Final Report
Our goal is to design a system that would be used to measure the temperature by changing resistances. The system would work in a solid medium but we would also try to make it functional in liquid mediums such as oil, fuels and such other liquids. This instrument should be able to measure the temperature of the liquids from -259.15 °C to 400 °C and we should have a resolution of ±0.018 V.
Instrument Details
The RTD is made of platinum coated nickel and should be attachable to the oil tank. i.e, it should be resistant to vibration, high pressure, oil and heat (e.g. temperature up to 1000 °C). The cost should not exceed AED 770.4 = $214.
The RTD we have selected is manufactured by Lakeshore Cryotronics and its product code is PT-111 (see figure 1). It is wire wound and high precision with maximum power dissipation of 10 µW. This sensor is suitable for our application both electrically and Wetted Area (see figure 2).
Instrument Analysis
PT-111 Temperature Resistance specifications of the instrument (see figure 3)
Parameter Value
Input voltage 6V
Sensitivity 0.45 ohm\k
Range -260 to 400 C
Repeatability ±10 mK (77 K to 305 K)
Stability
Short-term ± ±10 mK (77 K to 305 K).
Long-term (per year) (1) ±10 mK at 77 K
Accuracy (calibrated) ±20 mK at 100 K; ±35 mK at 300 K
Units range (ohms) 0 to 300
The characteristics of the output performance of the transducer are the range of output, linearity, repeatability, sensitivity and frequency bandwidth among other. The report will however focus on the output range, frequency bandwidth and linearity. From the information given, the output range of the device is a temperature of -259.15 Celsius to 400 Celsius. The device that is an RTD platinum coated with nickel is relatively large and practical for the case. The graph of resistance versus temperature is a near linear temperature coefficient (see figure 3). The non-linearity of the transducer at the initial temperature could increase the errors incurred during temperature measurement. The frequency bandwidth of the device ranges from 50 Hz to 100 Hz that is typical for platinum RTD. Secondly, when the transducer does not produce any voltage change, a current excitation is required. The current passed through the RTD creates a voltage change across the device. The RTD transducer is an active type of transducer. The RTD resistance varies as temperature changes. Therefore, as temperature increases from low to high the device detects a change. The process is called ‘Signal Conditioning’ (see figure 4). The voltage generated determines the resistance and hence the temperature change. Similarly, the circuit detail of the operation is also illustrated below (see figure 5). The power dissipation is calculated using Equation 1. Note that all the equations used below are in Appendix 1.P=V2R
P= (6X6X300)
= 10.8kW
The PT100 has a self heating value of 0.2KΩ/mW at 0°C. Applying the temperature coefficient the effect equates to 747°C/W as the S value.Alpha, = 0.0039 ohms/ohm/°C
Value of S = 770k1000(k) *(1- 0.0039)*1000mW/W
= 747°C/W.
Therefore using Equation 2, T= 0.75oC
Thevenin Voltage
Using equation (4) Vin= 5V
Using equation (5) Vout = 3.3 V
Thevenin Resistance
Using equation (6)
At 10oC RT = 0
At 400oC RT = 255k
Amplifier Design. The amplifier chosen is a two OP-Amplifier. The device has an ‘In Amplifier’ and an ‘Out Amplifier’. The amplifier has the following specifications: a resistance of 1kW, a current of 1mA, VREF of 2.5 V and RREF=2.5 V. The amplifier input resistance minimizes the amount of current flowing through the device for desired voltage input. The output resistance increases the signal of the RTD to ensure a higher quality signal. The overall gain of the amplifier increases the device accuracy as well. The gain bandwidth increases the frequency bandwidth for more precise and accurate designs. A simple amplifier for conditioning purposes is illustrated below (see figure 6).Input Resistance, Rin
Using equation 8
Rin= (6/1.8)
Rin = 3.33k
Output Resistance, Rout
Using equation 7
Vout=6[153/300-35/300]
Vout =2.36V
Rout = 2.36/1.8= 1.4k
Overall gain=2.36/6= 0.39
Thus the amplifier has a minimal noise ratio.
Conclusion
The type of RTD in use is the platinum-nickel coated type. The device is linear since its resistance changes linearly proportional to temperature. The RTD selected consists of fine platinum wrapped around a ceramic or glass core. The material of the wire is pure to increase the accuracy of RTD. The temperature range of device is -259.15 Celsius to 400 Celsius, which is a wider range compared to other devices such as a thermistor. The calibrated accuracy of the device at 100K is 0.002 while at 300K it is 0.035. The smallest voltage change detected by the sensor is 0.018V, and it contributes to the total accuracy of the device. Therefore, the device has a very high accuracy level and error incurred while measuring the temperature is minimal. The high accuracy of the device makes it a better selection compared to any other device.
The material of the RTD is platinum coated with nickel indicating a wide temperature range (see figure 3). The combination of the platinum and nickel is hence crucial and makes the device more practical. The unique properties of platinum coated with Nickel makes it desirable for temperatures between -259.15 °C to 400 °C, used in the sensors. The design is stable with thermal exposure of 77K to 305K for the short term. However, for the long term it is more stable at temperatures of 77K. Hence the device provides long term stability compared to short term stability. The RTD device is more stable at lower temperature compared to higher temperatures (see figure 3). The resistance of the device shows non-linearity when subjected to temperatures below 70K. On the other hand, at temperatures above 77K the curve becomes linear.
The performance of the amplifier is suitable. The calculated input resistance and output resistance varies slightly with the theoretical values of the suggested amplifier. The two wire system of the amplifier is an advantage as it reduces the sensitivity and accuracy of the whole system. The amplifier improves its performance by using three wire methods. The circuit makes the system linear by reducing the overall resistance, thereby increasing the overall gain. The device has a low sensitivity of 0.45W/K. The resistance range of 0 to 300 ohms allows measurement of both low and high temperatures. The dimensions of the RTD are standard which result in increased accuracy, precision and repeatability. The price of the device however is relatively high. However, since the device is practical, it is viable.
Appendices
Appendix 1: Equations
Value of S=heatingvalue1000(k) * (/oC)*1000mW/W (1)
=R400-R101000Ro (2)
T=P X S (3)
Vin= R1R2+R1 .V1 (4)
Vout= Vb2{R/2} 1+ R2R1 (5)
RT= RO [1+T] (6)
Vout=Vin[R2R1+R2-RgR1+R2] (7)
Rin= Vin/ (8)
Vin= cRC (9)
Gain=VoutVin (10)
Appendix 2: Diagrams and Graphs
Figure 1: Diagram of RTD (Lakeshore, PT-111)
Figure 2: Diagram of sensor suitability for application
Figure 3: PT specification graph (Resistance vs Temperature)
Figure 4: Signal Conditioning Process Diagram
Figure 5: Signal Conditioning Circuit Detail
Figure 6: Simple Amplifier Circuit (For Conditioning)
Figure 7: Current Generator Circuit
