RTD CHARACTERISTICS TRAINER

Transcription

1 RTD CHARACTERISTICS TRAINER (Model No : ITB - 06CE) User Manual Version 1.0 Technical Clarification /Suggestion : / Technical Support Division, Vi Microsystems Pvt. Ltd., Plot No :75,Electronics Estate, Perungudi,Chennai ,INDIA. Ph: , Mail : service@vimicrosystems.com, Web :

2 CONTENTS 1. INTRODUCTION 1 2. TEMPERATURE MEASUREMENT 1 3. TECHNICAL SPECIFICATION 1 4. FRONT PANEL DIAGRAM 3 5. FRONT PANEL DESCRIPTION 4 6. RESISTANCE TEMPERATURE DETECTORS 4 7. ELECTRICAL-RESISTANCE SENSORS 7 8. CIRCUIT DESCRIPTION DATA SHEET EXPERIMENTAL SECTION 16

3 1. INTRODUCTION Temperature measurement is one of the earliest areas of metrology and its use in control and instrumentation is significant. This being one of the most important concept, we have released this ITB-06CE card in our series of Instrumentation Trainer Boards, which would help the students to bring out their ideas in a very simple way. 2. TEMPERATURE MEASUREMENT There are, in general, four types of sensors based on the following physical properties, which are temperature dependent: 1. Expansion of a substance with temperature, which produces a change in length, volume or pressure. In it's simplest form this is the common mercury-in-glass or alcohol-in-glass thermometer. 2. Changes in contact potential between dissimilar metals with temperature, thermocouple. 3. Changes in radiated energy with temperature, optical and radiation pyrometers. 4. Changes in electrical resistance with temperature, used in resistance thermometers and thermistors. The fourth property is used in our design, to create a sensor. Resistance thermometry requires a resistor properly mounted to create a sensor and by means of measuring the resistance of the sensor. 3. TECHNICAL SPECIFICATIONS RTD SENSOR Type : Pt100 0 C) Material : Platinum (Protected by stainless steel sheath) Resolution : to 0.39 / C Accuracy : ±0.6@100 C Temperature Range : -200 to 850 C Tube size : 120mm 8mm Vi Microsystems Pvt. Ltd., [ 1 ]

4 Connection through two core retractable lead (1 Meter Extended) Thermowell material : Stainless steel Coating : Nickel, chromium ITB-006CE Operating Temperature : 10 C - 55 C Cabinet Material : Mild Steel with powder coating Accuracy : 0.7% Full Scale Deflection Linearity : 1.8% Full Scale Deflection Dimension : 370mm 280mm 90mm LED DISPLAY Size : mm Type : Common Anode Display : 3.5 Digit Segment : Seven Segment Colour : Red POWER SUPPLY Input : 230V AC / 50Hz Output : +5V / 1A -5V / 500mA +12V / 500mA -12V / 500mA Vi Microsystems Pvt. Ltd., [ 2 ]

5 4. FRONT PANEL DIAGRAM T1 T3 Sw1 T2 T4 POWER ON/OFF EXT INT Sw2 RTD CHARACTERISTICS TRAINER ( ITB - 06CE ) WHEAT STONE BRIDGE DIFFERENTIAL AMPLIFIER T6 T5 C GAIN AMPLIFIER ZERO Vi Microsystems Pvt. Ltd., [ 3 ]

6 5. FRONT PANEL DESCRIPTION Power ON/OFF Switch : To Switch ON / OFF the unit. T1 & T2 : These are used to connect the two terminals of RTD. SW1 : To select the resistance mode. T3 & T4 : To measure the resistance value of RTD. SW2 : This switch is used to select the internal / External mode of operation. INT - place the switch SW2 towards downward. EXT - place the switch SW2 towards upward. T5 : To measure the differential amplifier output voltage. T6 : To measure the signal conditioner output voltage. T7 : This terminal is the common GND terminal. 3.5 LED Display : It displays the RTD sensor output interims of C. 6. RESISTANCE TEMPERATURE DETECTORS Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger their resistance. They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second. They are among the most precise temperature sensors available with resolution and measurement uncertainties or ±0.1 C or better possible in special designs. Usually they are provided encapsulated in probes for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function, such as a temperature controller or precision thermostat. The advantages of RTDs include stable output for long period of time, ease of recalibration and accurate readings over relatively narrow temperature spans. They are active devices requiring an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated read-out device. Vi Microsystems Pvt. Ltd., [ 4 ]

