Recording is the other half of measuring diversions from streams, under California’s new water diversion measurement and reporting regulations. Diverters are required by law to measure flows at frequencies based on the volume of water diverted in a year. The flow has to be measured and recorded. Of course diverters may not care about the data – it costs money and it doesn’t add income. What you and I want in all of our purchases is the best value for the money.
For very small diversions, flows have to be recorded weekly. That may be easy to do depending on the location and access to the diversion.
For medium-sized diversions, flows must be recorded daily. This is possible, but
it doesn’t allow for the owner or employees to have time off, travel, and so on. At this level of recording, an automatic recorder of some type is necessary. Large diversions must be recorded hourly, and automatic recording is the only practical way to be sure flows are recorded. That is the subject of today’s post: automatic recording of flows, or what is really done most of the time, recording water levels and using equations to calculate the flow.
We will leave aside the discussion of propeller, acoustic Doppler, magnetic, and other in-line meters. If you have a diversion that goes through a long length of straight pipe, one of these devices can be bolted in or strapped on. This post is about open diversions into a ditch, where an instantaneous measurement device (weir, orifice, flume) already exists…or may be installed soon. These open devices do not measure flow directly, they measure the water level. An equation is used to convert that level to a flow.
There are hundreds of devices (ready to go) and components (connected parts) to measure water levels. There are also hundreds of loggers that collect data. Here, we will look at 4 water level sensors connected to data loggers, called water level loggers.
Onset has a neat Bluetooth Hobo water level logger. This may help to satisfy the Water Board’s telemetry requirements starting January 1, 2020; the data must be updated weekly on a website, and downloading data weekly is easier with this logger. We’ll see what the Water Board says as this rolls out. The MX-2001, with the cap removed, hooks up to the MX-2001-TOP with a cable, and once installed, is downloaded
with the free Hobomobile smartphone app. The app does everything you’d normally need a data shuttle and cable for – starting, setup, configuration, downloading, and stopping the logger.
The top unit with the Bluetooth radio has to be out of the water, so of course the top of the stilling well holding the unit has to be 1.0 feet or higher up out of the water. If the stilling well is galvanized iron pipe, you’ll need to get within a few feet to download it. If you are using PVC you might get a connection at 100 feet.
Will two units close to each other interfere? Nope, the app finds both and lets the user choose which unit to work with. As with any water level logger installation, keep a logbook or spreadsheet with the Serial Numbers for each location so you aren’t confused later.
What about barometric pressure? The TOP unit records barometric pressure, 
so you don’t need a second unit for atmospheric pressure, nor do you have to know the elevation difference between two separated units. The unit subtracts atmospheric from absolute pressure, then gives you all 3 values when you download: absolute, atmospheric, water only. That makes data processing much easier.
In California, you should be able to get one of these shipped to you for $750. Compare that to the regular Hobos, which need one in the air, one in the water, and a data shuttle and cable. It would put you back almost $1,000 to get the separate pieces shipped to you. If you have two or more locations to log, then the old style is less expensive as far as parts go. Still, the Bluetooth version is likely more cost effective when you consider the minutes saved each time the Bluetooth unit is downloaded, compared to unlocking or unscrewing the cap, getting the water unit out, downloading it, and replacing the cap or lock.
The next is a setup that rancher and retir
ed aircraft engineer Frank Crowe uses. Frank’s desire was to save him and his neighbors money, so he put together the Vegetronix Aqua-Plumb Water Level Sensor connected with the Logger-8-USB. Together these are $340, which is
the least cost of anything that I have seen. Add shipping, tax, 
and $60 in other parts and batteries, and for $450 you’ll have the parts you n
eed for moderately durable, reliable, and accurate water level logger. Not only that, but the Logger-8-USB has 8 channels altogether, so a diverter could measure up to 8 water levels at once by adding 7 more sensors at $95 apiece, not including tax and shipping.
Here is Frank’s latest setup with his comments: “Finally was able to put together a prototype package for the 
Vegetronix sensor. The box is a little bigger than needed, but seems to work. I’m trying to get the data to download into something I can analyze, but it seems to work very stable.
