3.1 Sensors
3.1.5 Measurement Resistors
When making temperature measurements using measurement resistors the electrical resistance of a sensor subjected to the temperature is the variable utilized.
The temperature dependence of the electrical resistance of metals, semiconductors and ceramics is used as the measurement value. The materials are divided into two groups based on the slope of the curve: NTC- and PTC-sensors.
PTC-sensors are materials whose resistance increases as the temperature increases (positive temperature coefficient) or “cold wire“. Included are the metallic conductors which are used in the manufacture of the measurement resistors described below.
NTC-sensors (negative temperature-coefficient) or “hot wire“ are usually semiconduc-tor or ceramic sensors, which are usually installed for specific requirements and tem-peratures.
Materials for Measurement Resistors
The are a number of requirements which must be met for the materials used as tem-perature sensors in order that good and reproducible measurements can be made.
• Large temperature coefficient,
• Minimal sensitivity to environmental effects (corrosion, chemical attack),
• Wide measurement range,
• Interchangeability,
• Long term stability,
• Easily processed.
For industrial temperature measurement technology, Platinum is the most used mate-rial for the resistors followed by Nickel.
It is for this reason that both of these materials will be described in detail in the follow-ing.
The Platinum measurement resistors with a nominal value of 100 Ω (Pt100) has be-come established in recent years as the industrial standard.
Nominal Values
The resistors are identified by the resistance at 0 °C (32 °F) (nominal value). Ni100 and Pt100 the most common types have a resistance of 100 Ω at 0 °C (32 °F), Pt500 or Pt1000 have 500 or 1000 Ω respectively at 0 °C (32 °F).
Fig. 3-3: Resistance Rt relationship to temperature for Platinum measurement resistors with different nominal values
Temperature Coefficient (Tc)
More precisely stated, the temperature coefficient of the electrical resistance. It defines the change in electrical resistance between two temperatures, usually between 0 °C and 100 °C (32 °F and 212 °F) with the units:
which is therefore dimensionless
For smaller temperature ranges a linear relationship can be assumed:
with
500 600 700 800 900 0
Where:
t: Temperature in °C
t0: Reference temperature ( e.g. 0°) Rt: Resistance at temperature t in Ω R0: Nominal resistance at 0 °C in Ω
α: Average temperature coefficient between 0 °C and 100 °C (32 °F and 212 °F) in K-1 Platinum Material
Its advantages include very pure producability, high chemical resistance, easy manu-facturability, good reproducibility of the electrical properties and a wide application range between -250 °C and 850 °C (-418 °F and 1562 °F).
The temperature coefficient of spectral pure Platinum is 0.003925 K-1 and is different than the value required for Pt-measurement resistors. The Platinum used for industrial Platinum temperature resistors is selectively produced.
Specified in EN 60751 for the Platinum sensors, among others, are the temperature relationship to the resistance, the nominal value, the allowable deviation limits and the temperature range.
Measurement Characteristics of Platinum Simplified:
It the range from 0...100 °C (32...212 °F) Platinum has a temperature coefficient of 0.00385 K-1, i. e. a Pt100 measurement resistor at 0 °C (32 °F) has a resistance of 100Ω and at 100 °C (212 °F) 138.5 Ω.
Expanded:
By definition the basic values are divided into two different temperature ranges:
For -200...0 °C (-328...32 °F) a third order polynomial applies
For the range from 0...850 °C (32...1562 °F) a second order polynomial applies
The coefficients according to EN 60751 are:
A = 3.9083 · 10-3 K-1 B = 5.775 · 10-7 K-2 C = 4.183 · 10-12 K-4
( )
[ ]
R
t= R
01 + ⋅ + ⋅ A t B t
2+ ⋅ − C t 100 ° C t ⋅
3[ ]
R
t= R
01 + ⋅ + ⋅ A t B t
2For temperatures above 0 °C (32 °F) the relationship between the temperature an the resistance can be described by the equation:
in which the resistance values for the basic value tables in EN 60751 are listed for tem-perature in steps of 1 K.
For the sensitivity, i.e. the resistance change according to K, for temperatures <0 °C (32 °F):
For temperatures above 0 °C (32 °F) the following applies:
t A
Tolerance Classes for Platinum
According to EN 60751 the Platinum resistance thermometers the deviation limits Δt are divided into two tolerance classes:
Class A:
Class B:
Tbl. 3-14: Deviation limit according to EN 60751 and expanded deviation limit
Fig. 3-5: Deviation limit for Platinum resistance thermometers in °C Tolerance
designation
Temperature range
Tolerance in K Deviation limit at 0 °C (32 °F) resistance
Deviation limit at 100 °C (212 °F)
Class A ±0.35 K
Nickel Material
It is appreciably less expensive than Platinum. Its temperature coefficient is almost twice as high, but it has a decidedly poorer chemical resistance. The measurement range is limited to only -60...250 °C (-76...482 °F) and the allowable deviation limits are greater than for Platinum. The Nickel measurement resistors are standardized in DIN 43760.
