The Temperature Measurement System

A temperature measurement system consists of the temperature sensor and the evaluation electronics. The sensor is usualy a temperature-dependent resistor or thermocouple and is normally fitted in a protection tube which is suitable for heat transfer. The two components of the measurement system is connected with cable. The sensor and evaluation electronics are often combined in a temperature probe (terminal head). The evaluation electronics can be either a transmitter (standardized output signal) or a direct-reading electronic indicating instrument (digital indication of the temperature). The temperature measurement system with thermocouple is shown on the Picture 1. On this picture, as example is shown "J" thermocouple (Iron-Constantan). This thermocouple has range from -336 to +2193°F, or from -204 to +1200°C, or from 68 to 1473 K (this values can vary). Another types of thermocouples are "K" (Chromel-Alumel), 'T' (Copper- Constantan), "E" (Chromel-Constantan), "R" (Pt-Pt-13%-Rh), "S" (Pt-Pt-10%Rh), "B" (Pt-13%-Rh-Pt-6%Rh), etc.

Picture 1: The temperature measurement system with thermocouple


A thermocouple is an element which always measures the temperature difference between the measurement point and the reference junction. The reference junction terminal, at the transition from thermocouple to copper cable, has the temperature T2. This temperature must be known in order to be able to measure the temperature T1. The connection between the thermocouple and the evaluation electronics is provided with a compensating cable. This cable has the same thermoelectric properties as the thermocouple itself, so, this way it prevents an additional thermoelectric emf arising at the junction. However, it is essential that the appropriate compensating cable is always used,and also plugs and sockets free from thermoelectric emf must be used.

Error components in Temperature Measurement System

In principle, when calculating the overall measurement uncertainty of a temperature measurement system, all the individual components, from the evaluation electronics, indication, cable, sensor, must be included.
The sensor provides a temperature-dependent signal that is fed via the cable to the evaluation electronics, where it is ultimately converted to a temperature indication or to a current signal (for example, 0 - 20 mA). The error sources of all three components have to be included in the consideration of measurement uncertainty. This means that additional measurement uncertainty components are introduced once again, when transmitters are used during the conversion into a temperature indication. So, every component from the temperature measurement system generates error, and in order to provide precise formula for the measured temperature every single error component must be included in this formula. The precise formula for the output signal of the temperature measurement system will look like this:

Ix = Tm + σMF** + σMD + σMA + σME** + σMTh** + σMRI** - CS x σt0S* + σVLX* + σRAL** + σV + σtM + σtW + σB + σLZ

Here, the Ix is the output signal from the measurement system, actually this is the indicated temperature or the temperature equivalent (for example, the current signal). The Tm is temperature at the measurement point. The rest of the components included in the formula depends on the system used for measurement. The components that are marked with * are included only when thermocouples are used, and the components marked with ** are included only when resistance thermometers are used.


Measurement uncertainties at the sensor and in the cables:

>> σMF - measurement signal deviation caused by the heat conduction error of the thermometer;
>> σMD - measurement signal deviation caused by the deviation of the sensor as per EN 60 751 (resistance thermometer) or as per EN 60 584 (thermocouple);
>> σMA - measurement signal deviation caused by insufficient stabilization;
>> σME - measurement signal deviation caused by the self-heating of the sensor (only with resistance thermometer);
>> σMTh - measurement signal deviation caused by thermoelectric emfs (resistance thermometer);
>> σMRI - measurement signal deviation caused by inadequate insulation resistance (resistance thermometer);
>> CS - sensitivity of the sensor, e.g. in °C/µV (thermocouple);
>> σt0S - temperature deviation caused by the reference temperature deviating from the assumed value;
>> σVLX - voltage deviation caused by the compensating cables (thermocouples);
>> σRAL - variation in the lead resistance (resistance thermometer);

Measurement uncertainties at the transmitter and indication:

>> σV - deviation of the evaluation electronics caused by supply variations (effect of voltage);
>> σtM - deviation caused by fluctuating ambient temperature (effect of temperature);
>> σtW - processing and linearization errors in the evaluation electronics;
>> σB - indication deviation caused by the influence of the input resistance (effect of burden);
>> σLZ - long-term stability of the evaluation electronics (deviation caused by component ageing);