FACTORS AFFECTING TEMPERATURE MEASUREMENT ACCURACY.
The reading accuracy of some temperature measuring devices like the thermometer are dependent on the size of the hole, the length of the tube and the fluid inside it. Other temperature measuring devices have bimetallic elements that can be extremely accurate.
The actual construction of the resistor material can also influence in the accuracy of temperature measuring devices.
Another major factor is the selection of the circuitry to read the device. If insufficient current is flowed thru the device, external noise will be a problem bec ause the signal levels will be very low. If the c urrent is too high, the probe will start dissipating heat, artificially shifting the temperature reading.
Another significant factor is the linearization of the met er. Most meters will use some type of polynomial curve fit algorithm to create a calibrated formula of temperature vs resistance.
2.
Some temperature measuring devices’ accuracy is influence by the purity of the w ire and the wire junctions.
LIQUID GLASS THERMOMETER
This thermometer uses the process known as we t viscosity. In this process it is important to know the precise temperature of the water bath. The glass thermometer is still used because of it extreme repeatability. These specialized thermometers have a bore that narrows at a particular point. In t his way it can expand a two degree temperature range in the middle of its scale to approximately two inches long, allowing readings down to a fraction of a tenth of a degree C.
3. FILLED SYSTEM THERMOMETER
The operation of a filledsystem thermometer is based on one of three principles: the th ermal expansion of a liquid, thetemperature dependence of the pressure of a gas, or o r th e temperature dependence of the saturated vapor pressure of aliquid.
4. THERMOCOUPLE
The thermocouple is an extremely versatile device. Since the measurement of the temperature occurs only at the actual interface between the two metals, the measurement area can be as large or as small as one chooses. Most thermocouples today are made from two pieces of dissimilar wire, welded together in a be ad. This junction can be as large or small as desired, simply by selecting the appropriate sized sized wire. Thermocouples can be created by physically connecting the two m etals together as well as welding them. The only requirement is that the two metals be in good physical contact. If one is not careful with wire insulation, a spot of missing insulation
Jeffrey Marvin E. Villasis
BSME 4
can quickly become the new thermocouple, rat her than the welded thermocouple that is inserted into the process. 5.
THERMOCOUPLE EFFECTS
Seebeck coefficient
is the change in material EMF with re spect to a change in
temperature.
Peltier effect describes
the temperature difference generated by EMF.
Thomson effect relates
the reversible thermal gradient and EMF in a homogeneous
conductor. 6.
THERMOCOUPLE RANGE AND CHARACTERISTICS
Since the thermocouple is actually just a pair of wires welded together, it is possible to use these outside the stated oper ating range. The physical limit is based on the melting point of the wire. There is no calibration for values outside the operating range, and field calibration will have to be used. Accuracy of t hermocouples is based on the purity of the wire and the wire junction. In previous years thermocouples were welded using a mercury bath. This has been replaced with car bon block welders operating under inert gas. Each type of wire has its own limits of error based on materials deviations. There are also special wires available that have been manufactured and tested at much tighter compositions.
