Resistance Temperature Detector

The two wires RTD is the simplest wire configuration. One wire is attached to each side of the element. A measure can be taken by any device equipped to measure resistance, including basic Volt Ohm Meters (VOM).

This is the least accurate way of measuring temperature, due to the fact that the lead wire resistance is in series with the sensing element. The lead wire is at a different temperature than the sensing element and also has different resistance verses temperature characteristics. The longer the lead wire greater will be the effect on the measurement.

RTDs are available with three different lead wire configurations. The selection of lead wire configuration is based on desired accuracy and instrumentation to be used for the measurement.

The outer wound RTD element is made by winding the sensing element wire around a center mandrill, which is usually made of ceramic. This winding is then coated with glass or some other insulating material to protect and secure the windings. The winding wires are then spot welded to extension leads and secured to the body with ceramic cement or epoxy.

Each of the types has their advantages. The thin film is the least expensive to manufacture and also the most rugged. They can also be manufactured in very small sizes. The inner coil wire wound style is the most accurate. It is however, more expensive to manufacture and does not perform well in high vibration applications. The outer wound element is similar in cost to the inner coil element. It is not as accurate as the inner coil style but is more rugged.

This type of element is normally manufactured using platinum wire. Very small platinum wire (0.02 mm) is coiled and then slid into a small 2 holes ceramic insulator. Larger extension leads are then spot welded to the ends of the platinum wire and cemented in place. Some manufacturers backfill the bores of the insulator with ceramic powder, once the coil have been inserted. This keeps the coils away from moving and shorting against each other. The end opposite the extension leads is capped with ceramic cement also.

TThe European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0ºC and a temperature coefficient of resistance (TCR) of 0.00385 Ω /Ω/ºC between 0 and 100ºC. There are three resistance tolerances for Thin Film.

The RTD elements nearly always require insulated leads attached. At temperatures below about 250 ºC PVC, silicon rubber or PTFE insulators are used. Above this, glass fiber or ceramic are used. The measuring point, and usually most of the leads, requires a housing or protective sleeve, often made of a metal alloy which is chemically inert to the process being monitored. Selecting and designing protection sheaths can require more care than the actual sensor, as the sheath must withstand chemical or physical attack and provide convenient attachment points.

There are two standards for platinum RTDs: The European standard (also known as the DIN or IEC standard) and the American standard.
The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0ºC and a temperature coefficient of resistance (TCR) of 0.00385 Ω /Ω/ºC between 0 and 100ºC. There are three resistance tolerances for Thin Film.

RTDs specified in IEC60751:
Class AA (Formerly 1/3B) = ± (0.1+0.0017×t) °C or 100.00 ± 0.04Ω at 0°C
Class A = ± (0.15+0.002×t) °C or 100.00 ± 0.06Ω at 0°C
Class B = ± (0.3+0.005×t) °C or 100.00 ± 0.12Ω at 0°C

Also, one special class not included in DIN/IEC60751:
Class 1/10B = ±1/10 (0.3+0.005×t) °C or 100.00 ± 0.012Ω at 0°C

Resistance thermometers offer the greatest benefits relative to other thermometer types in these situations:

• Accuracy and stability are the foremost goals of the Application.
• Accuracy must extend over a wide temperature range
• Area, rather than point, sensing improves control
• A high degree of standardization is desirable

The major application areas of RTDs are:

• Food Processing Industry
• Plastic Industry
• Pharmaceutical industry
• Air, gas and liquid temperature measurement
• Textile industry
• Exhaust gas measurement
• Industrial electronics
• Military & Aerospace

Platinum RTDs typically are provided in two classes:Class A and Class B.
Class A is considered high accuracy and has an ice point tolerance of 0.06 ohms.
Class B is standard accuracy and has an ice point tolerance of 0.12 ohms. Class B is widely used by most industries.

The accuracy will decrease with temperature. Class A will have an accuracy of 0.43 ohms ( 1.45°C) at 600°C and class B will be 1.06 ohms ( 3.3°C) at 600°C.Other accuracy classes like 1/3, 1/5, 1/10 DIN of class B are available.

Platinum resistance temperature sensors (PRT) They offer excellent accuracy over a wide temperature range (from -200 to +850°C).

Other resistance value options
RTD elements are also available with resistances of 200, 500, and 1000 Ω at 0°C. Such type of RTDs is normally known as PT200, PT500, and PT1000 respectively. These RTDs have the same temperature coefficients as previously described, but because of their higher resistances at 0°C, they provide more resistance change per degree, allowing for greater resolution.

