H49.0.02.6C-02
Operating Manual MH 3750
Page 5
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3 Some Basics Of Precision Temperature Measuring
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Probe Precision/Device Precision
The device is very precise (please refer to technical data). To be able to use this high precision, the connected temperature
probe has to be as precise as possible, too. The following precision classes are available as a standard at reasonable prices
(Platinum resistor thermometers according to EN60751):
Class
Error ranges
B
± (0,3 + 0,005 • |temperature|)
1/3 B (=1/3 DIN)
± (0,1 + 0,0017 |temperature|)
1/10 B (=1/10 DIN)
± (0,03 + 0,0005 • |temperature|
)
A
± (0,15 + 0,002 • |temperature|)
error of device and probe
0
1
2
3
4
-200
0
200
400
600
800
T [°C]
error [°C]
B
1/3B
1/10B
A
GMH37xx
error of device and probe
0
0,2
0,4
0,6
0,8
1
1,2
-50
0
50
100
150
T [°C]
error [°C]
B
1/3B
1/10B
A
GMH37xx
Error over measuring range Pt100
Error over range –50...150°C Pt100
For applications demanding higher precision than given by this classes we suggest to adjust the device to the used probe or to
get a calibration certificate for the device combined with the probe.
Attention:
if an adjusted or calibrated probe is replaced, also the adjustment or calibration certificate has to be renewed to
maintain the referring overall precision!
Be careful when buying third party temperature probes: Besides the standard EN60751 there are some other obsolete or
unusual standards on the market. If such a probe has to be connected, the user sensor curve (have a look to the referring
chapter) can be used to adjust the instrument!
•
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 this kinds of errors mainly caused by unwanted resistances. It is suggested to use
suitable probes and extensions only.
•
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 he 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 0.3mA. 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.
SIKA, Dr. Siebert & Kühn GmbH & Co. KG, Struthweg 7-9, 34260 Kaufungen, Germany
Tel: +49-5605-803-0, Fax: +49-5605-803-54, email: [email protected], web: www.SIKA.net