Calibration of Thermocouples and understanding transducers. e2v. This is a CCD. James Morrison. Clacton County High School

Transcription

1 Calibration of Thermocouples and understanding transducers e2v This is a CCD James Morrison Clacton County High School

2 Contents Abstract... 1 Introduction to e2v... 2 The Work Placement Temperature Transducers... 3 T-Type Thermocouple... 3 Hazards/Risk Assessments... 4 Method... 6 Calculation for Standard deviation Results Analysis Conclusion Step By Step Programming Method Glossary Glossary Appendix Acknowledgements Abstract At e2v the task provided was around testing and calibrating temperature transducers, which in this case were T type thermocouples. These transducers will then be used in the profiling of CCD (Charge couple device) projects. 1

3 Introduction to e2v e2v is a technology company which specialises in six different areas of science: Civil aerospace (Non-military aircraft) Defence (High performance Imaging) Space (Supplies NASA and ESA with sensors) Industrial (Radar systems and Navigation) Medical & Science (Radiotherapy Treatment machines) Security & Rescue ( Thermal imaging cameras) These are just a few of the products e2v make, test and distribute to their customers. They were established in 1947 to develop and manufacture radar components. e2v employ around 1,650 people and have annual revenue of 220 million. They have design and operational facilities across Europe, North America and Asia, and have a global network of sales and technical support offices. They have been involved in major projects such as the Hubble telescope upgrade, in which they fitted specialist sensors which allowed detailed pictures of the universe to be created, and analysed by NASA. In August and 2014 a Lander Rosetta supplied by e2v with five specialist sensors, landed at comet67p. This was a ten year journey and was the first rendezvous made with a comet. The sensors have to go through all specialist tests. e2v have lots of facilities used for testing the products, they have three different test chambers which are used to test products to extremes. This allows the company to simulate the environment of space and allow them to see if the sensors still worked after going through extreme conditions. As the sensors were launched they can experience a force upwards of 1500G, to compare this with a human, the average force a human can withstand is around 15G depending on the human. On a rollercoaster on a tight bend humans can experience upwards force of 4-6G. Therefore these facilities allowed them to simulate this, and let them see whether their sensors will withstand the force and fully restore to their usual selves. The chambers used liquid nitrogen to rapidly cool products and used electricity to rapidly to heat products from a range of around -160 C to 220 C; another chamber also allows them to change humidity and pressure of the air to simulate aircraft in flight. Finally In the year 2015, e2v provided sensors for a NASA satellite to be sent out into space and capture high resolution pictures of Pluto. Pluto is an ICY dwarf planet which we know virtually nothing about, so the images produced will enhance scientists understanding of the planet. 2

4 The Work Placement. The project e2v provided is used to highlight why calibration of equipment is important, how the company ensure the results are as accurate as possible and why they need accurate results. The transducers used within the Compliance Engineering department are known as thermocouples, these are sensors which measures temperature. There are many types of thermocouples for example K, J, G & T; each type has different properties (i.e. temperature ranges and costs). The Compliance Engineering department at e2v tend to use T type thermocouples. This is because it meets the department s requirements. They are very cheap and are relatively accurate. Temperature Transducers T-Type Thermocouple Specifications This type of thermocouple can withstand temperatures of: - Minus 270 C to 370 C or (-454 F to 700 F) It also has an accuracy of (whichever is greater): - Standard: +/- 1.7 C or +/- 0.5% - Special Limits of Error: +/- 1.0 C or 0.4% How it works A thermocouple is a temperature transducer consisting of two wires of dissimilar metals joined at both ends. If the two junctions between the metals are at different temperatures an electric current will flow around the circuit. This is called the Seebeck effect or otherwise known as the Thermoelectric effect. There are two junctions, one is connected to the substance you want to measure the temperature of, this is called a reference junction. The other junction is connected to a multiplex card and used to scan and record the data. 3

5 This chart shows the millivolts (voltage) flowing through the wire at specific points when it is heated to a certain temperature. When the temperature is at 0 C there are 0 millivolts running through the wire. This is known as its reference point. There is a relationship between the temperature and voltage, when the temperature increases, the voltage increases as well however, when the temperature decreases the voltage decreases. When heat is applied the voltage flows one way around the circuit and produces a positive readout however. When the heat is removed and the thermocouple junction goes below 0 C, the electrons flow the opposite way around the circuit, giving it a negative volts readout. This is shown in the table below. (Figure A) Figure A Hazards/Risk Assessments At e2v, like any other company, they have to perform a risk assessment. This was a nine page document identifying all the possible risks apprentices and employees could come into contact with. The biggest hazard is most likely LN2 which has an expansion rate of 600 times in air. This is a very dangerous substance. If a member of staff came into direct contact with it they would have serious injuries such as frostbite. When LN2 boils it produces gaseous nitrogen, which removes oxygen from the air. This results in low oxygen levels in the atmosphere, particularly if vented into a closed space, and can result in asphyxiation. Because nitrogen gas is colourless, odourless and tasteless, it cannot be detected by the human senses and will be breathed as if it were air; therefore labs and areas containing it will be fitted with an oxygen level detector. This will detect the levels of oxygen in the air which is usually around 21%, if it falls below this a alarm will sound indicating nitrogen gas is present. 4