Standard Platinum RTDs (SPRTs)

The maximum temperature rating for RTD’s is based on two different factors. First is the element material.Platinum RTD’s can be used as high as 650°C (1202F). Other materials are much lower in temperature rating and vary from material to material. The other determining factor for temperature rating is probe construction. There are construction considerations used in each of these different styles making them ideal for use in each of those ranges. Noone style is good for all ranges.

As shown in table, although copper is cheapest, it also has the lowest resistivity and therefore requires inconveniently large sensing elements. On the other hand, even as nickel and nickel alloy have high resistivity, their resistance versus temperature coefficients are non-linear. They are also sensitive to strain and their resistivity suffer from an inflexion around the Curie point (358ºC) that makes the deviation of their resistance/ temperature expressions more complicated.

This platinum which not only has a high resistivity (more than six times that of copper) but also a high degree of stability and a wide temperature range. Although platinum is expensive it can be drawn into fine wires or strips and we only require small amounts for manufacturing RTDs. As a noble metal, it has minimum susceptibility to contamination.

The presence of impurities is undesirable since diffusion, segregation and evaporation may occur in service, resulting in a lack of stability. The resistivity is also sensitive to internal strains. Thus, it is essential that the platinum should remain in a fully annealed condition i.e. it should be annealed at a temperature higher than the maximum temperature of service.

The criterion for selecting a material to make an RTD is:

• The material must be malleable so that it can be formed into small wires
• It must have a repeatable and stable slope or curve.
• The material should also be resistant to corrosion.
• The material should be low cost.
• It is preferred that the material have a linear resistance verses temperature slope.

Some of the common RTD materials are platinum, copper, nickel, Balco (an alloy of 70% nickel and 30 % iron). These metals have the advantage that they can be manufactured to a very high degree of purity and are, consequently, available with highly reproducible temperature/resistance characteristics. These metals can also be drawn to a fine diameter wires required in resistance thermometry.

At 0° C, A Platinum RTD has a resistance of 100 Ω & a temperature co-efficient of about 0.00385 Ω / Ω / °C. These non-linearties are described in Callender-Van Duesen equation. This equation consists of both a linear portion & a non- linear portion.

Range 200 to 0ºC: R (t) [Ω] = R (1 + At + Bt² + C (t – 100ºC) t³)
Range 0 to 850ºC: R_? (t) [O] = R (1 + At + Bt?)
With: R₀ is resistance at 0 ºC
A = 3, 9083 x 10ˉ³ °Cˉ¹
B = -5,775 x 10ˉ⁷ °Cˉ²
C = -4,183 x 10ˉ¹² °Cˉ⁴

• Select the right one among various RTDs on the basis of temperature range and accuracy requirement.
  • RTDs work best under the temperature range of -200ºC-850ºC.
  • If you need a higher level of accuracy then RTD is better option.
• RTD’s are commonly used in applications where repeatability and accuracy are important considerations. Properly constructed Platinum RTD’s have very repeatable resistance vs. temperature characteristics over time.
• RTD can be used where Stability, Sensitivity and Linearity is important parameter.
• RTD Design selection:
  • Good heat transition between sheath and the temperature probe to permit short response time and high measuring accuracy.
  • RTD designshould withstand the process vibrations.

Resistance thermometer use metals that alter their electric resistance when heated. Platinum is most commonly used material for industrial RTD. However copper and Nickel are also used for some applications. The resistance at 0ºC is called R and it is important parameter to be defined. The most commonly used RTD element is of Platinum with resistance of 100O at 0ºC. RTD has a positive temperature coefficient. Normally industrial RTD are used up to temperature range of 400ºC.

Working:

• In RTD, The change in resistance value is very small with respect to the temperature. So, the bridged circuit is used.
• Constant electric current is supplied to bridged circuit and voltage drop is measured across the resistor, through which resistance is measured. Thereby, the temperature can also be determined.
• This temperature is determined by converting the RTD resistance value using a calibration expression.

The three wires RTD is the most popular configuration for use in industrial applications. In order to minimize the effects of the lead resistances, a three-wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms. There is a lead resistance in each arm of the bridge so that the resistance is cancelled out, so long as the two lead resistances are accurately the same. This configuration allows up to 600 meters of cable.

When used correctly, the three wire configuration eliminates the series resistance. This permits an accurate measurement of the sensing element. Two of the leads are connected to one side of the sensing element and the single lead to the other side. The resistance in L1 and L3 should be matched as close as possible; this will cause the lead resistance to cancel them. The color code for a three wire RTD is two red wires and one white.

A four wire RTD is the most accurate method to measure an RTD. It is primarily used in laboratories and is seldom seen in an industrial application. The four-wire resistance thermometer configuration increases the accuracy and reliability of the resistance being measured: the resistance error due to lead wire resistance is zero.

