Sunday, November 6, 2022

Pool Drain Test

As mentioned in the previous blog post, a test was planned to position an AMOS equipped with a conductivity sensor in the pond / stream in back of our house, in order to see if there was any noticeable change in conductivity in the water due to draining of the pool. The pool uses salt and a chlorinator during the summer months to keep it clean, but we need to drain it every fall before the water freezes, to make sure there isn't too much ice build-up in the winter. The water within the pool had a conductivity of  4.395 mS/cm. 

The location where AMOS was placed unfortunately had a fair amount of tree cover:

but thankfully not many leaves were left, and there was a nice bit of sun on the afternoon when the bloat was deployed and this picture was taken (October 23). The depth of the water in this area was almost up to the top of my rubber boots. 

Unfortunately, the good weather did not last, and the rest of the week saw near constant rain, with scarcely any sun at all. On the evening of the 24th, I placed a sump pump into the pool with a garden hose connected it to it, and started pumping out water to the corner of our fence nearest the pool.  The pumping went all night, and I shut it off at around 8 am the next morning. Close to 1 m was pumped out of the pool. The pool has a diameter of 8.2 m, so this would correspond to a volume of:

Volume = pi * (8.2/2)^2 * 1 = 53 cubic meters, or 53000 L. 

Here is a picture showing the location of AMOS relative to the pool:

The distance from the pool to AMOS was approximately 45 m. 

AMOS was configured to collect data every hour, and go into low power mode between samples. The plan was to leave it in place for about a week or so, to evaluate the long-term effect of the pool draining. But low power mode wasn't nearly as low power mode as I had hoped, because I had forgotten that the temperature / conductivity sensor and the GPS board were powered continuously. Had a switching circuit been employed to turn these off when they weren't required, a fair amount of power could have been conserved. As it was, when AMOS was sampling it was consuming about 8 W, but when it wasn't sampling it was still consuming about 3.5 W, which over time, given the lousy weather, proved to be too much, and at 4:00 am on October 26, the main battery detected an under-voltage condition and flipped itself off. 

The data was downloaded from AMOS, and graphed:

The short-term resolution of the conductivity sensor is specified as 0.001 mS / cm. Based on the above graph, the long-term variation (hour-to-hour) appears to be a bit greater, something like  +/- 0.004 mS/cm. There does appear to be a weak upward trend in the above data after the pump was started, but the increase is only about 0.002 mS/cm. The specified stability of the sensor is 0.003 mS/cm/month, so I'm not sure the above data represent conclusive evidence that the draining of the pool affected the conductivity of the stream. The large amount of rain which started on October 24 and continued throughout the week greatly increased the flow through the stream, which might have reduced the amount of pool water that collected around AMOS. The soil in our backyard is fairly loose and sandy also, with good drainage, so perhaps very little pool water would have been transported to the stream, even without the rain. 

So to summarize, it looks like a re-do is in order for Fall 2023. 😊