7 The lead wires used to connect the RTD to a readout can contribute to the measurement error, especially when there are long lead lengths involved, as often happens in remote temperature measurement locations. Those calculations are straight forward and there exist 3-wire and 4-wire designs to help minimize or limit such errors, when needed. Often the lead error can be minimized through use of a temperature transmitter mounted close to the RTD. Transmitters convert the resistance measurement to an analog current or serial digital signal that can be sent long distances by wire or rf to a data acquisition or control system and/or indicator. RTDs, as mentioned above, work in a relatively small temperature domain, compared to thermocouple, typically from about -200 C to a practical maximum of about 650 C to 700 C. RTDs can be made cheaply in Copper and Nickel, but the latter have restricted ranges because of non-linearities and wire oxidation problems in the case of Copper. Platinum is the preferred material for precision measurement because in its pure form the Temperature Coefficient of Resistance is nearly linear; enough so that temperature measurements with precision of ±0.1 C can be readily achieved with moderately priced devices. Better resolution is possible, but equipment costs escalate rapidly at smaller error levels. All RTDs used in precise temperature measurements are made of Platinum and they are sometimes called PRTs to distinguish them. RTD works on the principle that electrical resistance of the most metals increases linearly with temperature. If a metal wire has a Resistance R 0 at 0 C, then the resistance at T C will be given by: The constant is called the temperature coefficient of resistance. Typical values are: METALS Platinum Copper Nickel A temperature transducer using the above principle is called Resistance Temperature Detectors (RTD). These are simple to use, requiring no special wiring, are highly stable and very sensitive. RTD's commonly use Platinum, nickel and copper to form the sensor (see Figure-2), although iron, tungsten and alloys can be used. The former metals have the advantage that they can be obtained to high degrees of chemical purity. Vi Microsystems Pvt. Ltd., [ 5 ]

8 COPPER SENSORS are an obvious candidate, as copper is readily available at high degree of purity and is quite linear. High temperature measurements are limited to about 100 C. Owing to the low resistivity of copper, long and hence bulky lengths of fine wire are required and the resulting relatively low resistance obtained makes accurate measurement difficult. Copper is also very susceptible to oxidation and corrosion. Figure - 2 NICKEL SENSORS are appreciably non-linear, restraining measurements to less than 300 C, although passive networks can be added to give linear outputs with temperature. However, temperature cycling through the point at 358 C causes resistance instability. The resistivity is higher allowing smaller devices to be reconstructed. As there is a little international agreement it's wider use is limited. Oxidation and corrosion is a problem. PLATINUM SENSORS dominate resistance pyrometer. Platinum is stable, resists corrosion and oxidation, is malleable, has high melting point and a high degree of resistivity, and can be made to a high degree of purity. All these are achieved at the expense of high manufacturing costs and careful mounting to avoid strain gauge effects. Figure - 3 Vi Microsystems Pvt. Ltd., [ 6 ]

9 Platinum-based resistance thermometers are probably the most widely used. Measurements can be made from -220 C to 850 C readily to a stability of hundreds of a degree over several years. RTD is specified in terms of its resistance at 0 C, and change in resistance from 0 C to 100 C. This is known as the "fundamental interval". Platinum RTDs are constructed with a resistance of 100 ohm at 0 C (and are often referred to as Pt100 sensors) is used in our design. This gives a resistance of ohm at 100 C, and hence a fundamental interval of 38.5 ohm. Pt100 sensors can be used over a temperature range of C to 800 C. RTD's are available in many shapes and sizes. Figure-4 shows various construction of RTDSs. These are designed to protect the wire from mechanical shocks while not applying any stress on the wire (which would cause resistance changes in a similar way to a strain gauge). Construction of the wire to be in direct contact with the fluid gives a faster response, but little protection against corrosion is necessary. The sensors in Figure-4 are totally enclosed, but the increased mass gives longer time constants. Figure ELECTRICAL-RESISTANCE SENSORS The electrical resistance of various materials changes in a reproducible manner with temperature, thus forming the basis of a temperature-sensing method. Materials in actual use fall into two main classes: conductors (metals) and semiconductors. Conducting materials historically came first and traditionally have been called resistance. Semiconductor types have been given the generic name thermistor. Any of the various established techniques of resistance measurement may be employed to measure the resistance of these devices, with both bridge and "ohmmeter" methods being common. Vi Microsystems Pvt. Ltd., [ 7 ]