A four wire RTD circuit removes the effect of mismatched resistances on the lead wires. A constant current is passed through L1 and L4, L2 and L3 measure the voltage drop across the RTD element. The color code for a four wire RTD is usually two red wires and two white wires. The following diagram illustrates a typical four wire measurement.

• It comprises of a thin-walled and flexible mineral insulated sheath cable made up of stainless steel.
• The cable contains low resistance inner wires made of copper embedded in pressed fireproof magnesium oxide.
• The temperature sensor is connected to the inner wires and fitted in a protective tube.
• Protective tube and sheathed cable are welded together hermetically.
• Good heat transfer between protective tube and temperature sensor allows fast response time and high measuring accuracies.
• The flexible probe tube allows temperature measurement at locations that are not easily accessible.
• They are used in difficult measurement application with strong vibrations as well as at all measuring positions where flexibility and ease of replacement are needed.
• Areas of application are to be found in chemical plants, power stations, motors, as well as in machine construction and building installation and in general industrial applications.

• RTDs in Industrial applications are rarely used above 660 ºC. At temperatures above 660 ºC it becomes increasingly difficult to prevent the platinum from becoming contaminated by impurities from the metal sheath of the thermometer. This is why laboratory standard thermometers replace the metal sheath with a glass construction.
• At very low temperatures, say below -270 ºC (or 3 K), due to the fact that there are very few photons, the resistance of an RTD is mainly determined by impurities and boundary scattering and thus basically independent of temperature. As a result, the sensitivity of the RTD is essentially zero and therefore not useful.
• Compared to thermistors, platinum RTDs are less sensitive to small temperature changes and have a slower response time. However, thermistors have a smaller temperature range and stability

Advantages of resistance thermometers are:

• High accuracy
• Low drift
• Wide operating range
• Suitable for precision applications

In summary, resistance thermometers offer the greatest benefits relative to other thermometer types in these situations:

• Accuracy and stability are the foremost goals of the Application.
• Accuracy must extend over a wide temperature range
• Area, rather than point, sensing improves control
• A high degree of standardization is desirable

Resistance thermometers readily adapt to most process control and thermal equipment designs. The user may specify cases with axial leads for circuit board mounting, flat packages for clamping to surfaces, miniature cases for embedment into metal blocks, and any sheaths and fittings which can be produced by a machine shop. In addition, wire windings may be configured to sense over large areas.

Unlike thermocouple junctions, which can be welded directly to metal surfaces, resistance thermometers present a certain amount of bulk; and heat losses to ambient air may affect readings. Small flat elements, such as thin films, may mount on surfaces, but fragile element and lead wire connections make installation difficult.

Figure shows a flexible resistance thermometer with a wire-wound sensing element sandwiched between insulating layers. It conforms closely to sensed surfaces, and has thin insulation to readily transmit heat to the sensing element. The wire element may be wound to nearly any size to average out temperature gradients, and the flexible construction can withstand extreme shock and vibration.

Selection depends on the nature of the medium being sensed and cost requirements. Direct immersion of a probe into a liquid requires a fitting with a pipe thread, which may be adjustable or welded on the probe.

Figure shows a typical assembly, with one thread for mounting the probe and another for a connection head. Connection heads provide a transition between probe leads and external signal wires.

Mounting in a solid material is best accomplished with a spring-loaded holder, which may be fixed or adjustable. Spring loading provides good contact of the probe tip against the bottom of the hole and dampens potentially damaging vibration. When liquids are particularly corrosive, under high pressure, or fast-flowing, a thermowell may be necessary. A thermowell is a tube, closed at one end, which protects the probe and allows its removal without breaking the liquid seal.

The encased probe is the standard resistance thermometer configuration for industrial process control and machinery protection. Most probe cases are stainless steel or Inconel to withstand high temperatures, although other materials offer advantages at intermediate ranges.

For example, the tip-sensitive probe of below Figure copper-alloy tip which conducts heat 20 times better than stainless steel. This design improves thermal contact with sensed surfaces and reduces errors from conduction along the sheath.
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Standard probe diameters range from 0.125 – 0.250”. Smaller probes respond faster when directly immersed, but larger probes may fit more snugly in standard Thermowells. Probe lengths range from a few inches to ten feet or more.

Sheaths and other structures surrounding resistive elements should maximize heat transfer from the sensed medium, minimize heat transfer from ambient which can alter readings, and provide necessary protection of the elements.

Proper materials and construction can dramatically improve reading accuracy. One strategy practicable only with wire-wound resistance thermometers versus thermistors, thermocouples, and solid-state devices are temperature averaging. An element may be wound to average temperature over lengths of up to 100feet.

