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Drift Mechanisms for Resistance Thermometers

In document ABB Temperature Handbook (Page 143-148)

3.5 Aging Mechanisms in Temperature Sensors

3.5.2 Drift Mechanisms for Resistance Thermometers

The effect of impurities on the temperature coefficient of Pt-resistor materials As already mentioned, the purity of the mandatory alloy compositions is an essential requirement for the thermal materials. The Platinum resistance wire for the manufac-ture of Platinum measuring resistors is no exception.

A rough differentiation can be made between application categories:

• For the manufacture of temperature sensors, in order for the requirements in ITS 90 to be satisfied, Platinum with pure quality is required. Temperature sensors of this type are used as definition and interpolation instruments for determining the Inter-national Temperature Scale between the fixed points in the temperature range from -189 °C (-308.2 °F) (N2-Point) to 961 °C (1761.8 °F) (Ag-Point).

• For resistance thermometers, as they are defined in EN 60751, physically pure Plat-inum is used, which, as a result of the addition of specific elements to the alloy, are

“set“ to the required temperature coefficient α.

For its temperature coefficient (which corresponds to the linearized temperature dependence of the material in the temperature range between 0...100 °C (32...212 °F)) the value 3.8506 x 10-3 K-1 can be calculated from the basic values in EN 60751.

Impurities, which may contaminate the Platinum during manufacture or during the operating period of the temperature sensor, can change the chemical composition of the material and thereby its temperature coefficient. The result is a deviation from the basic values in the standard. The Platinum resistance wire will be gradually “poisoned“.

The sensor drifts.

A typical problem, which also leads to the poisoning of the Platinum resistance wire, is the absorption of foreign materials from the thermomwell material, or from the sheath materials used for the mineral insulated cables. This absorption process is practically nonexistent or extremely slow at lower temperatures, but it accelerates dramatically at higher temperatures. For this reason, metallic thermomwells made of stainless steel should not be used when long term temperature exposure over approx. 420 °C (788 °F) is anticipated. For long term use above that temperature, thermomwell mate-rials such as quartz glass, high purity ceramic or mineral insulated cables with a Plati-num sheath should be used.

A typical indication that the resistance material is aging, which can be attributed to poisoning, is an increase in the Ro-value, accompanied by a decrease in the α-value.

The following table demonstrates the effects of impurities on the α-value for physically pure Platinum.

Tab. 3-25: Effects of contamination on the temperature coefficient (α) of Platinum

Drift effects due to mechanical stresses in the sensor element during operation Not only changes in the chemical composition of the resistor material due to contami-nation by foreign elements can cause instability in the temperature sensor, but also the presence of mechanical stresses in the sensor element or in the total assembly can lead to changes of the resistance values. Continuous mechanical vibrations, especially when combined with high operating temperatures, affect the temperature sensor sig-nificantly. There are two effects which can be initiated by the stresses described in the following.

In wire wound resistors, which are not solidly positioned in the carrier body for vibration resistant, short circuits between the individual windings can occur causing step change reductions in the Ro-resistance value.

The fine wire in the sensor element can be elongated at the connection point by strong vibration loads causing a reduction in the wire cross section. In an extreme case the fine wire can break off. A comparable effect can occur if the resistance thermometer is exposed to continuous large temperature changes and a temperature change resistant design was not used. In such applications, the sensor element, if the fit is too tight, ex-periences continuous tension and compression forces (alternating stresses) in the con-nection wires due to the different thermal expansions of the materials.

Element dα (ppm-1)

Fe -1.28 x 10-6

Ni -0.16 x 10-6

Ir -0.20 x 10-6

Mn -0.21 x 10-6

Rh -0.09 x 10-6

Cu -0.35 x 10-6

Pd -0.10 x 10-6

Ag -0.15 x 10-6

Au -0.07 x 10-6

Pb -0.90 x 10-6

Cr -3.25 x 10-6

Changes in the connection lead resistance

In resistance thermometers using a 2-wire configuration, the connection lead resis-tance is a direct component of the measured value. To correct the measured resisresis-tance value to its actual temperature dependent value, the connection lead resistance is usually specified so it can utilized by the user to correct the value measured. The con-nection lead resistance can be accounted by the manufacturer by using a resistor with smaller resistance value (negative actual value deviation from reference value).

If during the course of operation of the temperature sensor the resistance of the connection leads change (e.g. due to a cross section reduction of the wires, oxidation at the connection locations, etc.), then the deviation of the measured values appear as a drift, which often goes unnoticed. For resistance thermometers connected in 3- and 4-wire configurations this effect is automatically compensated.

Tab. 3-26: Measurement error due to connection lead resistance

Tab. 3-27: Wire resistance of Cu-mineral insulated cables at room temperature Material

in Ω/m uncompensated compensated

Cu 0.06 2.75 0.48 K 0.3 K

Changes in the insulation resistances

The design of resistance thermometers is essentially comparable to thermocouple designs. Comparable materials are also used. The electric insulation capabilities of the insulation materials can change in the application range of the resistance thermometer for a number of reasons. A change causes parasitic short circuits to be created, which act as resistors in parallel with the actual sensor resistance as shown in the circuit diagram below. Electrically they act as voltage dividers.

Fig. 3-64: Electrical circuit diagram for a real resistance thermometer

The resultant shunt current causes a lower, incorrect measurement signal. The effect of “poor“ insulation resistance increases for higher nominal resistances of the sensor (e.g. Pt1000 Ω). For resistance thermometers, which are to be used at high tem-perature, in certain instances it is better to avoid using resistance thermometers with Ro-resistance values of 25 Ω or 10 Ω.

RL1.2 RL1.1

RL2.2 RL2.1

Rins RS

RWth

RL = Connection lead resistance Rins= Insulation resistance Rs = Sensor resistance

Fig. 3-65: Relative negative measurement error caused by a parallel resistance, due to non-optimal insulation.

At this point it should be stressed, that a regular periodic check of the insulation resis-tance during the operating life of the resisresis-tance thermometer is one of the most impor-tant quality assurance measures which can be conducted. Especially since the measurement of Rins requires minimal expense and can be made under actual instal-lation conditions. The requirements according to EN 60751 relative to the insuinstal-lation resistance limits should, in reality, only be considered as minimal requirements. A decrease in the insulation resistance can also indicate a tear in the insulation, through which not only moisture but also other contaminants could penetrate changing the resistance thermometer curves.

R = 1000 Ohm0

R = 100 Ohm0

R = 10 Ohm0

1.0E+02

1.0E+02 1.0E+04 1.0E+06

1.0E+01 1.0E+00

1.0E+00 1.0E-01

1.0E-02

1.0E-02 1.0E-03 1.0E-04 1.0E-05 1.0E-06

Relative Negative Measurement Error (%)

Parallel Resistance (kOhm)

In document ABB Temperature Handbook (Page 143-148)