Measurement Characteristics of Nickel Simplified:
In the range from 0...100 °C (32...212 °F) Nickel has a temperature coefficient of 0.00618 K-1 i.e. the measurement resistor Ni100 at 0 °C (32 °F) has a resistance of 100Ω and at 100 °C (212 °F) 161.85 Ω.
Expanded:
The relationship between the resistance and temperature for Nickel in a temperature range -60...250 °C (-76...482 °F) is:
where
A = 0.5485 · 10-2 K-1 B = 0.665 · 10-5 K-2 C = 2.805 · 10-11 K-4 D = -2 · 10-17 K-6
According to DIN 43 760 the nominal value is 100.00Ω (therefore: Ni100). Additionally, resistors with R0= 10 Ω, 1000 Ω or 5000 Ω are also manufactured.
( )
R
t= R
01 + ⋅ + ⋅ + ⋅ A t B t C t
4+ ⋅ D t
6Ni100
0 50 100 150 200 250 300 R[]tΩ
In the standard the maximum allowable deviation limits Δt for Nickel resistors are defined by:
for 0...250 °C (32...482 °F)
for -60...0 °C (-76...32 °F)
Fig. 3-7: Maximum deviation limit in °C for Ni100
Nickel resistors are often found in the heating, ventilating and air conditioning sectors.
(
0.4 C 0.007 t)
t=± ° + ⋅ Δ
(
0.4 C 0.028 t)
t=± ° + ⋅ Δ
-100 -50 0 50 100 150 200
Ni100
250
Deviation Limit [°C]
Temperature [°C]
0.0 0.5 1.0 1.5 2.0 2.5
Measurement Resistor Designs
Only Platinum measurement resistors will be discussed in the following. They are divided into two categories, thin film and wire-wounded resistors. Ceramic, glass or plastic are used as the basic carrier materials.
Thin Film Resistors
The measurement coil is made of Platinum wires with diameters between 10 µm and 50 µm.
Wire-wounded Resistors
A precisely adjusted Platinum coil with connections leads is located in a ceramic double capillary. Glass frit powder is packed into the holes of the capillaries. Both ends of the ceramic body are sealed with glass frit. After the glass frit is melted the Platinum coil and the connection leads are fixed in place.
In another design, the Platinum coil is not placed in the holes of a ceramic cylinder, but is placed in a slot in the ceramic body. The outside dimensions are between 0.9 mm and 4.9 mm (0.035” and 0.20”) with lengths between 7 mm and 32 mm (0.28” and 1.25”).
Typical applications: demanding measurement and control requirements in the process industries and laboratory applications.
Fig. 3-8: Ceramic wire-wounded resistor
Ceramic double capillary Platinum coil
Drilling
Connection wires
Glass Measurement Resistors
In this design the measurement coil is wound in a bifilar configuration on a glass rod and melted into the glass and the connection wires attached. After it is adjusted, a thin wall glass tube is placed over measurement coil and both elements fused together. The geometric dimensions of the diameter are between 0.9...5.0 mm (0.035”...0.20”) with lengths varying between 7...60 mm (0.275”...2.35”).
Typical applications: chemical system engineering.
Fig. 3-9: Glass measurement resistor
Slot Resistance Thermometer
The Platinum measurement winding is placed stress free in a slot in a plastic band and connection leads attached stress free. The insulation body is surrounded, including the cable exit by shrinkable tubing. The geometric dimensions for the width can vary between 8 mm (0.31”) and 12 mm (0.5”), lengths between 63 mm and 250 mm (2.5”
and 10”). The thickness is 2 mm (0.08”).
Typical applications: temperature measurements in the winding of electrical machines and on curved surfaces
Foil Temperature Sensors
The Platinum measurement winding is embedded between two Polyimide foils and the connection leads attached. The thickness is 0.17 mm (0.007”).
Typical applications: Measurements on pipes Metal Film Resistors
In place of measurement wires thin platinum layers are used as the measurement ele-ment. The layers are applied to ceramic carriers. There are a number of methods for depositing thin layers, e.g. vacuum vapor deposition, sputtering or sintering a thick Platinum paste.
Glass rod
Glass tube
Platinum coil
Connection wires
Platinum Thick Film Measurement Resistors
In this design a Platinum paste is applied to a ceramic substrate using a silk screen process and fused. Then the resistance is trimmed to the nominal value, a glass pro-tection layer and connection leads attached and then stress relieved. The thickness of the Platinum layer is between 10 µm and 15 µm.