10 7.1 Conductive Sensors (Resistance Thermometers) The variation of resistance R with temperature T for most metallic materials can be represented by an equation of the form 2 R R ( 1 a T a T... a T ) where R 0 is the resistance at temperature T = 0. The number of terms necessary depends on the material, the accuracy required, and the temperature range to be covered. Platinum, nickel, and copper are the most commonly used. Sensing elements are made in a number of different forms. For measurement of fluid temperatures, the winding of resistance wire may be encased in a stainless steel bulb to protect it from corrosive liquids or gases. Open-type pickups expose the resistance winding directly to the fluid (which must be non corrosive) and give faster response. Bridge circuits used with resistance temperature sensors may employ either the deflection mode or operation or the null model. While the resistance/temperature variation of the sensing element may be quite linear, the output voltage signal of a bridge used in the deflection mode is not necessarily linear for large percentage changes in resistance. Resistance-thermometer bridges may be excited with either AC or DC voltages. The direct or rms alternating current through the thermometer is usually in the range 2 to 20mA. This current causes an I 2 R heating which raises the temperature of the thermometer above its surroundings, causing the so-called self-heating error. The magnitude of this error depends also on heat-transfer conditions and usually is quite small. Platinum can withstand high temperatures while maintaining excellent stability. As a nobel metal, it shows limited susceptibility to contamination. All metals produce a positive change in resistance with temperature. This, of course, is the main function of an RTD. The system error is minimized when the nominal value of RTD is large. This implies a metal with a high value of resistivity should be used for RTDs. The lower is the resistivity of the metal, the more material we will have to use. The requirements of a conductor material to be used in RTDs are: 1. The change in resistance of material per unit change in temperature should be as large as possible. 2. The material should have a high value of resistivity so that minimum volume of material is used for the construction of RTD. 3. The resistance of materials should have a continuous and stable relationship with temperature. The most common RTDs are made of either platinum nickel or nickel allows. The economical nickel wires are used over a limited temperature range. They are quite non-linear and tend to drift with time. For measurement integrity, platinum is the obvious choice. n n Vi Microsystems Pvt. Ltd., [ 8 ]

11 An examination of the resistance versus temperature curves of Fig.1 shows that the curves are nearly linear. In fact, when only short temperature spans are considered, the linearity is more evident. This fact is employed to develop approximate analytical equations for resistance versus temperature for a particular metal. 7.2 Linear Approximation A linear approximation means that we may develop an equation for a straight line which approximates the resistance versus temperature curve over a specified span. A straight line has been drawn between the points of the curve which represent 1 C and C with 0 C representing the mid point temperature. The equation of this straight line is the linear approximation of the curve from 1 C to 2 C. The equation of the straight line is written as: where R = R 0 (1 + 0 ) with 1 < 0 < 2 R = approximate resistance at C ; R 0 = approximate resistance at 0 C ; = - 0 = change in temperature ; C 7.3 Quadratic Approximation A quadratic approximation of the resistance temperature curve is more accurate representation of the curve over a limited range of temperature. The quadratic approximation relationship includes both a linear term as in Equation and an addition has a term which varies as the square of the difference in temperature. The approximation is written as: where 1 = linear fractional change in resistance, / C and 2 = quadratic fractional change in resistance, /( C) 2 RTD exhibits change in resistance with temperature. Before it can be used for measurement or control, this change in resistance must be converted to a change in voltage or current. The electrical power dissipated in the RTD for this conversion must be strictly limited to avoid errors due to I 2 R heating of the sensor. Typically 10mW dissipation will cause the temperature rise of 0.3 C, which implies low values current (less than 10mA) and voltage (below 1V). The commonest circuits, however are based on Wheatstone bridge of Figure-5. If the measuring circuit has high impedance (so that it does not load the bridge), simple circuit analysis shows that: Vi Microsystems Pvt. Ltd., [ 9 ]

12 Figure - 5 Unfortunately, because Rt appears in both the numerator and denominator of the left hand term, V does not change linearly with changes in Rt. There are three common ways of overcoming this non linearity ( Shown in Figure-6 ) This non-linearity can be reduced to acceptable levels by making R3 >> Rt and R1 >> R2 (typically by a factor of 100). This has the side effect of reducing the bridge voltage by a factor of 100 as well, but this can easily be re-established by a means of a DC amplifier. Figure - 6 Vi Microsystems Pvt. Ltd., [ 10 ]

13 Figure - 7 In Figure-7, the non-linear output from the bridge is processed by a suitable line arising circuit to give an output voltage which is linearly related to the temperature. The line arising can be performed by an op-amp circuit. In most industrial applications, the RTD will be situated remote from it's measurement electronics. If the connecting leads are more than a few meters in length, they will introduce an unknown resistance, "r", into each lead as shown in Figure-8. Figure - 8 Vi Microsystems Pvt. Ltd., [ 11 ]

14 Figure - 9 This unknown resistance is itself subject to change caused by temperature and strain gauge effects and is a possible source of error. This is overcome by using a four-wire connection to the RTD as shown in Figure-9. Each lead will experience the same conditions, so the changes introduced into the RTD will be matched by the changes in temperature of the RTD, not the lead. Figure-9 shows the commonest industrial circuit for RTD. Figure - 10 Vi Microsystems Pvt. Ltd., [ 12 ]

