enraf 862 MIT - 1 Introduction; 1.1 Product information; 1.2 Operating principle; 1.3 Measuring sequence

Introduction
Instruction manual 862 MIT Page 5
1 Introduction
1.1 Product information
The 862 MIT (Multi element Interface Thermo sensor converter) is used for interfacing an 864 Multi Thermo
sensor Thermometer (MTT). This unit is basically a solid state electronic element selector, containing all
necessary hardware for selecting and measuring an MTT with up to 16 spot temperature elements.
The 862 MIT must always be used in combination with an 854 level gauge, an 873 SmartRadar or an 877 Field
Display & Interface. These instruments must be equipped with an MPU, HPU or OPU option board to interface
the 862 MIT.
1.2 Operating principle
The 864 MTT contains of 16 spot elements (thermocouples) equally divided over the measuring range.
The lowest element acts as the cold junction and its actual temperature is accurately measured by a Pt100
(reference resistor).
The spots which are submerged in the product are used for the average product temperature calculation; the
remaining spots are used for the average gas temperature calculation.
The optional board, located in the 854 level gauge, 873 SmartRadar or 877 FDI, calculates the average product
temperature and the average gas temperature.
When the instrument is provided with a display the temperature can be displayed. Remote readout of the
measured temperature is possible via the 977 Tank Side Indicator (TSI), the 878 panel indicator (CPI) or via tank
inventory systems such as Entis Pro.
1.3 Measuring sequence
The measuring cycle of the 862 MIT takes approximately 2 minutes and 26 seconds. In that time the following
data is measured.
Cycle Measurement Cycle Measurement Cycle Measurement
1 R 5 R 23 U
ref TC0 TC0'
2 R 6 R 24 ÷ 38 U ÷ U
test TC0' TC15 TC1
3 R 7 ÷ 21 R ÷ R 39 U
Pt100 TC15 TC1 test
4 R 22 U 40 U
gnd TC0 ref
Each resistance measurement is measured with 1 mA current and without current. The measurement without
current is an offset measurement.
In measurement cycle 1 a conversion factor is determined: Hz/ohm. With this conversion factor all other
resistance measurements are calculated. This conversion factor is determined with every measurement
(2 min 26 sec). In this way an auto calibration of the 862 MIT is obtained.