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Goals of Calibration of the Thermistor Circuit.

Block Diagram of the Thermistors.

Tonight is time to zoom out and look at the block diagram of the thermistors.  When drawing a CAD drawing it is necessary to sometimes zoom in and work with details and sometimes necessary to zoom out and look at the overview.  When designing parts of a process to work together we must do the same thing.

What we are interested in at this point is how we get good consistent results if we have multiple thermistors all feeding into one computer recording the temperatures.  The red text in the picture lists each of the main components and the type of signal we hope to obtain from each. (Remember you can click on the picture to see an enlarged version… zoom-in… zoom-out. 🙂  Use your browser back arrow to return to this page.)   The sources of variability are listed in blue on the diagram.  I have yet to look up the tolerance specs on the A-D, but usually these devices are very accurate.

The computer in the drawing will actually be two computers.  The computer connected to the A-D will take data at a fairly slow sample rate.  Probably something like once every 10 minutes or so.  This will be stored on that computer until another computer asks for it and downloads a block of the data.

One or the other of the two computers will convert the raw “clicks”, a number from 0 to 1024 into the actual temperature.  It would be nice if all of the thermistors produced the same number for identical temperatures.  Most of our variability is due to the tolerance specifications of our thermistors and the scaling resistors used in the electronics.   The only way we can remove that variance is to add some adjustments within the electronics to compensate for the variability and then perform a calibration at known temperatures.  If we are lucky then all of the thermistors and associated circuits will produce identical values throughout the temperature range and the linearization done in the computer will be same for each circuit.  If we are a little less lucky we will have to write code for each A-D location.

Although we have went over a lot of hurdles and bumps getting to this point and getting the thermistors linearized good enough, we are now at the point where we will have to go into a major calculation frenzy to get the last part of this done so we can add the correct adjustments to calibrate the electronics.  That will be the subject of the next several of the electrical posts.

I will try to keep this from being really boring math yet provide you with enough information so you can do it all yourself.  That is a tough “row to hoe”, but I think I am up to it.

I have been working on the crane project and very soon will have a good post about the status of that project.  I have came up with a fixture to accurately hand cut the dovetail grooves on the do-nothing machine and will soon have a post on that.  Once we get past all the theory for the thermistor circuits I will be moving into prototype construction of the electronics and that will become more “hands-on”.  Soon we will be ready to take on new challenges.   In the electrical theory area I will be talking about capacitors and inductors, but I hope to spread the concepts to include some mechanical and thermal concepts as well.   Also, very soon we will start talking about thermal solar energy concepts.  I look forward to moving ahead.

If you have any suggestions or comments please e-mail me at the address listed on the “contact me” page.

Gary

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