User Manual
TRAQC-3 LL
TRAQC-3 LL 12/2018 EN Rev.5
page : 6
2.
Important knowledge
The factors which characterize a dry-block oven are:
Accuracy of the display compared to a standard
The stability over time
The axial homogeneity of the block
The boring to boring homogeneity
The European Association of National Metrology Institutes, Euromet CG-13 Guideline
describes this in detail. This paragraph will briefly discuss the parameters above. Please
note that the importance and pit-falls with these parameters are often overlooked. More
important is to specify these parameters correctly in order to know the measurement
uncertainty.
2.1
Accuracy of the display compared to a standard
Usually a dry-well block oven is used in combination with a standard temperature
measuring instrument. A Unit under test (UUT) is in that instance calibrated against the
said standard. Important then is to have a dry well which can hold its temperature very
stable.
2.2
Stability
Is defined as temperature variation as function of time. The Euromet CG-13 guideline
uses a 30 minute interval. This makes clear that a good stability over a long period of
time is actually a good stability.
2.3
The axial homogeneity
Is probably the most important (and forgotten) parameter of a dry-well block calibrator.
If one uses a probe with 5 mm length and measure at intervals the temperature while
pulling out this probe, one can determine the axial gradient. Due to its geometry,
isolation characteristics, the heat loss of the probe etc. the heating bloc is not
homogenise in temperature. Differences in the order of magnitude of degrees of Celsius
are often found. This parameter is very important because usually it is not known how
long a PT100 in a probe is, or at what position, in the probe (or UUT) the PT100 or
Thermocouple is placed. The Euromet CG-13 Guideline mentions a 4 cm homogenise
zone as a kind of a norm.
2.4
The boring to boring homogeneity
Most dry-well block calibrators have an insert with more than one boring. If there is one
boring this boring is usually in the centre. An ideal symmetric situation with internal
heaters elements is then automatically provided. With two borings a symmetrical position
of both borings can be reached if the position is of each boring is at equal distance to the
heating element. A mark of the insert should then be provided. This mark should be
placed in line with the mark on the block. Again this is often overlooked but an important
factor in determining the uncertainty of a calibration. Once one is aware of this issue one
can easily take appropriate counter measures.
