Technical information

How to improve the total uncertainty of a calibration with atemperature dry-well calibrator (industrial applications)
WIKA data sheet IN 00.32

For some calibration teams, the uncertainty of atemperature dry-well calibrator, indicated in the data
sheet, is not enough. The available solutions to improvethese values are explained in this document.
Calibration is essential to establish and maintain theaccuracy of any thermometer. It can be used to provide
traceability to national standards and compliance withquality assurance systems such as ISO 9000. Comparative
calibration is achieved by immersing thermometers in astable temperature environment together with a reference
thermometer.
The choice of calibration instrument depends, alongside
the temperatures, on the type of thermometer used in theprocess. For probes with equal and common geometry, atemperature dry-well calibrator is the ideal solution. In thesecases, the bores of the insert can be optimally adapted(minimum immersion depth: 70 mm [2.75 in]) and themeasurement uncertainties reduced.
For an accurate calibration, the thermal coupling of thetemperature probe to the dry well and insert is crucial.
With too large a bore diameter, the air gap between thebore wall and the probe diminishes the heat transfer.
Longer settling times and measured errors are the result. A
maximum clearance of 0.5 mm [0.02 in] is considered to be
a compromise between still-acceptable measured error andthe risk of the probe becoming jammed.
Since all temperature dry-well calibrators are closed at thebottom and open at the top, this inevitably results in an axial
temperature gradient in the dry well and insert. This leads tomeasured errors if the test item is not seated on the bottomof the sleeve.
As gradients over the first 40 mm [12.58 in] above thebottom provide the largest contribution to the measurement
uncertainty, these are therefore also specified in the datasheets.

CTD9350 as an application with an external reference
thermometer

If the measuring location of the test item is outside this zone,
the calibration is further distorted by an “axial inhomogeneityerror”.
If test items cannot be inserted down to the bottom of thesleeve, you should use an external reference thermometer.
Then, the reference and test item can be aligned to thesame temperature gradient. The inhomogeneity error is thus
largely compensated for and the measurement uncertainty
significantly reduced.

WIKA data sheet IN 00.32 ∙ 12/2022

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Further technical information:
Fixed-point calibration per ITS-90; see IN 00.38

But not only can this issue guide the customer to use anexternal reference. The most important topic is accuracy.
The deviation of WIKA temperature dry-well calibrators
depends on the model and temperature range used. To
trust the value on the display and to trust the accuracy, thecalibrator needs a traceable certificate. If a new calibrator
is calibrated and configured in WIKA's DAkkS temperaturelaboratory, WIKA can reduce the deviation to ZERO and thetotal uncertainty is only the measurement uncertainty of thelaboratory.
Due to different parts of the measurement uncertainty budget
the measurement uncertainty of accredited laboratories isnearly the same.
Total uncertainty of dry-well calibrators
1.20

The deviation of such precision thermometers is defined in
two steps:
Deviation of the electrical measuring instrument itself+ deviation of the temperature probe = deviation of themeasuring chain
Therefore the uncertainty of the laboratory has to be added
to the deviation of the measuring chain in order to calculate
the measurement uncertainty, for example:
ModelΔΔProbe

UlabTotal U

CTR3000

0.005 K

0.01 K

0.01 K

0.025 K

CTH7000

0.015 K

0.01 K

0.01 K

0.035 KBest case: ΔProbe = 0 K
Worst case: ΔProbe = UlabTo achieve the best possible performance of the precision
thermometers, the coefficients/characterisation of thetemperature probe have to be calculated and stored in
the measuring instrument channel being used (or if using
SMART probes into the probe connector).

1.00
0.80
0.60
0.40
0.20
0.00
-200

0+200+400+600+800

If for some applications the measurement uncertainty of> 0.2 K is not good enough, WIKA are able to provide arange of appropriate equipment: e.g. a temperature dry-wellcalibrator in combination with a precision thermometer andtemperature probe.
WIKA's precision thermometers offer maximum performanceand measure resistance ratios against an internal
high-stability reference resistor.
Comparative calibrations of platinum resistancethermometers (PRTs) typically involve measuring theresistance of the unknown thermometer after first
determining the dry-well temperature with a reference
thermometer. Both measurements are referenced to thesame internal precision reference resistor. With the “directcomparison” technique, the reference thermometer is used inplace of the reference resistor and the ratio of the unknown
probe resistance to that of the reference thermometer ismeasured directly.

WIKA data sheet IN 00.32 ∙ 12/2022

Precision thermometer model CTR3000 withmultiplexer model CTS3000

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WIKA recommends using an external reference incombination with a temperature dry-well calibrator up to atemperature of 500 °C [932 °F]. The reasons for this are as
follows:■ Different shapes of instruments to be tested can becalibrated.
■ Accuracy can be improved up to 95 %.
■ Flexible use for other applications.
■ The calibration is performed with the reference
thermometer, the temperature dry-well calibrator does not
require a calibration.

Outlook
To achieve a better result for the deviation of the temperatureprobe, we recommend calibrating the precision thermometerusing the fixed point method. The freezing, melting or triple
points of specific pure materials are used to define the fixedreference temperatures that are used in ITS-90 (InternationalTemperature Scale of 1990). This improves the measurement
uncertainties of laboratories to approx. 1 mK.
→ For information on the fixed-point calibration in accordancewith ITS-90, see technical information IN 00.38 at www.
wika.com.

Temperature dry-well calibrator model CTD9100 with
precision thermometer model CTR3000

12/2022 EN© 11/2014 WIKA Alexander Wiegand SE & Co. KG, all rights reserved.
The specifications given in this document represent the state of engineering at the time of publishing.
We reserve the right to make modifications to the specifications and materials.

WIKA data sheet IN 00.32 ∙ 12/2022

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WIKA Alexander Wiegand SE & Co. KG
Alexander-Wiegand-Straße 30
63911 Klingenberg/GermanyTel.
+49 9372 132-0
info@wika.de
www.wika.de