HND-T205
page 16
HND-T205 K05/0422
4-Wire-Measuring
When using resistance thermometers as the Pt100 a quite large measuring error
can be caused by inadequate cables and connections. Using 4wire measuring
avoids these kinds of errors mainly caused by unwanted resistances. It is
suggested to use suitable probes and extensions only. (For pin assignment
please refer to chapter
7.7 Probe connection
)
Heat loss caused by probe construction:
Especially when measuring temperatures which deviate very much from the
ambient temperature, measuring errors often occur if the heat loss caused by the
probe is not considered. When measuring fluids therefore the probe should be
emerged sufficiently deep and be stirred continuously. When measuring gases,
the probe should also emerge as deep as possible in the gas to be measured
(e.g. when measuring in channel/pipes) and the gas should flow around the probe
at sufficient flow.
Measuring Surface Temperature
If temperature of the surface of an object has to be measured, one should pay
attention especially when measuring hot (or very cold) surfaces, that the ambient
air cools (or heats) the surface. Additionally, the object will be cooled (or heated)
by the probe or the probe can have a better heat flow to the ambient temperature
as to the objects surface.
Therefore, specially designed surface probes should be used. The measuring
precision depends mainly on the construction of the probe and of the physics of
the surface itself. If selecting a probe try to choose one with low mass and heat
flow from sensor to handle. Thermally conductive paste can increase the
precision in some cases.
Allowable temperature Range Of Probes
Pt100 Sensors are defined over a wide temperature range. Depending on probe
materials and sort of sensor (e.g. hybrid sensors, wire wound resistors...) the
allowable temperature ranges have to be considered. Exceeding the ranges at
least causes a wrong measuring, it may even damage the probe permanently!
Often it also has to be considered, that the temperature range is just valid for the
probe tube, (plastic-) handles can’t stand the same high temperatures. Therefore,
the tube length should be selected long enough, that temperature keeps low at
the handle.
Self Heating
The measuring current of the instrument is just 0.3 mA. Because of this
comparably low current practically now self heating effect has to be considered,
even at air with low movement the self heating is <= 0.01 °C.
Cooling by Evaporation
When measuring air temperature, the probe has to be dry. Otherwise, the cold
due to the evaporation causes too low measurings.
