accommodate several sensors at once. The ice point is designated as a secondary standard because its accuracy is ±0.005°C (±0.009°F), compared to ±0.001°C (±0.0018°F) for primary fixed points. Comparison calibrations, commonly used with secondary SPRTs and industrial RTDs, the thermometers being calibrated are compared to calibrated thermometers by means of a bath whose temperature is uniformly stable. Unlike fixed point calibrations, comparisons can be made at any temperature between –100°C and 500°C (–148°F to 932°F). This method might be more cost-effective since several sensors can be calibrated simultaneously with automated equipment. These electrically heated and well-stirred baths use silicone oils and molten salts as the medium for the various calibration temperatures. 2.3.3. Element types There are three main categories of RTD sensors; Thin Film, Wire-Wound, and Coiled Elements. While these types are the ones most widely used in industry there are some places where other more exotic shapes are used, for example carbon resistors are used at ultra low temperatures (-173 °C to -273 °C). Carbon resistor elements are widely available and are very inexpensive. They have very reproducible results at low temperatures. They are the most reliable form at extremely low temperatures. They generally do not suffer from significant or strain gauge effects. Strain free elements use a wire coil minimally supported within a sealed housing filled with an inert gas. These sensors are used up to 961.78 °C and are used in the SPRT’s that define ITS-90. They consisted of platinum wire loosely coiled over a support structure so the element is free to expand and contract with temperature, but it is very susceptible to shock and vibration as the loops of platinum can sway back and forth causing deformation. Thin film elements have a sensing element that is formed by depositing a very thin layer of resistive material, normal platinum, on a ceramic ; This layer is usually just 10 to 100 angstroms (1 to 10 nanometers) thick. This film is then coated with an epoxy or glass that helps protect the deposited film and also acts as a strain relief for the external lead-wires. Disadvantages of this type are that they are not as stable as their wire wound or coiled
Repeatability: Ability to maintain R vs T under the same conditions after experiencing thermal cycling throughout a specified temperature range. Hysteresis: Change in the characteristics of the materials from which the RTD is built due to exposures to varying temperatures. Stem Conduction: Error that results from the PRT sheath conducting heat into or out of the process. Calibration/Interpolation: Errors that occur due to calibration uncertainty at the cal points, or between cal point due to propagation of uncertainty or curve fit errors. Lead Wire: Errors that occur because a 4 wire or 3 wire measurement is not used, this is greatly increased by higher gauge wire. 2 wire connection adds lead resistance in series with PRT element. 3 wire connection relies on all 3 leads having equal resistance. Self Heating: Error produced by the heating of the PRT element due to the power applied. Time Response: Errors are produced during temperature transients because the PRT cannot respond to changes fast enough. Thermal EMF: Thermal EMF errors are produced by the EMF adding to or subtracting from the applied sensing voltage, primarily in DC systems. 2.3.6. Construction Figure 2.6: Construction of an RTD (Wikipedia.org) These elements nearly always require insulated leads attached. At temperatures below about 250 °C PVC, silicon rubber or PTFE insulators are used. Above this, glass fibre or ceramic are used. The measuring point, and usually most of the leads, require 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. 2.3.7. Wiring Configuration Two-wire configuration Figure 2.7: Two-wire configuration of an RTD
heat transfer can be ventilation, reduction of copper layers between the SHT1x and the rest of the PCB or milling a slit into the PCB around the sensor (see Figure 3.7). Figure 3.7: : Top view of example of mounted SHT1x with slits milled into PCB to minimize heat transfer. 3.2.7. Light The SHT1x is not light sensitive. Prolonged direct exposure to sunshine or strong UV radiation may age the housing. 3.2.8. Membranes SHT1x does not contain a membrane at the sensor opening. However, a membrane may be added to prevent dirt and droplets from entering the housing and to protect the sensor. It will also reduce peak concentrations of chemical vapors. For optimal response times the airvolume behind the membrane must be kept minimal. 3.2.9. Interface Specifications Table 3.1: SHT1x pin assignment, NC remain float The supply voltage of SHT1x must be in the range of 2.4 – 5.5V, recommended supply voltage is 3.3V. Power supply pins Supply Voltage (VDD) and Ground (GND) must be decoupled with a 100 nF capacitor – see Figure 29. The serial interface of the SHT1x is optimized for sensor readout and effective power consumption. The sensor cannot be addressed by I2C protocol; however, the sensor can be connected to an I2C bus without interference with other devices connected to the bus. The controller must switch between the protocols. Figure 3.8: Typical application circuit, including pull up resistor RP and decoupling of VDD and GND by a capacitor.
tool is a compact, readable ANSI C code, with support to local extensions of the standard C, as well as automatically generated design documentation. MicroC uses an Operating System Implementation (OSI) definition to describe the implementation of the software and hardware target environment for a given design. Any one OSI might support only a subset of the design concepts referred to above. As a general rule, the tool tries to make use of any such design aspect/concept it encounters in the model. If the given OSI has no support for that design aspect/concept, an error message is produced. It is designed to provide the programmer with the easiest possible solution to developing applications for embedded systems, without compromising performance or control. Features Write your C source code using the built-in Code Editor (Code and Parameter Assistants, Code Folding, Syntax Highlighting, Auto Correct, Code Templates, and more.) Use included mikroC PRO for PIClibraries to dramatically speed up the devel opment: data acquisition, memory, displays, conversions, communication etc. Monitor your program structure, variables, and functions in the Code Explorer. Generate commented, human-readable assembly, and standard HEX compati ble with all programmers. 4.3. Basic Flow Diagram The flow diagram of the program is shown in figure 4.1:
8. REFERENCES 1. İbrahim Doğan (2011). Microcontroller for Students – The Theory. 2. İbrahim Doğan (2011). Microcontroller for Students – Practicals. 3. mikroelektronika microC pro for PIC user manual 5. 7. 8. 9. 10. 11. 12. 13.