Stability is long-term drift in thermometer readings. A typical specification would limit drift to 0.1ºC per year for rated operation. Normal services at points well within the temperature rating typically cause much less drift. Drift is a consequence of the element material, with platinum being the most stable; encapsulating materials, which could contaminate the element; and mechanical stress placed on the element by expansion of winding bobbins or other supporting structures.

The degree of accord between two successive readings with a thermometer is its repeatability. Loss of repeatability results from permanent or temporary changes to the resistance characteristics of the element and may be caused by exposing the thermometer to temperatures at or beyond the endpoints of its specified range. A repeatability test cycles the thermometer between low and high temperatures; any change to R is noted. A typical repeatability rating for 0ºC industrial platinum resistance thermometers is 0.1ºC.

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, and 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 specifies the amount of resistance a thermometer is allowed to deviate from a standard curve (such as the curve produced by the Callendar-Van Dusen equation).

A tolerance at the reference temperature is usually 0ºC, and a tolerance on the slope or TCR. Below shown figure states that a resistance thermometer conforms most closely to its curve at the reference temperature, while the resistance fans out above and below this reference.

Interchangeability between two thermometers is no more than twice the value of their Conformity. Commercial platinum resistance thermometer elements are available with extremely tight tolerances, to within 0.026ºC 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 0ºC.

Mineral-Insulated resistance thermometers (M.I.) are equipped in general with Platinum-measuring resistors Pt100 Ω to DIN IEC 751. The inner (Cu) conductors are embedded in a closely compacted, inert mineral powder (MgO); the measuring resistor will be connected to the inner conductors, is also embedded and is surrounded by the metal sheath to form a hermetically sealed assembly. Sometimes inner conductor of constantan and nickel are also used.

The sheath functions as a useful protective cover in many situations. They are applied in locations where fast response, reduced space and or vibration resistance is a need. They can be furnished with a fixed cable or with a special plug which allows rapid fitting or exchange.
Mineral-insulated RTD temperature probes consist of a flexible, thin-walled stainless steel mineral insulated cable, in which low ohmic conductor copper wires are embedded in pressed, heat resistant magnesium oxide.

The temperature probe is connected to the wires of the internal conductors and accommodated in a stainless steel sheath. Thermowell and mineral-insulated cable are welded to one another.

The good heat transition between the sheath and the temperature probe permits short response times and high measuring accuracy. The vibration resistant (shake proof) design guarantees a long operating life. Temperature measurements at measuring points difficult to access, are possible, thanks to the flexible mineral-insulated cable. The smallest bending radius is 5 times the outer diameter.

If the sensing element and leads are not completely insulated from the case, a shunting effect occurs in which the case becomes a parallel resistor and lowers apparent readings. In most industrial thermometers, with specified insulation Resistances in the 100-MΩ ranges, error approaches zero. The manufacturer must take care to seal water-absorbing materials. The shunting effect decreases with low-resistance elements, which accounts for the use of 25.5 PRT’s in laboratory measurements.

The resistance change per degree change in temperature is a function of base resistance and TCR (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.

A resistance thermometer is a passive resistance sensor; it requires a measuring current to produce a useful signal. Because this measuring current heats the element wire above the true ambient temperature, errors will result unless the extra heat is dissipated. Self-heating is most often expressed in mW/ºC, which is the power in milliwatts (1000 I²R) required to raise the thermometers internal temperature by 1ºC. The higher the mW/ºC figure, the lower the Self Heating.

As an example, assume a 5 mA measuring current is driven through 100 platinum RTD at 100ºC.

Self-heating is specified as 50 mW/ºC in water moving at 3 ft/sec. The amount of heat generated is:
1000 mW x (0.005 A)2 x (138.5) = 3.5 mW
The self-heating error is:
(3.5 mW) / (50 mW/ºC) = 0.07ºC

The generated heat increases with higher sensor element resistance (when a constant current measurement device is used), or with increasing measuring current.

The resulting error is inversely proportional to the ability of the thermometer to shed extra heat; which in turn depends on thermometer materials, construction, and environment.

The worst self-heating occurs when a high resistance is packed into a small body. Thin film elements, with little surface area to dissipate heat, are an example. Self-heating also depends on the medium in which the thermometer is immersed. Error in still air may be over 100 times greater than in moving water.

A time constant indicates the responsiveness of a resistance thermometer to temperature change. A common expression is the time it takes a thermometer to reflect 63.2% of a step temperature change in moving water. Response speed depends on the mass of the thermometer and the rate at which heat transfers from the outer surface to the sensing element. A rapid time constant reduces errors in a system subject to rapid temperature changes.