## Using Circular Headgates as Submerged Orifices

There you go. Based on the change of stem height, you can calculate how much the headgate is open. Convert that to a percentage. For example. A 2.0′ diameter headgate open 0.5′ is 25% open. Then use the table’s 50% row to where it intersects the 2.00 foot diameter column to get an area of 1.913 square feet.

## H-S Flumes For Accurate Measurements Of Small Flows

What if you have a small diversion, but grass or debris would interfere with a standard weir?  A weir has to have unobstructed, free-flowing water over

the crest so measured depths accurately relate to a calculated flow.  A weir with debris problems has to be cleared whenever flow is measured, which increases the time requirement.

When weirs have low flows, they trap debris more frequently, and they are less accurate when the depth over the crest drops below  0.2 feet (2.4 inches).  Then the only way to measure flow is with a narrow suppressed weir, or with a contracted weir, typically half or less the maximum width.  A V-notch weir can be used for measurement of low flows.

Changing the weir boards for different flows requires someone with experience,

who will recognize when the depth over the weir is 0.2 feet or less and then use a contracted weir board.  However, people are busy when irrigating, and even busier when flows drop.  Weirs are often neglected during the time they need more frequent maintenance visits.

A good flume for passing debris and measuring low flows is the HS flume.  These are accurate right down to zero flow.  For the maximum flow, they require more

material than a rectangular Winflume, Montana, or Parshall flume.  However, they are more accurate than other flumes at very low flows – testing by the University of Minnesota found an average accuracy to be +/- 3.2% for ideal approach conditions.  They will pass debris down to zero flow – the flume shown here has an opening of 0.05 feet, or 5/8 inch at the flat bottom, and the opening increases with height.

Why aren’t HS flumes common in California?  I suspect that the early adoption of Parshall flumes here established the standard.  I have seen a few hundred flumes, but I had never seen an operating HL (wide, high flow), H, or HS flume, prior to my installations.

Why go to the trouble of using an HS flume, if Parshall flumes are readily

available?  A Parshall flume may be +/- 10% accurate down to perhaps 5% of its maximum flow.  Below that, the accuracy decreases.  An HS flume is +/- 10% accurate down to 1% to 2% of its maximum flow.  If the flow regime is predominantly low with occasional high flows, it is important to measure those low flows with the best possible accuracy.  Some places where low flow measurement is critical include field runoff where pollution is proportional to flow, small water rights, and dam leakage.

HS flumes are easier to construct than a Parshall, too.  The HS flume bottom is flat, and it has 3 vertical planes.  The photos of the Parshall flume here show

that it has 3 horizontal planes, and 5 vertical planes.  An HS flume takes less time to build, and can be put together fairly quickly in any farm or ranch shop.  Parshalls are complex enough that they are purchased, including design and shipping costs.

## Do Something Wrong, Instead Of Nothing Right

Do something wrong, rather than nothing at all. Have you ever heard that before? I have heard it from Army veterans, a boss, even an elder of a church.  George Patton said, “A good plan violently executed now is better than a perfect plan executed next week.”  A non-military way to say that is, “A poor plan now is better than no plan at all.”

What it means to you and me is, if action is necessary, do something, maybe ANYthing, rather than freezing in place or ignoring a problem. This is obvious when you see a tornado 5 miles away, for example; either drive away from it if you are in a car, or take shelter if you are on foot. If you have a plumbing leak in the house and no parts to replace broken pipe, then put a bucket under it, or turn off the valve, and call a plumber. All of us have seen a TV show (or maybe had it happen to us) where the bad guy pointed a rifle and said, “Don’t move”. What do we all say to the TV? “Don’t just stand there, run!”. Doing nothing is a much worse choice, if the result for freezing in place is death or injury.

What about water rights – how does doing something wrong help? Everyone knows by now that surface water diverters need measurement devices, so put in a weir box and boards and measure your flow before the threats come from the Water Board, your watermaster, your ditch tender, or your neighbor.  Even just stick horizontal boards in a ditch and seal the sides with plastic – something to take positive action to reduce future pain.

Remember to file the information for the measurement device with the Water Board, either via your annual report of diversions, or using the Water Right Form and Survey Submittal Portal.