15 Figure - 11 Vi Microsystems Pvt. Ltd., [ 13 ]

16 8. CIRCUIT DESCRIPTION In ITB-006CE unit the Pt100 type of RTD is used to measure temperature in terms of resistance. Platinum sensor is preferred here, because it highly stable, resists corrosion and oxidation, it is malleable and has high melting point and also has high degree of sensitivity compared to other RTD sensors. As said earlier the output of the RTD is in the form of resistance. The resistance of the RTD varies with temperature. This is then converted in to voltage or current. The output of RTD is interms of resistance. The resistance of RTD varies with temperature. This resistance is then converted to voltage by means of a wheat stone bridge. The wheat stone bridge gibes the output voltage interms of MV. Since the output is interms of mv, it should be amplified to a comfortable range of 0-5V by using the signal conditioner amplifier. The output of this amplifier is given to the LED display, which display the output in the form of temperature with C. The block diagram is given below. POWER SUPPLY RTD SENSOR Wheat stone Bridge mv Signal Conditioner 0-5V DC Display Figure - 12 Block Diagram of ITB-006CE Unit Vi Microsystems Pvt. Ltd., [ 14 ]

17 9. DATA SHEET Sensor Type ohm Platinum RTD Celsius Temp Resis Temp Resis Temp Resis Temp Resis Temp Resis Vi Microsystems Pvt. Ltd., [ 15 ]

18 10. EXPERIMENTAL SECTION EXPERIMENT-1 AIM To study the temperature Vs resistance characteristics of RTD (Pt100) APPARATUS REQUIRED 1. ITB-06CE 2. RTD sensor 3. Water Bath 4. Thermometer 5. Multimeter (optional) PROCEDURE 1. Patch the wires of RTD to the T1 and T2 terminal of the RTD input block and switch ON the unit. 2. Place the RTD and thermometer into the holes provides in the waterbath. 3. Keep the SW1 in right direction. 4. Place the multimeter in the resistance mode across T3 and T4 terminals. 5. Switch ON the waterbath and note the temperature in thermometer and corresponding resistance value in multimeter. 6. Plot the temperature Vs resistance graph. This gives the characteristic curve of the RTD. Refer to the model graph. TABULAR COLUMN Temperature ( C) 0 C C Resistance ( ) Vi Microsystems Pvt. Ltd., [ 16 ]

19 MODEL GRAPH The graph between Temperature and Resistance are drawn. TEMPERATURE ( C) VS RESISTANCE TEMPERATURE ( oc ) VS RESISTANCE RESISTANCE ( ) TEMPERATURE 0 o C 100 ( ) TEMPERATURE 100 oc 138 TEMPERATURE ( oc ) SAMPLE READING Temperature 0 C 100( ) Temperature 100 C 138.5( ) RESULT Thus the study of Temperature Vs Resistance characteristics was studied and graph is plotted. PRECAUTIONS 1. Gradually heat the water and note the corresponding resistance simultaneously. 2. The multimeter / ohmmeter should be in the range (0-200) to measure for (0-100) C. Vi Microsystems Pvt. Ltd., [ 17 ]

20 EXPERIMENT - 2 AIM To study the temperature Vs voltage characteristics and the accuracy of the signal conditioning board. APPARATUS REQUIRED 1. ITB-06CE 2. RTD sensor 3. Water Bath 4. Thermometer 5. Multimeter (optional) 6. Power FORMULA TO BE USED PROCEDURE Actual Temperature Displayed Temperature % Error * 100 Full Scale Division 1. Patch the wires of RTD to the T1 and T2 terminal of the RTD input block. 2. Switch ON the ITB-06CE Unit. 3. Keep the switch SW1 in left direction and switch SW2 in external mode. 4. Now adjust the Zero Potentiometer to read 0 C at the display. This is done for initial setup of the unit and this adjustment should be left undisturbed. 5. Place the multimeter in voltage mode across the T6 and T7 terminals. 6. Insert the RTD and thermometer into the waterbath and note the temperature without any heating at ambient condition. 7. Switch ON the waterbath and note down the actual temperature in thermometer, output voltage of the unit and the displayed temperature simultaneously. 8. Plot the graph for Actual Temperature Vs Voltage. 9. Calculate the % error and plot the graph for Temperature Vs % Error. The first graph measures the linearity of the signal conditioning unit and the second graph Vi Microsystems Pvt. Ltd., [ 18 ]

21 measures the accuracy. TABULAR COLUMN Actual Temperature ( C) Output Voltage (V) Displayed Temperature ( C) %Error MODEL GRAPH 1. The graph between temperature Vs voltage are drawn. TEMPERATURE ( oc ) VS OUTPUT VOLTAGE ( V ) OUTPUT VOLTAGE ( V ) TEMPERATURE : 100 o C OUTPUT : 5 V TEMPERATURE ( oc ) 2. The graph between temperature and % Error are drawn. Vi Microsystems Pvt. Ltd., [ 19 ]

22 RESULT Thus the study of Temperature Vs Voltage and the accuracy of signal conditioning board was studied and the graph is drawn. Vi Microsystems Pvt. Ltd., [ 20 ]