg, the NTC resistor value is about 8K, so conduct the temperature is 33deg, about 5 deg error.
So what is the B constant value here? How to conduct accuracy temperature?
Best,
Tony Liu

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#### KlausST

##### Super Moderator
Staff member
Hi,

In the datasheet there are 72 occurances of "3%"
* most of them clearly give the tolerance of the "B constant"
* the other clearly give the tolerance of the nominal "25°C initial resistance value".

Which one exactly do you mean? And what is unclear with it?

Klaus
--- Updated ---

I just did an internet search for you "ntc b value calculations formula"
I came to this explanation: https://www.ametherm.com/blog/thermistors/thermistor-beta-calculations

Klaus

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### tony_lth

points: 2

#### FvM

##### Super Moderator
Staff member
Your type specification doesn't include the letter for resistance tolerance which can be either 3 or 5% according to datasheet. Exact B value and tolerance doesn't matter near 25 °C. For 8k you can calculate 31°C NTC temperature, deviation from expected 27 °C is much higher than explainable by resistor tolerance. Possible explanations
- inaccurate measurement
- NTC self heating. What's the sensor current?

### tony_lth

points: 2

#### tony_lth

Sorry, Gurus,
Finally we found the bug, there are some pull-up and pull-down resistors which should be removed, but not.
So after do so, we tested room tempereature 26.2 deg, and compared to a air-conditioner sensor's 26.4deg.
I am satisfied with the result.
BTW, do you think this TDK NTC can reach 1% measurement accuracy? from results, it seems very good.
But in datasheet, there are so many factors, so how to evaluate the accuracy from theory?
Best,
Tony Liu

#### Easy peasy

you would have to calibrate it at 3 differing temps to get 1% or better ...

### tony_lth

points: 2

#### FvM

##### Super Moderator
Staff member
You didn't mention the temperature range. 1 K accuracy around 25 °C is possible without calibration.
--- Updated ---

In addition, you can buy NTC sensors like NTCLE203E3103SB0 https://www.vishay.com/docs/29118/ntcle203.pdf with 2.2% R and 0,5 % B tolerance that achieve 1 K accuracy over the full application temperature range (-55 to +150°C) without calibration. Although it's simple error propagation calculus, 1st years engineering stuff, but you'll find supplementing documents with accuracy calculations on the TDK or Vishay web sites.

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points: 2