Technical Information
Platinum Resistance Temperature Detectors
For precise temperature measurement, Platinum RTDs offer the best overall advantage of any other device.
According to BIPM First Edition, Technology for Approximating the ITS 90, many thermometers are formed by winding a fine platinum wire or coil on a glass support and embedding the winding in glass. This can introduce strain into the wires at higher operational temperatures. Of the many techniques devised, the configuration that offers the best stability with the lowest hysteresis, is one in which a platinum coil is supported inside the capillaries of a two or four-bore, high-purity alumina insulator. Either the use of cement to clamp one side of the coil to the capillary wall, or, preferably, the insertion of soft alumina powder to prevent the platinum coil from vibrating freely helps achieve good reproducibility in industrial applications. Detectors supplied by SDI are of this wire-wound type, in which the platinum winding is partially or fully supported (application dependent) by a high temperature alumina powder inside a ceramic tube. This construction provides a detector in which stability, interchangeability, and accuracy are the highest available for industrial and scientific applications.
By concentrating on the manufacture of this type of detector, SDI is able to provide unparalleled service by offering the largest possible selection of sizes for various requirements.
Tolerance & Accuracy
SDI supplies detectors with tolerance values of Bands 1 to 5, In accordance with International Specifications, or to other specifications requested by the customer. Band 5 detectors have a tolerance of plus or minus 0.01 ohms a 0°C, equivalent to plus or minus 0.025 °C. The following table illustrates how SDI Bands relate to DIN/IEC International Standards.
Detector Chart
This chart illustrates the tolerance applicable for international specifications together with the SDI BANDS 1 to 5. At higher temperatures, the tolerance deviation is most affected by variation of ALPHA values, rather than accuracy at 0°C. For detectors manufactured by SDI, typical ALPHA deviation is less than ±0.000003 °C-1.
| Equivalent Tolerances @ 0°C | ||||
| DIN/EC Class B | ±.12% | 0.12 Ohm | 0.30 °C | 0.54°F |
| SDI Band 1 | ±.10% | 0.100 Ohm | 0.26 °C | 0.47°F |
| 1/2 DIN/EC Class A | ±.06% | 0.06 Ohm | 0.15 °C | 0.23°F |
| SDI Band 2 | ±.05% | 0.050 Ohm | 0.13 °C | 0.23°F |
| 1/3 DIN/EC | ±.04% | 0.04 Ohm | 0.10 °C | 0.18°F |
| SDI Band 3 | ±.03% | 0.030 Ohm | 0.08 °C | 0.14°F |
| 1/5 DIN/EC | ±.02% | 0.024 Ohm | 0.06 °C | 0.11°F |
| SDI Band 4 | ±.02% | 0.020 Ohm | 0.05 °C | 0.09°F |
| 1/10 DIN/EC | ±.01% | 0.012 Ohm | 0.03 °C | 0.05°F |
| SDI Band 5 | ±.01% | 0.010 Ohm | 0.03 °C | 0.05°F |
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Tolerance and Interchangeability
SDI supplies detectors with tolerance values in accordance with Bands 1 to 5, in accordance with International Specifications, or to other specifications requested by the customer. Band 5 detectors have a tolerance of ± 0.01 ohms at 0 °C, equivalent to =/- 0.025 °C.
Temperature Range
SDI detectors are manufactured for use between the -200 °C to +661 °C. To avoid contamination at temperatures above 300 °C, special care should be taken, particularly with use of metal sheaths.
Point of Calibration
0.43 mm diameter platinum 0.7 milliohms/mm
0.28 mm diameter platinum 1.9 milliohms/mm
0.15 mm diameter platinum 6.3 milliohms/mm
For close tolerance applications, care must be taken in positioning the connecting wires. For a detector with 0.28 mm diameter platinum leads, connecting wires at a point 6 mm from the ceramic body instead of 5 mm will result in a 3.9 milliohm error.
Stability
As part of the manufacturing process, SDI detectors are automatically aged to ensure the highest levels of stability. Detectors operated over the range -50 °C to +450 °C have stability levels unattainable from any other type of detector. Typical resistance at 0°C will not change by more than 0.04% after 10 consecutive shocks from -200 °C to +600 °C.
Self-Heating
When tested in a well-stirred ice bath, the rise in temperature will not exceed 0.3 °C, with 10 milliwatts dissipated in the detector. A 1.6 mm diameter x 25 mm long detector has a self-heating characteristic of 0.015 °C/mW in water flowing at 1 meter per second. In air, with only natural convection, this can result in a reading 20 to 40 times greater. A detector passing a current of 1 mA dissipates 0.1 mW of heat at 0 °C. This will give a self-heating effect of 0.6 milliohms in flowing water and 12 to 24 milliohms in naturally circulating air. Thus a reasonable figure for the measuring current in a naturally circulating air application would be 0.3 mA. Smaller detectors require even less current for the same error level. In general, it is recommended the measuring current not exceed 1 mA.
