Presys T-25N, Техническое руководство

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PRESYS

Instruments T-25N
Page 15
3 - Recommendations as regards Accuracy of Measurements
PRESYS
dry block temperature calibrators are instruments of high accuracy level,
requiring the observation of all the procedures described in this section, in order to
achieve the necessary conditions to get the accuracy levels during the calibrations.
 Special attention should be paid in relation to the insert cleanliness. When necessary,
it should always be washed with water and soap, well rinsed and dried. Oil, grease,
solid particles can hinder the heat transference to the insert and even jam the insert
inside the block.
 The sensor to be calibrated must fit snugly into the appropriate well. In case the
sensor is loose, the measurement accuracy meaning can be completely senseless.
The meaning of clearance between the sensor and the respective well should be
understood in a subjective way and the common sense is very important. Thus, the
sensor should enter the insert well (both completely clean) in such a way to stay
snugly enough so that it can not move or swing inside but it should not enter by force
to get jammed.
 In the specific case of generating negative temperature with the T-25N model, the
calibrations should be performed in a descending temperature order. This procedure
is useful because in negative temperatures ice is formed in the insert surface and
between the insert and the thermo-element being calibrated. This humidity change the
thermal contact and cause error in the calibration. After finishing the use of the insert
below 0  C, increase the setpoint to a positive temperature value, withdraw the insert
from the block and the sensor, dry completely all these parts before continuing the
calibration. This procedure guarantees an accuracy next to  0.1  C. In case you can
accept higher values of accuracy, like  0.2  C or more, these cares can be left apart.
3.1 - Getting a Better Accuracy from the Dry Block
The temperature control is based in the temperature measurement of an internal
sensor placed in the block.
This control sensor is adjusted in factory by means of another sensor with high
accuracy (probe) connected to a superthermometer, as described in item
6.2 - Probe
Calibration
. So, at factory, the accuracy of the superthermometer is transferred to the dry
block calibrator. The transference will be well performed only when there is a perfect
temperature equilibrium between the internal control sensor and the superthermometer
probe. Therefore, it should not have any clearance between the insert and the block or
between the probe and the insert. Both the internal sensor and the probe should also be
at the same depth.
The user will get the best accuracy from the dry block, provided in this technical
manual, in case one succeeds in reproducing the same conditions of the factory
adjustment, that is, the same insert used in factory, no clearance, same depth etc.
Summing up, the important thing to get the best accuracy from the dry block is to
reproduce the process used by the factory to adjust the dry block itself.
When a higher accuracy than that mentioned in the technical manual is
necessary, one should use an external superthermometer as a reference or standard to
compare with the thermo-element under calibration.