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
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.
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.
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.
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.
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.
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.
No leads, ambient
With Leads, ambient
Pressing thumb on thermistor ~33.1c
Heat with long leads, No sink
Heat with short leads, No sink
Heat w/ long leads, w/ sink
Heat w/ short leads, w/ sink
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 ;).
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.
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:
This is a very interesting process, hopefully I can make use of it in the future.