Platinum Thin Film Measurement Resistors Flat types
A Platinum layer 1 µm to 2 µm thick is vapor deposited or sputtered onto a ceramic sub-strate. The desired geometric shape is formed by cutting with a laser or structured using photolithography. The Platinum traces are between 7 µm and 30 µm wide. A laser trimmer is used to adjust the resistance to the nominal value. For protection against mechanical damage (scratches) a 10 µm thick glass ceramic insulation is applied using a silk screen process and fused. After the connection leads are attached by welding the connection spots are covered by a fused glass coating applied in a stress free manner. The geometric dimensions of the flat types vary from 1.4 mm x 1.6 mm (0.05” x 0.06”) to 2 mm x 10 mm (0.08” x 0.40”), the substrate thickness from 0.25 mm to 0.65 mm (0.010” to 0.026”)
Typical applications: all application ranges, surface temperature measurements
Thin Film Tubular Types
In addition to the flat type thin film measurement resistors a round form is available. In this design the flat measurement resistors are inserted axially in cylindrical ceramic tubes. The ends of the tube are sealed by melting glass frit across them which also seals and positions the ends of the measurement resistor together with the connection leads. The end result is a round shape. The ceramic also provides protection for the thin film measurement resistors. The outside dimensions of the diameter are 2 mm to 4.8 mm (0.080” to 0.20”) and the lengths are 5 mm to 14 mm (0.20” to 0.55”) . Typical applications: process engineering
Fig. 3-11: Thin film tube type (Installation principle)
Thin Film Platinum Measurement Resistors with Solder Connection Pads
In this design the connection pads are coated with a solderable metallization. The design has adjacent connection pads with solder depots suitable for direct connection of insulated cables. Measurement resistors with connection pads at opposite ends are called “Surface Mounted Devices“, SMD, which can be directly soldered to circuit boards or hybrid circuits.
Typical application: On circuit boards.
Fig. 3-12: Thin film and metal wire measurement resistor designs
Selection Criteria and Application Limits
The application limits of the sensors are restricted by numerous parameters. The most important, without question, is the temperature. Exactly defined temperature limits are difficult to specify. In addition to the temperature, they are also influenced by the medium to be measured, mechanical factors (different expansion coefficients) and the accuracy and reliability requirements.
It is not possible to specify a universally applicable conclusion as to which resistance thermometer design represents the best solution. The best construction solution is in a high way depending on the application conditions. Selection criteria are:
– Temperature range
It is rare, that for a specific application the entire range specified in the standards is required. For high temperatures (greater than 600 °C (1112 °F)) sensors with special connection leads (NiCr) are used. For applications with temperature shocks wire-wounded types are preferred.
– Required accuracy and long term stability
The accuracies are derived from the tolerance classes of from the actual individual measurement values; for long term stability, specific ambient conditions must be considered. Particularly, for industrial conditions above 400 °C (752 °F) caution should be exercised, carefully weigh versus thermocouples.
– Sensitivity and self heating
Sensitivity is defined by the resistance changes according to K and for the Pt100 it is 0.385 Ω/K and for the Pt1000 it is 3.85 Ω/K. Since the measurement signal is derived directly from measured current, the resistance to the current (voltage drop at the measurement resistor is U = RxI) in the circuit causes self heating in the measurement resistor, which increases as the square of the current (P = I2 xR). For accurate measurements the self heating must be kept small and therefore the cur-rent has to be limited.
It can be stated simply that for industrial applications, using the measurement currents of modern transmitters, the following needs not to be considered.
Expanded:
where
Iallow : Allowable measurement current EK: Self heating coefficient in W/K ΔTallow: Allowable temperature increase R0: Nominal resistance
α: Temperature coefficient
Typical values for the voltage sensitivity for an allowable temperature increase of 0.1 K for Pt100 film type measurement resistors are approx. 0.1 mV/K and for a Pt1000, ap-prox. 0.4 mV/K for measurements in flowing water.
In air, the values for a Pt100 are approx. 0.03 mV/K to 0.09 mV/K. The maximum allow-able measurement current for flowing water for a Pt100 is approx. 6 to10 mA and for a Pt1000 approx. 3 mA. In air for a Pt100 it is approx. 2 mA and for a Pt1000 approx.
1 mA. Wire resistors have somewhat lower self heating coefficient than the film resistor types and therefore can be operated with higher allowable measurement currents (for Pt100 Iallow. is approx. 4 mA to 14 mA in water and 2 mA to 3 mA in air). Their nominal value is however limited to 100 mW.
2
– Response time
The response time of the bare resistors is usually of little concern because the design of the thermometer into which they are installed is the dominant factor in determining the response time. The following values, however, are of importance in laboratory applications.
The small geometric dimensions of the film type measurement resistors and their associated minimal heat capacity results in short response times, at T0.5 approx.
0.1 s in water and approx. 3 s to 6 s in air. For wire type resistors the response time T0.5 is between 0.2 s and 0.5 s in water and between 4 s and 25 s in air.
– Geometric dimensions and connection wire resistances
The assigned basic values and their allowable deviation limits apply to the measure-ment resistors including the connection wire resistances (generally 10 mm...30 mm (0.4”...1.2”) long) or for longer connection wires up to a defined sensor point. All additional connection wires and junction resistances must be considered or compen-sated using special circuits.