Assuming there is force applied to the thermistor as a result of crushing the ice, are you checking the calibration of the thermistor between each run? Many thermistors are made of molded sintered metal oxide and applying a crushing force may affect their response to temperature changes.
We are calibrating between runs, but it's worth reminding others who might use this information that it is critical to calibrate the thermistor between runs.
I wanted to update this thread with some photos and results. I wasn't the one running the experiments so I may not be able to answer specific questions about the setup. I was helping to get these temp measurements so the experimenter would know precisely the temperature of the sample at each run.
I've attached a couple of photos of parts of the experiment. One photo shows our breadboard circuit. The results show here (in plot figure) were gathered with our circuit on the breadboard area of the Demo Board. The analog output from the second Op-Amp (for this particular run) was read into a 24-bit NI data logger. This was to assure we weren't adding to our troubles with the MCP3208 part of the design. It worked well enough that we're moving to a prop-only based system.
One photo shows the sample after being prepped and before being placed into the "crusher". One thermistor is embedded into the sample (dirty ice), the other is placed at one of the pushing surfaces where a small groove has been milled out to allow the wires and thermistor head to reach into the center of the surface. One photo shows the resistance being measured on one of the thermistors and the volt meter shows the resulting voltage.
Finally, I've attached a plot that shows the voltages recorded converted into temperatures. This is a single run. The temperature (from the surface not the embedded thermistor) shows a steady rise and slowly crosses zero. This is consistent with the melt water observed at the interface.
Thanks all (especially Tracy) for helping out with this.
Peter
Comments
We are calibrating between runs, but it's worth reminding others who might use this information that it is critical to calibrate the thermistor between runs.
I've attached a couple of photos of parts of the experiment. One photo shows our breadboard circuit. The results show here (in plot figure) were gathered with our circuit on the breadboard area of the Demo Board. The analog output from the second Op-Amp (for this particular run) was read into a 24-bit NI data logger. This was to assure we weren't adding to our troubles with the MCP3208 part of the design. It worked well enough that we're moving to a prop-only based system.
One photo shows the sample after being prepped and before being placed into the "crusher". One thermistor is embedded into the sample (dirty ice), the other is placed at one of the pushing surfaces where a small groove has been milled out to allow the wires and thermistor head to reach into the center of the surface. One photo shows the resistance being measured on one of the thermistors and the volt meter shows the resulting voltage.
Finally, I've attached a plot that shows the voltages recorded converted into temperatures. This is a single run. The temperature (from the surface not the embedded thermistor) shows a steady rise and slowly crosses zero. This is consistent with the melt water observed at the interface.
Thanks all (especially Tracy) for helping out with this.
Peter