I need to log the interval between drips at a fieldwork site in a cave without physically interacting with the drip
(1) A capacitance based sensor is definitely possible.
IC's that allow you to measure capacitance changes far smaller than you require and with automatic self trimming are available at reasonable prices.
Example only. AD7150 - datasheet here.
As little as 1 femtoFarad detection.
Response time is 10 ms but a falling drop may be able to be detected as it falls through the sensor (window). Would need to be investigated.
Can be operated "stand alone" without microcontroller support and MAY be able to be made to provide input to logger without other 'conditioning'.
(2) IR! However, you can very easily [tm] achieve the time scales you want using IR techniques, and may find this easier. The enabling "trick" is to modulate the IR beam at a low duty cycle at a fast enough rate to not miss any drops.
Battery mAh capacity and drop speed are relevant.
Assume for now a 5 m/S drop speed and 2000 mAh battery capacity.
I'll also assume that the drop path does not diverge horizontally "too much" and that it can be persuaded to fall though a small-ish area circle such that an IR beama can be interrupted noticeably without too much effort. Falling through an IR dark space will help.
These assumptions can be modified as desired.
For example a 3600 mAh 3.7V mean LiPo battery would very happily exceed the above capacity with enough voltage to run an IR LED based system.
6 months ~= 4000 hours so average battery drain = 2000 mAh/4000 hours = 0.5 mA.
0.5 mA with DC IR drive
5 mA at 10% IR on
50 mA at 1% IR on
5 mA may be marginal in some cases.
50 mA should be more than adequate.
At 5 m/s a water drop falls 5mm in 1 millisecond.
You can probably detect the beam interuption of a drop over a physical 5mm distance, but if not, the distance and thus the time can be reduced.
The IR source has to be on for at least one "transmit period" during the time the water drop is present in the sensor "aperture".
At 1 mS and 10% modulation you need a 100 uS transmit period and 900 uS off period.
At 1mS and 1% modulation you need 10 uS transmit and 990 uS off.
If you want to reduce the 'aperture to say 1 mm you need a frame time of only 200 uS (for a water drop at 5 m/s)
10% on = 20 uS, off = 190 uS.
1% 0n = 2 uA, off = 198 uS.
The 2 uS transmit time is starting to become mildly challenging but still very achievable with basic off the shelf componentry.
The above requirements can be reduced to the following formula.
Tcycle = A/V milliseconds (= pulse repetition period)
Ton = D x A / V milliseconds.
- D = duty cycle ( 0 <= D <= 1)
A = aperture distance (fall distance ) in mm
V = drop velocity in m/s
If there was a known minimum inter-drop time this could be used to allow the detector to be powered down between drops BUT this should not be necessary.
Data logging is "just a matter of doing it".
The logger may be integrated with the detecttor in some implementations but is conceptually a different subsystem.
You need to specify what you want to log.
eg drop to drop time - all data recorded.
and/or drop to drop time, saved as a distribution.
And or drops per minute / hour/day ... - with what resolution.
Modern memory capacities are such that logging every even is doable if necessary.As an extreme example, 1 drop per second for 6 months = 4000 hours x 3600 s/hr = 14.4 million drops. 128 MB of memory would allow about 9 bytes per drop. Logging time absolutely to 1 second accuracy over 6 months requires about 3 bytes of data per drop. Doing it incrementally requires less. A 32 kB static RAM or FRAM or Flash memory would handle the task with ease with minimal mean current for the whole logger compared to IR sensor needs with careful design.
An off the shelf logger may be able to be used or a custom design could be implemented. Actual concept is very simple. As ever, practical implementation will usually have a few traps along the way but the logger is a simple project.
Max drop speed = ?
Max 1D (horizontal) or 2D (area) of drop dispersion at sensor plane.
General environment? dark/light, windy/calm. (Assumjed dark and calm in cave, but maybe not)
Mimimum interdrop period?
How big a battery is acceptable?
Logging specs (as above)
Do you want/need 1/10/100/1000000 or these (Call me if 1000000 :-) ).
How many would the internation cave ater drop logging fraternity buy in a typical year and what would they be prepared to pay?