1M Ohm Thermistor Circuit voltage drops not adding up

Question

The circuit is wired as follows

vcc --- R1 (1M ohm) ---|Thermistor --- Ground
                       |                    
                    Voltmeter
                       |   
                    Ground

The Thermistor has a resistance of 1M ohm when it is at 25 C. I am around that temperature now and indeed my ohm meter shows it has a resistance around 1M. I am seeing voltage drops of about 1.1V across the resistors but VCC is 3.3V. This blows my mind but I really don't know enough fundamentals to understand how this is the case.

If I switch the voltmeter out for something like an analog pin on an arduino I get very noisy input which does NOT match the voltmeter. On the arduino I am sampling about once a second.

One last point is that the probe I am using is this probe. It seems that grounding the metal sheath might help reduce the noise I am not yet certain.

Improvements to noise. It seems I could benefit from using a capacitor and multisampling to improve the quality of the signal. Information related to my Microcontroller

Answer 1

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Circuit (drawn using the built-in CircuitLab schematic editor).

If your multimeter DC voltage range has an input of 1 MΩ which is typical - although 10 MΩ is also common - then you have 1 MΩ in parallel with the 1 MΩ thermistor giving 500 kΩ for the parallel combination. With R1 as shown that creates a 3:1 voltage divider so 1.1 V would be the expected meter reading.

Answer 2

You should determine whether your voltmeter is disturbing the behavior of the circuit, due to a low equivalent resistance.

Try this: put two 1 megohm resistors in series between 3.3 V and ground. Use your voltmeter to measure the power supply voltage (should be 3.3 V) and then measure the voltage across each resistor. These voltages should be about 1.65 V each, and should add up to be equal to the measured supply voltage. If not, then the voltmeter is disturbing the circuit behavior.

The input impedance of an Arduino A/D pin is probably going to behave even worse than your voltmeter. You will probably need to add a unity-gain buffer amplifier between the thermistor circuit and the Arduino. Another option would be to use a thermistor with a much lower resistance, say 10 kilohm or less.