Siphon Testing - Round 2, Brief Findings

The standpipe and funnel assembly

I ran several tests on my experimental siphon, trying different diameter drain pipes and different lengths.  I also tested slow-fill, to determine whether or not the standpipe funnel actually helps.

Effects Drain Pipe Diameter and Length on Drain Times

The standpipe is 3/4" in diameter.  I ran several tests with various configurations (with and without snorkel, with and without the standpipe funnel).  Unfortunately I could only run one or two tests per series, so please take these results with a grain of salt...

As you can see on the chart above, the 3/4" drain pipe length appears to affect drain times fairly predictably.  Unfortunately the drain rate increases were not as high as I was hoping them to be.  The first test (left-most dot) is without any additional drain pipe.  The X axis is pipe length, the Y axis is the seconds to drain - that is, the time from siphon-start to the siphon-end burp.

The fastest drain time was recorded while testing without the funnel in place.  I only ran one test of that sort, however, so that result might be an outlier.  I noticed that in only one or two tests the drain rate would be extremely fast, but in the remainder of tests it seemed to hold fairly constant.

For the next series of tests, I used a bushing to convert to 1" after the bulkhead fitting.   The idea for this was to reduce static pressure and allow the water to flow faster.  Thus, 1" drain pipe would be used.  As this was larger than 3/4", technically it would have operated at slightly higher pressure.

The majority of the 30" tests were done with a snorkel bell.  The snorkel was used primarily to watch the pressure inside the bell.  The most important things to note here are that the drain rate remained constant for most of the tests, between 35 and 40 seconds for nearly every test, and seemingly regardless of drain length.  Again, in the chart above, the X axis is for length, the Y axis for drain time (in seconds).

I would hypothesize, based on these data, that the ideal solution is to maintain a constant pipe diameter throughout the bell siphon.

The snorkel test bell assembly.

Bell Water Level Observations

My snorkel bell rises quite high above the top of the standpipe.  The siphon had no problem starting, given sufficient flow - more on that later.  The interesting thing I noticed was that after the siphon started, the water level in the snorkel rose by at least 1", usually 2", and in the case of using 3/4" drain pipe it rose by over 4"!  This level would usually slowly drop as the water level in the source reservoir was depleted.  The informal relation seemed to be: the faster the siphon, the higher the level reading on the snorkel.

Compared to the 1" drain pipe, the 3/4" drain pipe appeared to deliver significantly more suction once siphoning began.  I had marked my bell with inch indicators up to 11", but the water level in the snorkel tube quickly shot above where the 12" would have been.  All 1" pipe tests delivered consistent results: snorkel level rose by about 1.5" from the top of the standpipe, and drain times were consistent.

It was also interesting to watch the water level shoot up once the siphon started in earnest, and to drop as it was breaking.   I was able to observe the transition from spillover to drain with ease.  Another interesting test would be to verify if the water level in the snorkel matches that inside the bell.

Siphon in action: Note that the level of the tank is around 7.5", whereas the water level in the siphon appears to be nearly 10"

Slow Fill Observations

For the slow-fill tests, I rigged the supply to provide approximately 0.0358 cubic centimeters per second of water.  This figure was calculated based on observing the rise of the water in the tank and calculating it against the estimated tank geometry.  The drain was left at 30" of 3/4" pipe for all the tests.  I used the standard bell for all but the final test.

I first observed the standpipe without the bell, to ensure that the rate would not quickly flood the standpipe.  Having observed this, I replaced the bell and waited to see if the siphon would start.  It did not.  I then added the funnel back onto the standpipe.  The water seemed to flood into the standpipe a little bit better, but the rate was still much too slow to trigger a full siphon.

I added one 90-degree bend to the bottom of the standpipe, but it had no effect.  I added a second 90-degree, and finally it achieved siphon.  I repeated this test without the funnel, and came up with generally the same results.

Slow Fill Conclusions

Fill rate is key for siphon start.  I hypothesize that the fill rate must overcome the non-siphon drain rate in order to build a solid column of water in the pipe.  One the water column has been established, the siphon will start.

If the fill rate cannot be altered, then adding fittings to create back-pressure also works.  I did not use a trap-style drain configuration, as I prefer to let the bell breathe while the tank fills.  I am also not yet convinced that the trap is superior to simply two downward-trending bends.  The two 90-degree bends - added back-to-back to the very end of the drain - added sufficient back-pressure in my experiments.  They also did not immediately appear to harm siphon drain rates.

We must realize that the siphon is a dynamic system and governed by flow rates.  As such, the addition of snorkels, bends, reducers, etc, to "fine tune" the siphon will work only so long as the flow rates are appropriate.  In other words, these things do not guarantee a better (or even a functional) siphon.  You are, in effect, simply moving numbers around.

The standpipe funnel also does not necessarily yield a better siphon, though I suspect it did allow the siphon to trip faster, moving from spillover to drain much quicker than with the straight, unadorned standpipe.  This is, after all, the reason people claim to employ a funnel on their standpipes.

In future experiments, I would like to examine the rates of inflow and outflow, to understand better how the addition of back-pressure solves siphon-start problems.  I would also be curious to see if the drain pipe length has an effect on siphon-start; in all my slow-start tests, I kept the drain pipe length constant.  Finally, it would be interesting to see if the water level inside the bell is an indicator of the rate of drain.

In no cases did the 1/8" emergency drain hole in the standpipe inhibit siphon operation or tank fill.  

All Drain Test Results

Below are the results from all the tests.  The last two columns are the calculated cubic centimeters per second of drain rate, and the calculated time it should have taken for draining to complete.  As can be seen, the wide variability in the results suggest further testing.  The 1" tests are also curious, in that they were extremely consistent and always significantly longer than the calculated times.  I suspect the reason for this is the 3/4" standpipe, and/or the bushing to go from 3/4" to 1" pipe.  I plan on performing additional tests using 1" standpipe, once I have a 1" bulkhead fitting.

3/4" Diameter Drain Pipe Tests
Test #	length	drain time (seconds)	calc cm3/s	calc t
1	0	44	n/a	n/a
2	11	26	665.93	29.90401379
3	28	21	1,062.46	18.74333258
9	13	21	723.94	27.50769186
10	7.5	26	531.23	37.48666515
11	30	19	1,099.75	18.10777959
16	30	22	1,099.75	18.10777959
17 - no funnel	30	15	1,099.75	18.10777959
1" Diameter Drain Pipe Tests
Test #	length	drain time (seconds)	calc cm3/s	calc t
4	6	44	874.35	22.77575217
5	12	40	1,236.52	16.10488881
6a	30	20	1,955.11	10.18562602
6b	30	41	1,955.11	10.18562602
7a - snorkel	12	36	1,236.52	16.10488881
7b - snorkel	12	36	1,236.52	16.10488881
7c - snorkel	12	36	1,236.52	16.10488881
7d - snorkel	12	35	1,236.52	16.10488881
8	6	35	874.35	22.77575217