Ed,
Electrolytic capacitors make a poor choice for RC time constant
measurements. First their capacitance is at best within 10% of their stated
value. It is typically an under estimate of the value because of the way the
are used in filtering. Second they have a low leakage resistance ( the order
of a megohm and voltage dependent). The leakage resistance can be viewed as
a built in resistor in parallel with the capacitor. In addition some of the
Pasco units also have a low internal resistance (the order of a megohm) that
can act like a leackage resistance as well if your using it to measure the
voltage across the capacitor. A third effect is a high ESR (effective series
resistance) that as the name implies can be view as a resistance in series
with the leads of the capacitor. This one is especially true for some of the
"super capacitors" that have capacitances in the order of Farads.
I have founds the most reliable results with film capacitors (such
as mylar caps that come in sizes up to ten microfarads or more.
I hope this helps.
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU