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Math Help - What is the equation of this semi-log graph ??

  1. #1
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    Oct 2008
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    What is the equation of this semi-log graph ??

    Must have been away the week we did log's at school

    The problem in a nutshell......

    I have a 'voltage' vs 'current' semi-logrithmic graph.


    I need to be able to calculate a current given a paricular voltage from this technical specification.
    So I need to be able to determine the equation that represents this response so as I can calculate the current, right ???
    Is there someone out there that could help ?

    Detailed explanation of the problem......

    I have in front of me a device called a 'Travelling Wave Tube' (TWT).
    Its a device used, in this instance to amplify a small Radio frequency signal into a larger Radio Frequeny signal. Commonly used in communciations, radar applications etc
    To do this, inside the tube an ultra-high vacuum environment must exist.
    After 12 months or so, bits within the tube begin to ionize, creating gas which reduces the ultra-high vacuum, which in turn, reduces the over all performance of the amplifying tube.
    So what I'm doing is 'pumping' these tubes as a general maintainence which involves using a small power supply that provides a high voltage source ~3kv to the tube which de-ionizes the tube, therefore re-creating the ultra high vacuum within the tube.
    The HV power supply gives a monitoring output for the current draw, which is what I'm interested in.
    The current draw of the tube is represented as a dc voltage (as per the graph).
    So I need to be able to calculate what current draw the tube has by the monitoring voltage coming from the high voltage supply.
    Now my logrithmic theory is VERY rusty and I can't seem to come up with an equation that represents this plot that I can use to calculate htese currents.
    All help appreciated.
    Steve
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  2. #2
    Flow Master
    mr fantastic's Avatar
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    Quote Originally Posted by mr-grumble View Post
    Must have been away the week we did log's at school

    The problem in a nutshell......

    I have a 'voltage' vs 'current' semi-logrithmic graph.


    I need to be able to calculate a current given a paricular voltage from this technical specification.
    So I need to be able to determine the equation that represents this response so as I can calculate the current, right ???
    Is there someone out there that could help ?

    Detailed explanation of the problem......

    I have in front of me a device called a 'Travelling Wave Tube' (TWT).
    Its a device used, in this instance to amplify a small Radio frequency signal into a larger Radio Frequeny signal. Commonly used in communciations, radar applications etc
    To do this, inside the tube an ultra-high vacuum environment must exist.
    After 12 months or so, bits within the tube begin to ionize, creating gas which reduces the ultra-high vacuum, which in turn, reduces the over all performance of the amplifying tube.
    So what I'm doing is 'pumping' these tubes as a general maintainence which involves using a small power supply that provides a high voltage source ~3kv to the tube which de-ionizes the tube, therefore re-creating the ultra high vacuum within the tube.
    The HV power supply gives a monitoring output for the current draw, which is what I'm interested in.
    The current draw of the tube is represented as a dc voltage (as per the graph).
    So I need to be able to calculate what current draw the tube has by the monitoring voltage coming from the high voltage supply.
    Now my logrithmic theory is VERY rusty and I can't seem to come up with an equation that represents this plot that I can use to calculate htese currents.
    All help appreciated.
    Steve
    Read this: Semilog graph - Wikipedia, the free encyclopedia and this: http://www.physics.pomona.edu/sixide...s/LRM/LR13.pdf.
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  3. #3
    Grand Panjandrum
    Joined
    Nov 2005
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    someplace
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    4
    Quote Originally Posted by mr-grumble View Post
    Must have been away the week we did log's at school

    The problem in a nutshell......

    I have a 'voltage' vs 'current' semi-logrithmic graph.


    I need to be able to calculate a current given a paricular voltage from this technical specification.
    So I need to be able to determine the equation that represents this response so as I can calculate the current, right ???
    Is there someone out there that could help ?

    Detailed explanation of the problem......

    I have in front of me a device called a 'Travelling Wave Tube' (TWT).
    Its a device used, in this instance to amplify a small Radio frequency signal into a larger Radio Frequeny signal. Commonly used in communciations, radar applications etc
    To do this, inside the tube an ultra-high vacuum environment must exist.
    After 12 months or so, bits within the tube begin to ionize, creating gas which reduces the ultra-high vacuum, which in turn, reduces the over all performance of the amplifying tube.
    So what I'm doing is 'pumping' these tubes as a general maintainence which involves using a small power supply that provides a high voltage source ~3kv to the tube which de-ionizes the tube, therefore re-creating the ultra high vacuum within the tube.
    The HV power supply gives a monitoring output for the current draw, which is what I'm interested in.
    The current draw of the tube is represented as a dc voltage (as per the graph).
    So I need to be able to calculate what current draw the tube has by the monitoring voltage coming from the high voltage supply.
    Now my logrithmic theory is VERY rusty and I can't seem to come up with an equation that represents this plot that I can use to calculate htese currents.
    All help appreciated.
    Steve
    You can you not just look up the current from the dc voltage on the graph?

    CB
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  4. #4
    Newbie
    Joined
    Oct 2008
    Posts
    2
    I have the voltage data feeding into a spreadsheet and I need to know how to tell the spreadsheet to manipulate it to output the current calculation.
    As I'm running upwards of 3000 sample readings, I really need it to be an automated process

    As for the Wiki pages, I'd started off there and well to be honest, it may as well have been in Chinese

    Steve
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