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

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

The problem in a nutshell......

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

http://i124.photobucket.com/albums/p...e/techinfo.gif

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