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.
Process temperature
|
Fixed-point material
|
Fixed-point
temperature
|
Recommended
upper operating temperature
|
130-150°C
|
In, pure metal
|
156.6°C
|
260°C
|
180-225°C
|
Sn, pure metal
|
231.9°C
|
350°C
|
390-410°C
|
Zn, pure metal
|
419.5°C
|
450°C
|
530-540°C
|
Al-Cu, eutectic
|
548.2°C
|
650°C
|
545-565°C
|
Al-Si, eutectic
|
578.7°C
|
680°C
|
610-630°C
|
Al-In, monotectic
|
638.4°C
|
680°C
|
630-650°C
|
Al, pure metal
|
660.3°C
|
760°C
|
740-760°C
|
AgCu, eutectic
|
779.6°C
|
800°C
|
>
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.
>
Fig. 2: Hardware overview (single station)
>
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
*)
General:
|
|
Power supply:
|
230V AC/0.12A
or 24V AC/DC 1A
|
Dimensions (L x W x H) in mm :
|
220 x 310
x 90
|
Operating temperature:
|
-5 ... 60 (80)°C,
80°C with limited accuracy
|
Degree of case ingress protection:
|
IP 65
|
Weight:
|
appr. 4.5 KG
kg
|
Thermocouple measuring channels:
|
|
Range:
|
-4.5mV...39.062mV
(best for TC-type N)
|
Measuring time:
|
0,4s
|
Resolution:
|
18 Bit
(0.15µV, corresponding to 4mK at 600°C/type N)
|
Accuracy:
|
2µV
|
max. linearity error:
|
2µV
|
max. temperature drift in the range 0...60°C:
|
0,1µV/°C
|
Cold end junction measuring channel:
|
|
Internal sensing element:
|
Pt100, class A, acc. to
DIN EN 60751
|
Resolution:
|
18 Bit
(0,001 ,,
corresponding to 3mK at 25°C/Pt100)
|
Range::
|
-5 ...
80°C
|
Measuring time:
|
0,2 s
|
Accuracy (at
Tamb = 0...60°C):
|
0.2 K
(after warming-up time)
|
Warming-up time at room temperature:
|
30 min
|
Warming-up time after ambient temperature change of 30K:
|
3h
|
Heating element
|
|
Pulse width resolution
|
10 Bit
(0...1023)
|
Pulse interval
|
120 ms
|
Pulse amplitude
|
14 V (18 V on demand)
|
max. current
|
0,8 A
|
Interface
|
|
Seriell
|
RS232 or RS485 (adjustable)
|
*) technische Änderungen sind vorbehalten
|