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WIKA operating instructions, model CTR3000
14191761.01 05/2019 EN/DE
4. Design and function
4. Design and function
The CTR3000 is a high-accuracy instrument designed for laboratory and industrial temperature measurement and calibration
applications.
Features include:
■
Unique dual capability for both thermocouple and resistance thermometer measurements
■
Number of input channel can be expanded from four to forty-four channels
■
Large graphic touchscreen for temperature measurement values as well as configuration settings and statistical results
■
Advanced functions include differential measurement, programmable scanning routines, programmable timer, data logging,
statistical reporting
■
USB and Ethernet (RS-232 as an option) communication interfaces available for automated monitoring and calibration
applications
The CTR3000 will operate with all 3 and 4-wire Pt100 (100 Ω) platinum resistance thermometers as well as most standard
international thermocouple types. Temperature measurement units are selectable by front panel operation; °C, °F, K. Base
measurement units mV, Ω are also displayed.
Resistance accuracy is better than ±2 mΩ equivalent to temperature measurement precision of ±5 mK for Pt100
thermometers.
Standard miniature sockets allow convenient connection for thermocouple inputs. Connection sockets incorporate integral
temperature compensation sensors making high-accuracy thermocouple measurement possible without the use of an external
reference junction.
4.1 Principles of measurement
4.1.1 PRT measurement
The CTR3000 measures the voltage (Vt) developed across the unknown sensor resistance (Rt) and the voltage (Vs) across a
stable internal reference resistance (Rs) connected in series and passing the same current. The voltages are in proportion to
the resistances so the thermometer resistance is derived from: Rt = Rs x Vt / Vs. This technique achieves immunity from slow
moving time and temperature drift in the electronics as it is not affected by voltage measurement gain variations or current
source fluctuations.
In the same way that AC resistance measurement eliminates thermal EMFs, switched DC achieves a similar advantage.
Switched DC works by reversing the current flow on alternate measurement cycles and taking the average value, thereby
cancelling any thermal EMF offsets from the measurement.
For PRTs, the relationship between resistance and temperature varies slightly from one PRT to another. Therefore, no matter
how accurately the CTR3000 measures the PRT resistance, if the relationship between resistance and temperature for a
particular PRT is not known, accurate temperature measurement is not possible.
The CTR3000 uses PRT calibration data to overcome this problem and calculates temperature from temperature conversion
functions stored in internal memory. This method enables the CTR3000 accurately to convert resistance to temperature,
uniquely for each PRT used. It is very important therefore that a PRT is used on the correct and properly configured input
channel.
4.1.2 Thermocouple measurement
As well as the PRT resistance measurement facility the CTR3000 also functions as a precision millivolt-meter. Designed
for high-accuracy measurement over the EMF voltage range of all standard base and precious metal thermocouples, the
CTR3000 achieves a basic voltage accuracy of better than ±0,004 % + 2 μV over the full measurement range.
Thermocouple EMFs are converted to temperature using the EN 60584 linearisation functions.
The voltage input connection is specially designed to minimise the thermal gradient between the terminals. This is particularly
important when the internal reference junction compensation is used, as any temperature difference at the connection junction
will influence the measurement result.
