You are on page 1of 4


Todays tolerance classes grew out of the practical need to be able to install and
replace probes and have them work the same from probe to probe, plug-and-play, so to
speak. Think Interchangeability.
RTD standards - SAMA, GOST, OIML, JEMIMA, BS, JIS, DIN, ASTM, IEC - most of
these specs have identical class names, but not always identical tolerance values! Not
to worry - all you need to know is right here.
The first thing is to understand the meaning of the percentage values often used to
differentiate tolerance classes. First off, the percentage statement is only a shorthand
label. It originated for grouping the platinum resistors into tolerance classes, much in
the same way common electronics resistors are grouped by 20%, 10%, 5%, etc. This
provides quick identification and comparison at the resistor component level. Well, over
time this nomenclature carried over into the classification of industrial RTD probes for
choosing the tolerance class of the probe.
These percentage ratings understandably cause confusion: Class B is .12% and Class
A is .06%, so at 100C class B will vary at the most 0.12C and Class A 0.06C the
most, Right? Answer: Not even remotely! The actual values are 0.80 and 0.35,
respectively (in a moment, well show how to get the right values in a snap).
Wheres that come from? Answer: These requirements (originally applying to the
platinum 100 resistor as a component) ensured that something labeled Class B would
have a max variation of .12% in base resistance value (at 0C only), Class A ditto, but
.06%. But when translated into a temperature variation, it becomes .3C! Why?
Answer: at 0C .12% translates into .3C and .06% delivers .15C (again,
only at 0C mind you).
One more thing on the percentage ratings. They are still commonly referred to because
they have been around, so it seems, forever. Even now, when we hear Class B we
automatically think .12%. We hear Class A, and we immediately envision .06%.
But actually the current specs no longer use percentage statements. (More of this in a
So to recap, the percentage figures are shorthand to differentiate the tolerance classes,
not to give a percentage tolerance of temperature over a range.
Now, how to get correct temperature tolerance values in a snap.

Here are the current international tolerance classes for RTD thermometers (the three
relevant to our industry):
IEC 60751 ed. 2.0 (IEC 60751:2008) specifies these classes for industrial RTD probes:
Applies To:

( 0.1 + 0.0017*| t |)
( 0.15 + 0.002*| t | )
( 0.3 + 0.005*| t | )

Temperature Range Tolerance

-100/150C (32/302F)
-100/300C (-22/572F)
-196/500C (-58/932F)

To calculate the allowable for a temperature, simply add-up the two values found in
the parentheses for that temperature. The symbol | t | means absolute temperature,
i.e., ignore the + or sign (but do make sure to stay within the negative temperature
limit shown). Example: for both 100C or - 100C, class AA tolerance is 0.1 for the
fixed value and .17 ( .0017*100) for the multiplied value. Add the two and you get the
max deviation of 0.27C.
To further illustrate, here are some temperatures with the corresponding max at those
Temperature C
- 50




Note that the AA and A classes default to class B tolerance outside their temperature
range limit (tolerance-wise only, this is not an operating range limitation) for their


Class B is the standard thats used for most industrial applications. Class A is often
used in sanitary processes, and is the standard for our Sanitary RTD product line.
But there is a trend for still higher accuracies than these. Indicators and transmitters
have become more sophisticated. The need for more critical temperature control
increases as products and production processes grow in complexity and effectiveness.
The Class AA option is available, but rarely requested, likely due to not being much
tighter than the class A.
The most accurate commercially available class is called 1/10 B (often carrying the
older reference of .01%). The interchangeability typically tightens in practice to .2C but
only within an abbreviated temperature range, such as 0/50 C. It is not named
specifically in the current spec. But the spec refers to special tolerances recommended
to be a fraction of class B and requires that the manufacturer state the temperature
range to which stated tolerance applies.
One more method though exists, that delivers the highest accuracy, which we offer as
the option "AC", standing for "High Accuracy" option.
It works by leaving behind class designations, which apply to the general groups of RTD
sensors, and focuses only on an individual sensor on its own (not it being part of a
bigger group).
By performing an intensive calibration with high-end metrology level equipment on an
individual probe, its unique curve can be found, and be characterized by a few
"constants" that belong to that probe only. Yes, every RTD has a unique personality all
its own!
The IEC spec we started out with gives the three "constants" that all industrial RTDs
must fall within for the tolerance classes to work. You don't need to know them because
they are programmed into every transmitter, indicator, meter, etc., to provide the plugand-play interchangeability we've discussed. You can trust the temperature reading
your getting because the device and the probe have everything under the hood - no
gyrations on your part.
Let's illustrate using cars on a super highway. All industrial class B RTDs will be on the
correct 8-lane highway, switching lanes seamlessly all along the way. You can't know
which lane, just that it's on the right highway at all times. The tighter classes travel this
same highway, but shrink the route to stay within inner 7 lanes or 5, according to the
various classes, because the standard constants keep them on track. Again, you don't
know which lanes, but that it will stay in the inner 7 or 5 that apply to its class.


But if you do the high accuracy calibration to find the individual curve of that particular
RTD, your transmitter, indicator, etc., can track where your sensor is on the super
highway down the very lane its in, during the entire ride!
How? By finding the constants which only that RTD possesses, and plugging them into
your transmitter, etc., your transmitter now will tell you right where your RTD is at any
given moment. Many devices have the option to replace the default constants with the
constants of your RTD in their setup utility. With the AC option, we provide you with a
certificate giving the constants, along with the reference temperatures and measured
resistances. We can even provide an in-head transmitter with your probe's individual
constants already plugged into it! And this high accuracy holds good from all the way
down to the liquid nitrogen temperature of -195.798C all the way up to the freezing
point of zinc - 419.527C! It doesn't get any better than that!
If you need the best accuracy available from your RTD, this is the way to go.