Vibration
Properly supported, detectors will withstand a vibration level of 30 g over the frequency range of 10 Hz to 1 kHz.
Lead Extension
The additional benefit offered by these sub-assembly services is the ability to improve overall product reliability by allowing SDI experts to assure proper connection to the fine diameter leads of a platinum resistance element.
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Pressure
The ceramic-body construction of SDI detectors protects them from large changes of pressure. However, since elements are not normally hermetically sealed, they should be protected from contamination by liquids or gases through the use of suitable protection sheaths. Hermetically sealed detectors are available by special order.
Insulation Resistance
A 10,000,000 ohm shunt resistance will cause an error of 1 milliohm at 0 °C, equal to 10% of the total tolerance. Error will increase with detector temperature.
Thermal EMF's
Band 5 detectors have an interchangeability of ±0.025 °C at 0 °C. At an assumed current of 0.3 mA, this represents ± 3 microvolts, or a 10% error in tolerance. In addition, any junctions of dissimilar metals within the element, or between the element and measuring instrument, must be within a temperature variation within ± 0.01 °C. AC energization largely eliminates any EMF problems.
Screening and Electrical Interference
Electrical interference from AC supplies or other sources may also affect detector accuracy and is almost entirely dependent on the installation and type of instrumentation in use.
Immersion Errors and Stem Conduction
Assembly
Platinum RTD Standards
Sensing Devices manufactures Platinum Resistance Temperature Detectors to these Thermometry Standards for RTDs.
| Organization | Standard | ALPHA: Average Temperature Coefficient of Resistance(/°C) | Nominal Resistance at 0°C (Ohms) |
| British Standard | BS 1904: 1984 | 0.003850 | 100 |
| Deutschen Institut f¸r Normung | DIN 43760 : 1980 | 0.003850 | 100 |
| International Electrotechnical Commission | IEC 751: 1995 (Amend. 2) | 0.00385055 | 100 |
| Scientific Apparatus Manufacturers of America | SAMA RC-4-1966 | 0.003923 | 98.129 |
| Japanese Standard | JIS C1604-1981 | 0.003916 | 100 |
| American Society for Testing & Mat’ls | ASTM E1137 | 0.00385055 | 100 |
Standard Callendar – Van Dusen
CVD Table
The relationship between resistance (R) and temperature (t) is generated from the Callendar-van Dusen equations.
For the temperature range -200° to 0°C;
W(t90) = R(t90)/R(0) = { 1 + At90 + Bt290 + Ct390(t90 – 100)} …(1)
For the temperature range 0° to 661°C:
W(t90) = R(t90)/R(0) = { 1 + At90 + Bt290} …(2)
The constants A, B, and C are determined by the properties of the platinum wire used in the construction of the detector types Pt100, F100, and D100. The wire is specially selected to give nominal values of the constants as in the table below. The listed values for Alpha, Delta, and Beta are also listed.
TYPICAL CALIBRATION COEFFICIENTS – International Temperature Scale 1990 (ITS-90)
| Pt100 | F100 | D100 | |
| A (°C-1) | 3.90830 x 10-3 | 3.95834 x 10-3 | 3.97869 x 10-3 |
| B (°C-2) | -5.77500 x 10-7 | -5.83397 x 10-7 | -5.86863 x 10-7 |
| C (°C-4) | -4.18301 x 10-12 | -4.29000 x 10-12 | -4.16696 x 10-12 |
| Alpha (°C-1) | 3.850 x 10-3 | 3.900 x 10-3 | 3.920 x 10-3 |
| Delta (°C) | 1.49990 | 1.49589 | 1.4971 |
| Beta (°C) | 0.10863 | 0.11000 | 0.10630 |
(NOTE: “Pt” Alpha value is 3.85055 x 10⁻-3 for calculation purposes)
The constants A, B, and C can be written:
- A = Alpha x { 1+ (Delta/100)} °C-1
- B = -Alpha x Delta x 10-4 °C-2
- C = -Alpha X Beta x 10-8 °C-4
Alpha (α) is the temperature coefficient of resistance obtained by measuring the detector resistance at both 0° & 100°C and is defined as:
- Alpha = (R100 – R0)/(100 x R0)
Delta(δ) is obtained by calibration at a high temperature, for example, the Freezing Point of Indium, Tin, Zinc, or Aluminum (156.5985°, 231.928°, 419.527°, and 660.323°C respectively).
Beta (β) is obtained by calibration at a negative temperature, for example, Triple Point of Mercury and Argon (-38.8344° and -189.3442°C respectively) or Liquid Nitrogen (approximately -196°C).
Choosing the high and low temperature point which best suites your application range improves the R vs. T correlation when applying the formulas.
Using Alpha, Delta, and Beta, the Callendar-van Dusen equation can alternately be written:
W(t90) = R(t90)/R(0) = { 1 + α{t90 – δ(t90/100)(t90/100 – 1) – β(t90/100)3(t90/100 – 1)]}
(β is equal to 0 when t90 is greater than zero)
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