The pipe is 3/4″ mounted to the box, with the sensor wire going down to about an inch from the bottom and then returns up over 12″, therefore doubling the sensitivity. The end is held by some wire at the moment, but would probably work better with a stainless steel spring. The top of the pipe is not sealed, but should be to keep the humidity out of the box. Of course if the data logger were in a separate box, the seal would not be necessary.
To exercise the thing, it is stuck into a 3″ pipe with a water drip going in and a drain at the bottom. The overflow hole is 13″ above the bottom.”
So, what is the trade-off? If you are handy, somewhat experienced with electronic components, and willing to spend some hours, you can set this up yourself. Frank can help a few of his neighbors, but he has his family and ranch requiring his time, too. Otherwise, it is going to cost a couple hundred dollars or so for someone to set this up for you. It needs to be checked, maintained, and adjusted more often than the integrated water level loggers, too, so the maintenance and downloading cost can be $50 to $100 per year if everything is working well.
Next, the Onset Hobo U20L-04 Water Level Logger is $300 before shipping and tax. The DWR Groundwater folks I worked with for years, use these in groundwater wells. They are easy to set up – program one and place it in a stilling well. Take it out once or twice a year to dow
nload the data. The battery life is 5 years, maybe more.
Why aren’t these automatically the cheapest option? They may be the cheapest if a diverter has 2 diversions or more, or several neighbors are using the same Hobo U20L-04. However, they are not vented, meaning that as atmospheric pressure changes due to low pressure areas and storms, the device’s pressure reading will not be as accurate. Therefore Onset recommends having a second U20L-04 set up outside the water to measure the pressure change over time. The second device can be some miles away, so one outside calibration device could be used for several in the water within a 100-square-mile area.
What I heard from colleagues is that these did not last for 10 years, and often not for five years. Durability and reliability of a device are important for uninterrupted data, and therefore compliance with the Water Board’s regulations. The more often 
a device has to be replaced, the more it costs over time.
A download shuttle and cable are also required to get the data from the Hobo to your computer – delivered cost about $300. In summary, the delivered cost of two Onset Hobo U20L-04 devices and the download kit is about $1,000. This cost may be reduced somewhat if the cost of a calibration device can be shared between several diverters, or several diversions.
The third device discussed here is the Global Water WL-16. This is an integrated, vented device, designed to program and set in a pipe. Watermasters have used these for years at various diversions. The delivered cost is about $900.
The WL-16 has a stainless steel casing and is fairly tough. They should last a good 5 years. The problem is at the sensor end – it is relatively easy to clog up in warm-water conditions, with algae and/or silt. In cool flowing water, it might operate for the whole irrigation season. In warmer or still water, it will have to be checked and sprayed clean every 1 to 3 months. Watermasters have put the sensor ends in distilled water in baby-bottle bags, and rubber-banded the tops of the bags closed to keep the sensors clean for the entire irrigation season.
One other concern which I have not discussed with the manufacturer – the manual for the WL-16
was updated in 2009 and refers to Windows XP, not the current Windows 10. I am sure that a newer manual is sent out with the device when it is purchased. Overall, with some care to check the sensor end and clean it as necessary, this is a great drop-it-in-and-turn-it-on option.
The fifth water level logger discussed here is the PMC Versaline VL2111 – WLS-31 Water Level Datalogger. This looks much like the WL-16, but instead of a silicon bladder at the end of sensor, it has a non-fouling ceramic sensor. At $1,370 before tax and shipping, it has the highest purchase cost of the 4 listed in this post, but it is my recommendation for durability, reliability, and low maintenance.
The Versaline is made for wastewater; in other words, for sewer lines. The datalogger end is vented and it is not supposed to be submerged, same as the Vegetronix components and the WL-16. However, it is made to put inside manholes where it is very warm and humid. The PMC guys have maintained the sensor end in rough environments with the equipment lasting 8 to 12 years. If the sensor gets completely covered with algae (or something worse), it still works. It can be cleaned off with a toothbrush if it seems so clogged it might prevent water from getting to the ceramic end. The data logger and sensor are fairly new but are improvements on the older, long-lived versions.
The VL2111 – WLS-31 is three times the cost of the least-expensive option. However, it might be the least expensive in the long run…it sure is the most worry-free of all the options listed here!