Interchangeability based Design of Resistance Temperature Detectors P M V Subbarao Professor Mechanical Engineering Department Connection and Use of Active Temperature Detectors……

Potential Sources of Error with RTDs Resistance thermometer systems are susceptible to three types of errors: The inherent tolerances built into the thermometers, Gradients between the thermometer and the medium to be sensed. Errors introduced along the path between the sensor and readout or control instrument. Some sources of error are electrical; others result from the mechanical construction of the thermometer.

CONFORMITY For example, IEC 751, Class B, requires calibration within 0.3°C at 0°C, but allows TCR to deviate from nominal 0.00385 by ±0.000012 /°C. The tolerance spreads to 0.8°C at 100°C, 1.3°C at 200°C, and on up to 3.8°C at 700°C. Conformity specifies the amount of resistance a thermometer is allowed to deviate from a standard curve. A tolerance at the reference temperature, usually 0°C, and a tolerance on the slope or TCR.

Interchangeability Interchangeability between two thermometers is no more than twice the value of the Conformity. Commercial platinum resistance thermometer elements are available with extremely tight tolerances, to within 0.026°C in some cases. When interchangeability is an overriding consideration, the specified may consider other means to achieve it. For example, manufacturers may alter their calibration procedures to fix the reference temperature and tightest tolerance at a point other than 0°C. Or if the difference between two thermometers is more important than absolute temperature, matched pairs measured to agree within a certain tolerance may is less expensive than calibrating each thermometer within a small range of nominal.

It is important to note that: The Caution!!! It is important to note that: Conformity/interchangeability specifications only denote the relative accuracy of two otherwise identical thermometers mounted side by side in the same environment. They do not include errors acting equally upon both thermometers.

SENSITIVITY The resistance change per degree change in temperature is a function of base resistance and Temperature Coefficient of Resistance. Although a thermometer with higher sensitivity is not necessarily more accurate, a larger signal simplifies output electronics and is less susceptible to lead wire effects and electrical noise. In addition, a larger resistance produces the same voltage output with less measuring current, which helps to limit self heating of the thermometer element.

Uncertainty & Accuracy of a Pt RTD The Callendar Van Dusen equation analytically addresses the tolerance and accuracy of a Pt RTD at any point within its operating temperature range independent of alpha and ice point resistance. Sensor resistance interchangeability as a function of temperature is defined as The Resistance Limit-of-Error function. This can be calculated by taking the differential of the Callendar Van Dusen equation w.r.t. R0, a and d and applying the associated uncertainties. Design engineers are typically interested only in the Limit-of-Error (LOE) function since it characterizes worst case behavior.

The LOE function for resistance for T > 0°C is: Similarly, obtain the Temperature Limit-of-Error (i.e. temperature interchangeability), by Multiplying the derivative of RT by the uncertainty DRT

PRTD Temperature Accuracy

Resistance Measurement Circuits The common values of resistance for a platinum RTD range from 10 ohms for the bird-cage model to several thousand ohms for the film RTD. The single most common value is 100 ohms at 0°C.

RESISTANCE THERMOMETER PRACTICE The Problem of terminating the Resistance Thermometer. Fundamentally, every sensing resistor is a two wire device. In the sensing resistor, the electrical resistance varies with temperature. Temperature is measured indirectly by reading the voltage drop across the sensing resistor in the presence of a constant current flowing through it using Ohm’s Law: V= R.I

The measuring current should be as small as possible to minimise sensor self-heating; A maximum of around 1mA is regarded as acceptable for practical purposes. This would procedure a 0.1V drop in a Pt100 sensing resistor at 0C; The voltage dropped which varies with temperature is then measured by the associated circuitry. The interconnection between the Pt100 and the associated input circuit must be compatible with each other. It is essential that in any resistance thermometer the resistance value of the external lead wire be taken into account. If this value affects the required accuracy of the thermometer, its effect should be minimised.

Better Practice to Measure Resistnace The bridge output voltage is an indirect indication of the RTD resistance. The bridge requires four connection wires, an external source, and three resistors that have a zero temperature coefficient. To avoid subjecting the three bridge-completion resistors to the same temperature as the RTD, the RTD is separated from the bridge by a pair of extension wires:

These extension wires recreate the problem that we had initially. The impedance of the extension wires affects the temperature reading. This effect can be minimized by using a three-wire bridge configuration.

If wires A and B are perfectly matched in length, their impedance effects will cancel because each is in an opposite leg of the bridge. The third wire, C, acts as a sense lead and carries no current.

3-Wire Bridge Measurement Errors If we know VS and VO, we can find Rg and then solve for temperature. The unbalance voltage Vo of a bridge built with R1 = R2 is:

If Rg = R3, VO= 0 and the bridge is balanced. This can be done manually, but if we donʼt want to do a manual bridge balance, we can just solve for Rg in terms of VO:

4-Wire Ohms The output voltage read by the dvm is directly proportional to RTD resistance, so only one conversion equation is necessary. The three bridge-completion resistors are replaced by one reference resistor. The digital voltmeter measures only the voltage dropped across the RTD and is insensitive to the length of the lead wires.

ADOPTION OF Pt100 THERMOMETERS The practical range of Pt100 based thermometers extends from –200°C to 650°C although special versions are available for up to 962°C. Their use has in part taken over form thermocouples in many applications for a variety of reasons: Installation is simplified since special cabling and cold junction considerations are not relevant. Similarly, instrumentation considerations are less complex in terms of input configuration and enhanced stability. Instrumentation developments have resulted in high accuracy, high resolution and high stability performance from lower cost indicators and controllers; such accuracy can be better exploited by the use of superior temperature sensors. The availablility of a growing range of sensing resistor configurations include miniature, flat-film fast response versions in addition to the established wire wound types with alternative tolerance bands.

General RTD assembly Sensitive elements of RTDs are made of a thin wire 1 with outside diameter equal to 0.025 mm (platinum RTD) and 0.1 mm (copper RTD) double wounded (non-inductive) on a micaceous or porcelain stem 2. For mechanical strength the sensitive element is placed in the ceramic insulator tube 3 filled by extremely fine granular powder; extension wires are placed in the ceramic insulator 4, and entire assembly is covered by a protective sheath of stainless steel 5.

The space between the sheath and ceramic insulator is filled by ceramic packing powder 6. To avoid contact of sensitive element with environment, sensitive assembly is protected by high-temperature hermetic seal 7. The contact between the wire of the sensitive element and the ceramic encapsulation permits a rapid speed of response.

Industrial RTDs

FLEXIBLE RESISTANCE THERMOMETERS