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7 Method Equipment used: - Multiplex Card (MUX) - Thermocouples - Platinum Resistant thermometers (PRT S) - Data Logger - Climatic Chamber - Terminal Screwdriver - Label Machine 1. Learn about all equipment; understand T type thermocouples how they are used to calibrate results. 2. Wire up multiplex card (MUX) with a terminal screwdriver With 10 un-calibrated thermocouples. Multiplex Card 2.2. Two four wire platinum resistant thermometers (PRT S) were inserted into channels 15 and 5, as shown in the diagram below. Multiplex Card Showing PRT S in channels 5 and A PRT is a device which determines the temperature by measuring the resistance of the platinum wire. This piece of platinum wire acts as the temperature sensor thereby changing temperature will change the resistance of the wire. 6

8 3. It was then inserted into a Data Logger unit. A data logger unit records and stores data which has been specified by the user. For example it can log a lot of different variables such as temperature, resistance, DC volts, AC volts and frequencies To set up the data logger firstly the parameter that was needed to be measured was selected. In this process that parameter was temperature. (See figure 1). The step by step diagram can be seen on page After selecting the measurement parameter, the next step was to select the type of temperature transducer, i.e. Thermocouple, 2 or 4 wires Resistance Temperature Detector (RTD) or Thermistors. For this calibration process both thermocouples and 4 Wire RTD s were used. (See figure 2) 3.3. After selecting the transducer type as thermocouples the next step was to select what type of thermocouple was being used, which in this instance was a T-type thermocouple, this was used to measure the temperature. These were inserted into channels In figure 2 a T-type thermocouple was selected (See figure 3), therefore this type of thermocouple had to be used as the data acquisition unit needed to know the reference of what type thermocouple was used. This is because different thermocouples have different voltages flowing through them therefore if we used and selected the wrong type thermocouple an inaccurate result would be shown The next step after selecting what was going to be measured was to select the unit of measurement that was going to be used. The company always measures its devices to the unit of degree Celsius. (See figure 4) 3.5. A unit of accuracy is then needed. The unit of accuracy that was chosen to 0.1 C, this is important as 1 C was not accurate enough. (See figure 5) 3.6. A 4W RTD was then selected (See figure 6) this increases accuracy as it measures the resistance and then disregards or minuses the resistance of the wire. All wires have an internal resistance running through them therefore this resistance has to be accounted for otherwise the accuracy of the reading will be compromised. As you can see in the diagram each wire is producing a resistance. Two wires are producing a positive resistance and two are producing a negative resistance, these therefore cancel each other out A PRT 100 was used so the resistance that was measured was to 100Ω (Ohms). This was used as a reference point. (See figure 7) By using a PRT 100 more accurate results are gained as shown in the table below. When 0 C is passing through the wire Ω are passing through the wire. However when 1 C is passing through the wire there is Ω, it changes by 0.39Ω each time the temperature is increased by 1 C. This detects more minor changes than a PRT 1000(See figure 1a). A PRT 1000 is much less accurate (See figure 1b) as you can see in figure 1b when changing the temperature by 1 C the resistance increases by 3.9Ω which is ten times greater than that of a PRT

9 PRT 1000 PRT 100 Figure 1a Figure 1b 4. The data was then downloaded onto the company s software, Agilent Bench Link Data Logger, then it was then downloaded on the computers hardrive so the information could be analysed. Loading the data from the datalogger onto the computer 4.1. The data was then transferred over across to Excel. The data was then condensed, three columns of information were displayed however the only information that was wanted was the results table. Therefore the other two columns of data were deleted. This allowed the company to view the data more easily; it makes the table less messy therefore the data is easier to interpret A blank row was then inserted between the different calibration temperature stages. This will make it easier to locate the data points. 100 points of data will be taken at 9 different temperature stages of: 150, 120, 80, 40, 0, -40, -80, -120,-150 This is to ensure the accuracy of the data The temperature stages of thermocouple 1 are then inserted into a new tab labelled TC1. These 100 points are then condensed into nine different means. This is to get the average temperature across the thermocouple at each stage of temperature increase The next step was to find out the standard deviation. Standard deviation is a quantity expressing by how much the members of a group differ from the mean value for the group. 8

10 Calculation for Standard deviation. For example if asked to find the standard deviation of the heights of five dog they were then measured 600mm, 470mm, 170mm, 430mm and 300mm. Step 1: First you will have to find the mean of the dogs so all their heights are added up together and then divided by how many dogs there are Mean = = = This calculates the average height of the dogs combined as shown by the green line. The variance then has to be calculated. The variance measures how far each number in the set is from the mean. To calculate the variance, take each height of the dogs, square it, and then find the mean of the results. Then the mean heights of the dogs are taken away from the overall mean of all the dogs. e.g = = = == =

