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Sasha Brownsberger (Unlicensed)

Christopher Stubbs

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Electronics 

All thermistors are The detailed circuit diagram, and corresponding derivations, are available here: Circuit Diagram and Voltage to Temperature Derivations

Thermistors:

235-

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1625-ND: FP 07 FAST TIP PROBE, 150 Kohm, at 25C

AAS-920-267D-Thermometrics-NTC-TypeFP07-041216-web.pdf

self-heating is 1C per 50 uW of power dissipation. Resistance is 150K at room temperature. Still-air thermal time constant is 100 msec. 

plot of temperature increase vs. applied voltage across thermistor:

Image Modified

The problem with self-heating is that we are increasingly sensitive to local windspeed rather than temperature. We definitely don't want to exceed one volt across one thermistor, or 2 volts across both of them. 

Paper on thermal time constant measurements in still and moving air: [15200426 - Journal of Atmospheric and Oceanic Technology] Time Constant Estimates for Radiosonde Temperature Sensors.pdf

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Reference Resistors: 

  • Replace thermistors in test lines 
  • All are 5% 150 kOhm resistors 

Wiring:

Note: for the two reference lines (thermistor + resistor and resistor + resistor), a resistor might replace a thermistor in the description below.  The wiring scheme is the same for all 5 lines, whether thermistor/thermistor, resistor/thermistor, or resistor/resistor. 

  • All lines are twisted pair, shielded wires
  • For main line (that which includes both thermistors):
    • low wire (black wire) is connected to low end of power supply (which can be ground)
    • high wire (red wire) is connected to high end of power supply
    • Top thermistor in voltage divider configuration is soldered directly into high (red) wire
    • Bottom thermistor in voltage divider connects high and low (red and black) wires 
    • Shield is connected to ground (unattached at the end with the bottom thermistor)
  • For the sample line (running off of midpoint of voltage dividers) 
    • High (red) wire is soldered to the middle side of the top thermistor (side between thermistor pair) 
    • Low (black) wire is shorted to shield
    • Low (black) wire and shield are connected to ground on measurement end of array, unconnected on end connecting to thermistor

ADU Board 

Jim's Arduino-based hardware

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