In-Process Stormer Viscometer – Damn near complete

  The viscometer is nearing full completion! YAY! To mark this occasion to mark the end of a long two years, I am placing a small gallery of the almost finished product. The only thing missing is the outside cover which consists of a 4″ od aluminum tube. Also, the springs I’ve ordered have also not arrived as yet, however, for now, the elastics will suffice. Here are some of the features and facts:
 

  • Can be used in-lab or in-process
  • Selectable RPM with a tight tolerance on RPM +/-0.5RPM
  • User calibration routines. This allows the end user to calibrate with 3 fluids of known viscosity
  • 16 key keypad, used for calibration and settings, also for running special tests
  • Can be used as a laboratory gel-timer
  • Can be used for custom tests besides stormer viscometry
  • Low power consumption <100ma or <2.4W
  • RS-485 Serial output
  • Control electronics have complete galvanic isolation
  • 24VDC supply required
  • repeatability (requires further testing) +/- 1.5%
  • Modbus protocol (not yet implemented)
       
      Here’s the Gallery!

       
       Yes, a few too many pictures, oh well.
       
       This thing took me quite a while and what I learned from it was immeasurable. Thankfully now that everything works as expected I can focus on my other projects without having this thing hanging over my head. Here’s to completion!
       
      As an aside, here’s an interesting document on viscosity.
      here

Spring Stormer Viscometer, proceeding, the board works.

Well, I recently recieved my boards back from ap circuits in Calgary and I’m pleased with the result. I kind of fucked up by ordering 4 of the same board and not 2 of the control board and 2 power boards. Oh well! I can use the current boards for the new ones. Anyways, here are some images of the board.


The two boards together, unpopulated and populated
The two boards together, unpopulated and populated

The board, populate with SMD's
The board, populate with SMD's

An experiment in heat transfer

  After being apprised of an issue with heat transfer through PCB traces, being the stubborn idiot I am, I had to try and solve the problem. The issue is with a particular PCB that has a cutout section with a thermistor placed in a partially isolated section within the enclosure, unfortunately the traces themselves are serving as a heat transfer conduit especially so since copper is a particularly good conductor of heat.
 
  After thinking about it for a while I came up with a couple of ideas like calibrating the calculated output to accommodate for the temperature discrepancy, however that probably wouldn’t work without a second thermistor closer to the heat source, accounting for the difference. Another idea I had was to use an infrared thermopile but they’re kind of large and expensive and would require a heat channel mounted above it to prevent measuring the temperature of the case.
 
  After thinking for a while about it, the simplest answer I could come up with would be a heatsink to mitigate the heat transferred to the thermistor. While it is theoretically impossible to totally remove all heat difference, I believe that a large portion of the heat transfer can be eliminated. So I wanted to see if heatsinks actually help equalize heat much at all so I devised a bit of a rough experiment.
 
The Experiment
 
  I decided the quickest way to test this would be with materials I already had. I have a selection of thermistors so I used a glass axial thermistor.

This is the thermistor board used in the experiment. the thermal leads aren't attached.
This is the thermistor board used in the experiment. the thermal leads aren't attached.

The circuit used for the test. Beautifully illustrated :)
The circuit used for the test. Beautifully illustrated

 
The thermistor used is a glass 10kOhm NTC Thermistor in series with a 1k resistor in order to detect current/divide voltage. With the setup above, the voltage detected from GND to the divider is 438mv. the temperature in my basement is pretty steady since the furnace isn’t running. So now I attach the leads.
 
This is the board with the two 22AWG leads attached.
This is the board with the two 22AWG leads attached.

I also performed the test with the leads closer to the thermistor.
I also performed the test with the leads closer to the thermistor.

 
  I used Teflon coated wire to prevent conduction between the two sides and used my soldering iron as a heat source since it’s temperature controlled. I let the heat soak up through the lines for 10 minutes to ensure that the heat stabilizes at the thermistor. I also performed a test with the soldering iron closer to the sensor.
 
DSCN9550

 
Finally, I decided to place a heatsink on the line. I was going to solder aluminum shims to the leads in order to provide for heat sink. That turned out to be difficult at best, so I used coiled up copper wire and soldered it to the leads. I performed the same two tests with the position of the soldering iron. Anyways, here are the results of the test.

Test Type Voltage Recorded
No leads, ambient 438mV
With Leads, ambient 438mV
Pressing thumb on thermistor ~33.1c 554mV
Heat with long leads, No sink 496mV
Heat with short leads, No sink 625mV
Heat w/ long leads, w/ sink 476mV
Heat w/ short leads, w/ sink 544mV


 
  It should be said that this test is far from perfect and doesn’t prove anything quantitatively with any real degree of accuracy, however I wanted to see how effective even a rudimentary heat sink would be in a situation like this. It should be noted that while thermistors are inherently non-linear, we have voltage drops of 43% and 34% with the short and long leads respectively. I simply thought it was an interesting experiment. I have a few ideas on how to sink the heat or account for it but I think the heatsink is the easiest plan.
 
  Here’s a beautifully rendered image of the idea to reduce thermal linkage between the PCB and the thermistor ;).
 
The thermal isolation idea. By placing a heatsink on the exposed traces one could mitigate the heat transferred to the thermistor.
The thermal isolation idea. By placing a heatsink on the exposed traces one could mitigate the heat transferred to the thermistor.

 
  If there were a heatsink over the exposed traces and the traces made as long as possible, it should be possible to bring the traces fairly close to ambient. Like the experiment showed, even a rough heatsink was able to reduce the heat going to the thermistor by a substantial degree.
 
Feel free to comment.

FDM Part Prototyping

This post is simply to show how neat the FDM process is. FDM stands for Fused Deposition Modeling and is a neat tool for prototyping plastic parts. I was able to see the results of FDM after a customer had one of my models done with the process, he was nice enough to let me take some pics of it 🙂 . Frankly I think this kind of prototyping opens up a wide variety of interesting possibilities in terms of being able to develop plastic enclosures and various other mechanical projects.

Here are some images:


The two peices in FDM, this is a deposited ABS IIRC.
The two peices in FDM, this is a deposited ABS IIRC.

I altered the levels to show how the process fills in non critical areas. Of course it would depend on the machine performing the action.
I altered the levels to show how the process fills in non critical areas. Of course it would depend on the machine performing the action.

Corner Detail FDM
Corner Detail FDM

This is a very interesting process, hopefully I can make use of it in the future.

Here’s the wikipedia article http://en.wikipedia.org/wiki/Fused_deposition_modeling