11 The variance is then calculated. Finally to find the standard deviation the root of the variance is taken. Standard Deviation: σ = 21,704 = = 147 So, using Standard Deviation allows having a "standard" way of knowing what is normal, and what is extra-large or extra small. This is good for humans, for example, scientists may be measuring the magnitude of an earthquake. Therefore is there was an earthquake of 2.1 they would know it is reasonably low one and can predict it will not cause a lot of damage. However if there was an earthquake with a magnitude of 9.1, then they would know it would be very serious. If this scale was not there then people and authorities will not be able to predict the scale of damage. Therefore it also saves resources as scientists know if it will have a big impact upon an area or do a small amount of damage, so relevant resources can be dispatched to deal with the scale of the damage. 5. The final step is to calculate the uncertainty of the result. This is done by doing the standard deviation divided by the square root of n (how many thermocouples) This is a calculation of the standard deviation and uncertainty of results for thermocouple 1, at the point 0 C. As seen the uncertainty of result will be roughly 0.03 this is the chance of having a wrong, inaccurate or unreliable result. (See figure 1D) Figure 1D 10

12 The uncertainty of Thermocouple 1 was then calculated by adding all the standard uncertainty answers at each point the temperature were measured. Therefore the overall uncertainty was a 0.88 chance. This was then completed for the other nine thermocouples and two PRT s. The thermocouples were then either given a pass or a fail depending on their uncertainty value. If the overall uncertainty was over 1.5 then the thermocouple would have been failed, this is due to the fact that the uncertainty would have been too great. This test was then repeated for thermocouple 2-10 and PRT S 1 and 2. This shows all the calculations done for thermocouple 1. By analysing this data E2V can check if the thermocouple is working to a good standard. Thermocouple 1 Thermocouple 1 Data. 11

13 6. The Thermocouples were then labelled using the label machine, the labels displayed, when they were tested, what week of the year it was in and what number thermocouple it was. This is important because if they were used again then the department can refer back to the data. If a thermocouple failed this test then it would be disposed of. These thermocouples are not very expensive and only cost around 5 each. 7. The test is then complete and the thermocouples can be used in climatic chambers in the profiling of CCD S. 12

14 Results Results for thermocouple 1 Results for thermocouple 2 Results for thermocouple 3 Results for thermocouple 4 Results for thermocouple 5 Results for thermocouple 6 Results for thermocouple 7 Results for thermocouple 8 13

15 Results for thermocouple 9 Results for thermocouple 10 Analysis Here are the results for thermocouples 1 to 10, they have all passed as their uncertainty is 1.5. If this was any higher they would have failed. These thermocouples will now be used in the profiling of CCD S. All the thermocouples are now calibrated and have passed the test; this was expected as they were new products so we expect that the supplier will not provide the company with faulty or damaged thermocouples Conclusion The calibration of thermocouples allows the Engineering Department to measure and monitor the temperature in the climatic chamber, this is important as products are sent to this department for testing by customers. If the customer requires a specific temperature range for the product to go to, they will need calibrated thermocouples to monitor the temperature. This is to ensure the peaks and troughs are being met. Customers will also send Ramp and Dwell times. This is accomplished by gaining as much testing experience and performance data as early as possible during the products lifecycles. Simulated environmental testing usually involves creating artificial environments for test items to determine their expected resilience to their anticipated environments. This is important because if they did not test to a product, to see if it could survive in specific conditions, then it may fail during its use. 14

16 Step By Step Programming Method. Figure 1 Figure 4 Figure 2 Figure 5 Figure 3 Figure 6 Figure 7 15

17 Glossary CCD- (Charge Couple Device), this is a light-sensitive integrated circuit that stores and displays the data. This captures light and converts it into digital data. A CCD will be used in a camera; the quality of the image will depend on the resolution of the CCD. Transducer- A transducer is an electronic device that converts energy from one form to another, for example a speaker, this takes electrical energy and converts it into sound waves. MUX- (Multiplex Card) this is inserted into a data logger unit. PRT S- (Platinum Resistant Thermometers), A PRT is a device which determines the temperature by measuring the resistance of the platinum wire. A Multiplex Card Thermistor- A thermistor is a component in a circuit that s when resistance changes its temperature changes. As the temperature increases the resistance decreases. Data Logger Unit- A data logger unit records and stores data which has been specified by the user. This is an advanced hard drive and is also known as a data acquisition unit. LN2-Liquid Nitrogen, harmful to humans. Calibration- One device is calibrated and another isn t. The measurements on the calibrated device will be correct so both devices can be compared with one another to see if each device is working Uncertainty of measurement- The chance of a result being corrupted or ending up with a wrong or inaccurate result. Troughs- Lowest point Dwell- Dwell time is how long something stays at a constant temperature Ramp- How quickly the temperature increases in degrees per minute A Data Logger Appendix Risk assessment- Supplied and made by Colin Weetch 16

18 Acknowledgements The Compliance Engineering Department -George Williams -Colin Weetch -Neil Johnson For organising the work placement, mentoring me throughout my four weeks and selecting me for the work placement. The Nuffield Foundation Elizabeth Crilly Melanie DeSouza The STEM Team For getting in touch with the school and suggesting placements, and also organising the whole placement. 17