precise temperature control,
no one can give you the
level of accuracy, repeatability
and reliability you demand
than SDI. SDI elements are
manufactured and tested to meet
the most rigorous international
standards, so your product
or process performs the way it
was designed to - every time.
Product Information
- Tolerance and Interchangeability
- Temperature Range
- Point of Calibration
- Stability
- Self-Heating
- Vibration
- Pressure
- Insulation Resistance
- Thermal EMF's
- Screening and Electrical Interference
- Immersion Errors and Stem Conduction
- Assembly
- Lead Extensions
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
Detectors are supplied with standard 10 mm length leads of platinum or platinum group metal. Resistance value and tolerance is realized 5 mm from the ceramic body. The resistance of typical leads are:
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.
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.
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.
Properly supported, detectors will withstand a vibration level of 30 g over the frequency range of 10 Hz to 1 kHz.
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.
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.
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
For a 25 mm long detector contained
in a stainless steel sheath, varying the immersion length from 3 to 4 inches
in flowing water causes a resistance change of 3 milliohms. The differential
for a variation from 4 to 5 inches is reduced to 1.4 milliohms. Therefore
a minimum immersion depth of 6 inches is recommended.
Recommended methods for connection to the lead wires are welding, and tin or silver soldering. Solder or braze materials containing lead should not be used. Risk of flux contamination restricts the use of brazing techniques. All materials used must be capable of operating over the temperature range required.
All of SDI's RTD elements can be supplied with extended leads in 2, 3, or 4-wire configurations for easy installation. For manufacturers this translates into lower assembly, inventory and material costs.
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.