Using cFP-AI-110 to measure rpm:capacitance in the card?

I am attempting to measure the rpm of a number of different augers. The augers have a magnet on them that closes a magnetic switch when the magnet passes the switch. I have built a circuit with a very small amount of power traveling through a resistor and measure the voltage across the resistor. The magnetic switches are in series with voltmeters. I have attached a picture.
The idea was that by measuring the time between closed circuits, I could calculate the frequency of the augers. When I use multimeters, the simple circuit I built works perfectly; a closed switch will cause a meter to read (say) 9v, an open circuit reads 0. However, when I swap in the analog input card for the multimeters I encounter a problem. When the circuit is closed MAX instantly shows the proper voltage, but when it becomes open the voltage decays slowly. In other words I get a triangle shaped waveform, and since some of the augers spin rather quickly the peak to trough distance is almost non-existent.
Does anyone know why the voltage is measured properly as soon as I close the circuit, but why it decays slowly once it is opened? This problem only occurs if I use the NI card.
Thanks for your time
Attachments:
rpmcircuit.jpg ‏9 KB

If the drawing is right, a pull up seems more reasonable to me
You don't want floating inputs at your cFP.
 If the contact opens, the - input floats and need to be pulled to to + input to measure 0V
Greetings from Germany
Henrik
LV since v3.1
“ground” is a convenient fantasy
'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'

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