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SKTE 2
Fixed-Point Thermocouple
 
SKTE2_neu_klein.jpg

 

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).

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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


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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 *)

 

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