Take a look at the blog posts below.  There is enough information and how-to directions, that you should be able to do it well enough to satisfy the Water Board.  Check out these posts:

There is a philosophy based in law and a lot of experience, that says don’t put any controls on yourself until the court or government makes

you. Why remodel your house to accommodate the wiring or plumbing, if you aren’t selling the house and everything works okay right now? Who would put a lot of money into an old truck to make it pass smog, if it just might pass a smog check the next time it has to be done? What farmer would change how he irrigates or ranches if everything still operates and the bank will keep making operating loans?

All of the Water Board deadlines have passed to install measurement devices, or file Alternative Compliance Plans.  If you haven’t got your device or plan done yet, get a Request For Additional Time done as soon as possible.

Be proactive.  Take some inexpensive, temporary action.  Educate yourself for free with some time in the Internet. Even a small, less-than-perfect improvement in your measurement device, flow and water use record keeping, can pay back a lot more when you have to deal with potential Water Board fines, a court case, or even just an angry neighbor in the future.

## Converting Logger Pressure to Depth & Storage/Flows

If you have a pipeline as part of your diversion, then an in-line meter with an integrated data collector can be installed.  The data files from these units are

easily readable in Excel, and the files can be sent directly to the Water Board to meet the requirements of SB 88.

What if you don’t have a pipeline?  Then your flow needs to be measured in the open ditch with a weir, flume, or orifice.  These devices measure the flow but they don’t record the data.  To continuously record data, a submersible logging instrument must be used to measure the water pressure at the bottom of the box.  These logging instruments are commonly put into stilling wells that are inside or outside the measurement device.

How are water pressure logger measurements converted to diverted flows or reservoir storage?  Why does anyone even have to have an electronic pressure logger?  Onset ComputerPMCIn-Situ, , and other manufacturers sell data loggers and water level loggers, not pressure loggers, so why is this post talking about measuring pressures at all?

Loggers record pressure, because that is the easiest physical attribute to measure.  A data logger in water does not know how deep it is, and it does not

know how much flow is going by, or how much water is being stored in a reservoir.  Pressures relate directly to static (standing) water depths, and then equations convert the depths to flows, or to reservoir storage volumes.

How is pressure converted to depth?  It’s an easy calculation – water that is one foot deep has a pressure of 0.4335 psi at the bottom.  So, if your logger measures 1.60 psi, then the calculation to get depth is 1.60 psi / 0.4335 psi per foot = 3.69 feet of depth.

Note that water level loggers can be of two types.  The least expensive are completely submersible, and do not compensate for barometric pressure.  For an idea of  the readings of barometric pressure in a measurement device, a 2 foot deep logger records a pressure of 0.8670 psi.  Atmospheric pressure at sea level is about 14.7 psi, and high in the mountains may be 12.0 psi.  Air pressure is much greater than those measured in ditches.  Usually two of these loggers are used at once, one in the water, and one out of the water measuring only air pressure.  This also eliminates the variability in pressure due to weather changes.

The second type of data logger compensates for barometric pressure at the same time water pressure is being recorded.  That way, the water and air pressure data sets do not have to be combined before conversion to depths.  These loggers were always more expensive until the Bluetooth Hobo water level logger came along; as of February 2019 I found that it is the least expensive option for a single location.

Now that you can calculate any depth, how do you convert depths to reservoir storage?  That requires an Area-Capacity curve, also known as an Elevation-Storage curve.  The points can be picked off the curve.  For example, in the curve below, a depth of 8.5 feet would correspond to an elevation of 2,802.5 feet, and a reservoir storage volume of 30 acre-feet.

An owner of a reservoir with a capacity over 10 acre-feet must collect monthly storage values.  That’s easily done by hand.  However, a reservoir with a capacity of 50 AF requires weekly measurement; over 200 AF requires daily measurement; and over 1,000 AF requires hourly measurement.  That is really tedious to do by hand.

This is where an Excel spreadsheet can make the task a whole lot easier!  The spreadsheets shown below are just for this.  The first sheet helps translate a graph into a table of elevations and storage volumes.  The second sheet translates collected pressure values into depth and storage values, for as many data points as needed.

