EN
WIKA operating instructions, model CTR3000
67
14191761.01 05/2019 EN/DE
7. Technical information about temperature
7.3 Measurement
7.3.1 Thermocouple
7.3.1.1 Introduction
Very broadly the thermoelectric effect occurs when an electrical circuit consisting of dissimilar metal conductors is subjected
to a temperature gradient. An electric potential or voltage is developed along the conductors. This voltage potential varies
proportionally with temperature and provides a means by which to measure temperature.
There are two categories of thermocouple:
■
Rare metal, Platinum based types
Rare metal, platinum types are mostly used for high-temperature precision thermometry. Maximum temperatures of
1,700 °C [3,092 °F / 1,973.15 K] and measurement uncertainties of up to 0.4 °C are possible. The sensitivity of platinum-
based thermocouples is usually in the region of 10 µV / °C, which means that high-accuracy, high resolution measurements
require sensitive instruments such as the CTR3000.
■
Base metal, Nickel based
Base metal thermocouples operate over a wide temperature range with high-temperature types designed for use
up to 1,600 °C [2,912 °F / 1,873.15 K] . Temperatures above 2,300 °C [4,172 °F / 2,573.15 K] are possible with new
high-temperature tungsten rhenium types. Typical sensitivity figures of > 30 µV / °C characterise most of the base metal
thermocouple family.
These are easily affected by contamination effects which results in recalibration and drift. This is especially pronounced
at high temperatures where drift figures of the order of 10 °C [50 °F / 283.15 K] are possible. It is important to be aware of
the particular contamination effects and to select the correct thermocouple for the measurement environment. The type
N thermocouple offers the best performance in terms of reproducibility and measurement uncertainty, operating up to
1,250 °C [2,282 °F / 1,523.15 K]. It is the best choice for most general measurement applications, calling for accuracy with
low time and temperature drift.
7.3.1.2 Connection
Thermocouples measure temperature difference. As all practical thermocouples consist of at least 2 junctions, it is important
when performing absolute temperature measurement that one of the junctions is referenced to a known temperature.
The reference junction and voltage measurement precision significantly influence the overall temperature measurement
accuracy. Intermediate connection junctions such as connectors and extension cables between the measurement
thermocouple and the CTR3000 also influence the measurement result.
7.3.2 Resistance thermometer
The CTR3000 will operate with a range of 3- and 4-wire 25/100 Ω PRTs. The best performance will be achieved only
where good quality PRTs are used from reputable, proven sources. As with any measured parameter, the performance of a
measurement system depends upon its stability and repeatability. Low quality PRTs are likely to reduce system performance.
The relationship between temperature and resistance depends on several factors, including the alpha value and the PRT
calibration. Consequently more than one equation is required for resistance to temperature conversion. Calibration data for the
PRTs takes the form of Callendar-van-Dusen coefficients.
WIKA provides a range of proven PRTs of the series CTP5000 especially for use with the CTR3000, as well as offering a
service to provide customised PRTs to meet individual customers’ requirements.
High “alpha” PRTs:
The best possible system accuracy is achieved using high “alpha” (
α
) PRTs, or more correctly, PRTs using high
α
(high purity)
platinum wire.
Low “alpha” PRTs:
Low
α
PRTs contain a higher level of impurities in the platinum resistance wire used. This affects the resistance value at
a given temperature (the temperature coefficient). As impurities already exist in the platinum resistance wire, additional
contamination has a reduced effect and hence low
α
PRTs are more immune to contamination and are therefore better
for industrial applications. To ensure a robust PRT, the detector within the PRT is contained within materials, which can
