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Fixed-Point Thermocouple
Measuring System
The
central part of this industrial device system consists of a
thermocouple with an additional, integrated fixed-point cell and a
miniature heating element. The fixed-point cell contains a specific
high-purity substance. When this substance exceeds its melting or
freezing point, it generates a reproducible holding signal by
consuming or producing the latent phase change transition energy,
with this signal being used for calibration purposes (Figure
1).
>
Fig. 1: Signal of a fixed-point thermocouple measured during melting and freezing of
the integrated fixed-point substance (aluminium-silicon alloy)
If the fixed-point temperature lies slightly above a constant operating temperature, any increase in the
process temperature or any activation of the integrated miniature heater integrated into the sensor causes the melting of the fixed-point substance. At the same time, a recalibration of the sensor is
performed.
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Process temperature
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Fixed-point material
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Fixed-point
temperature
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Recommended
upper operating temperature
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130-150°C
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In, pure metal
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156.6°C
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260°C
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180-225°C
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Sn, pure metal
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231.9°C
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350°C
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390-410°C
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Zn, pure metal
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419.5°C
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450°C
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530-540°C
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Al-Cu, eutectic
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548.2°C
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650°C
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545-565°C
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Al-Si, eutectic
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578.7°C
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680°C
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610-630°C
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Al-In, monotectic
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638.4°C
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680°C
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630-650°C
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Al, pure metal
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660.3°C
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760°C
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740-760°C
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AgCu, eutectic
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779.6°C
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800°C
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>
The table opposite shows some of the metals tested so far. Furthermore, other materials presenting
specific calibration temperatures are available.
Each fixed-point sensor is equipped with a specially developed precision transmitter and is connected to a PC whose software controls the process of measuring data acquisition, heating cycle triggering, signal analysis and recalibration (self-calibrating system). The PC controls up to 8 measuring modules and is able to communicate via different plug-in cards with superordinate process control systems.
The most important advantage of such an in-situ calibration is that any systematic long-term errors of the whole thermocouple measuring circuit can be recognised. Thus, for temperatures close to the respective fixed point, measuring uncertainties of
<1 K are achievable.
Application possibilities can be found in all industrial fields where a higher accuracy is required over a longer time period, with the advantages offered by the robust thermocouple measuring technology being exploited at the same time.
Description of the system components
The basic equipment of a single gauging station includes
(Figure 2):
• Fixed-point thermocouple (SKTE-F)
• Precision transmitter (SKTE2-S)
• SKTE2-M – a LINUX based Master-PC with different communication interfaces (RS232/RS485, ProfibusDP, ModbusRTU, Ethernet)
The hard- and software equipment can be extended to manage up to 8 measuring points per PC.
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Fig. 2: Hardware overview (single station)
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Bild 3: Fixed-point thermocouple SKTE-F
Combined fixed-point thermocouple
This robust construction contains a thermocouple TC1 with integrated fixed-point cell, a miniature heating element, and an auxiliary thermocouple TC2. In the case of self-calibration, an excess temperature of up to 100K develops at the sensor tip (TC1). The auxiliary thermocouple records the temperature above the fixed-point cell, thus serving as substitutional measuring element.
Precision transmitter SKTE2-S
This device serves for the process-coupled acquisition of the measuring data supplied by a thermocouple of the type SKTE-F and for their transmission to a master PC. Furthermore, it is used to drive the sensor-integrated miniature heating element.
Due to the separate housing of the power supply unit and the precision measuring electronic system, a high measuring stability is guarantied. The aluminium die casting casings used permit a design up to a degree of protection of
IP65.
Technical data of SKTE2-S
*)
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General:
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Power supply:
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230V AC/0.12A
or 24V AC/DC 1A
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Dimensions (L x W x H) in mm :
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220 x 310
x 90
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Operating temperature:
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-5 ... 60 (80)°C,
80°C with limited accuracy
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Degree of case ingress protection:
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IP 65
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Weight:
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appr. 4.5 KG
kg
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Thermocouple measuring channels:
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Range:
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-4.5mV...39.062mV
(best for TC-type N)
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Measuring time:
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0,4s
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Resolution:
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18 Bit
(0.15µV, corresponding to 4mK at 600°C/type N)
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Accuracy:
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2µV
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max. linearity error:
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2µV
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max. temperature drift in the range 0...60°C:
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0,1µV/°C
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Cold end junction measuring channel:
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Internal sensing element:
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Pt100, class A, acc. to
DIN EN 60751
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Resolution:
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18 Bit
(0,001 ,,
corresponding to 3mK at 25°C/Pt100)
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Range::
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-5 ...
80°C
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Measuring time:
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0,2 s
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Accuracy (at
Tamb = 0...60°C):
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0.2 K
(after warming-up time)
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Warming-up time at room temperature:
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30 min
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Warming-up time after ambient temperature change of 30K:
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3h
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Heating element
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Pulse width resolution
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10 Bit
(0...1023)
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Pulse interval
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120 ms
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Pulse amplitude
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14 V (18 V on demand)
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max. current
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0,8 A
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Interface
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Seriell
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RS232 or RS485 (adjustable)
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*) technische Änderungen sind vorbehalten
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Description of the system components