For diversion ditches from a stream, how are pressures converted to flows?  The logger is in a stilling well, usually a pipe connected to the inside or outside wall of the weir, flume, or orifice.  It measures pressure, which is easily converted to depths.

As with reservoirs, Excel spreadsheets make the conversion process a whole lot easier.  The sheets below have the rating curve for a suppressed weir, and the second sheet converts pressure to actual water depths over the weir boards.  Even for thousands of hourly readings, the hourly flow volumes are quickly calculated and are ready to send to the Water Board:

SB 88 requires diverters to measure diverted water flow and/or volume, then report the measurements.  For small to medium-sized diversions and reservoirs, there is a often transducer measuring and recording pressure.  The pressure data has to be converted to depth and flow, or depth and volume.  Data may be hourly, daily, weekly, or monthly.  Whatever the frequency, the Water Board wants data files uploaded with annual Reports and Supplemental Statements.

Where’s the manual for how to do this, for any of several data loggers, and for  meters, weirs, flumes, and orifices, and flumes?  It exists in pieces and parts.  Each data logger manufacturer has a manual for each product.  Sometimes products are similar, and sometimes very different, as are the manuals.  The long-existing measuring devices, weirs, flumes, and orifices, are described and general measurement instructions listed in the U.S. Bureau of Reclamation Water Measurement Manual.

When it comes right down to it, a person has to be a “data-head” to enjoy collecting the data, and going through all of it to find bad results, missing data, and odd trends.  Then stage and flow have to be calculated and checked against periodic readings taken during visits to the reservoir or stream.  Data have to be listed in a format to upload with the Report or Supplemental Statement to the Water Board, and summed monthly to fill out the online form.

If you enjoy educating yourself and taking on new tasks, then you can be a data-head.  If not, then you’ll need to have an employee do it, or more likely hire an expert.

Who are the experts?  There are engineering firms, manufacturers, vendors and others who can download data for you.  It still comes down to the person helping you That person who does the work has to have done data reduction, calculations, checking, and quality control in the past.

Make sure you get help from someone who knows data inside and out!  If the Water Board has any questions, your data-head can explain and defend every bit of it for you.  He or she will already know the answers to any questions that come up.

## See Your Diversion With Social Distancing

Farmers and ranchers don’t spend much energy “implementing social distancing”.  For many folks, they do that already in their daily work.  However, I have a few clients that have to be really careful not to get sick – no shaking hands, keep a dirt road width between us.

That brings up a question:  do I always have to visit your place to see your diversion or reservoir the first time?  Not necessarily.  If I can see some live or recorded video, and some photos, that may be enough to advise you on what needs to be done, and a rough idea of what the options and costs are.  Especially if you are doing part or all of your installation, remote advice may meet your needs.

Do you have a smart phone?  That can take all the field videos and photos, and photos of necessary documents, too.

So, call and tell me what we’re looking at, and let’s try the video and photo option, especially if you are trying not to get sick!  If that just won’t work, then I’ll need to come out anyway, but I’ll have more information from what I have seen already.

## H-S Flumes For Accurate Measurements Of Small Flows

What if you have a small diversion, but grass or debris would interfere with a standard weir?  A weir has to have unobstructed, free-flowing water over

the crest so measured depths accurately relate to a calculated flow.  A weir with debris problems has to be cleared whenever flow is measured, which increases the time requirement.

When weirs have low flows, they trap debris more frequently, and they are less accurate when the depth over the crest drops below  0.2 feet (2.4 inches).  Then the only way to measure flow is with a narrow suppressed weir, or with a contracted weir, typically half or less the maximum width.  A V-notch weir can be used for measurement of low flows.

Changing the weir boards for different flows requires someone with experience,

who will recognize when the depth over the weir is 0.2 feet or less and then use a contracted weir board.  However, people are busy when irrigating, and even busier when flows drop.  Weirs are often neglected during the time they need more frequent maintenance visits.

A good flume for passing debris and measuring low flows is the HS flume.  These are accurate right down to zero flow.  For the maximum flow, they require more

material than a rectangular Winflume, Montana, or Parshall flume.  However, they are more accurate than other flumes at very low flows – testing by the University of Minnesota found an average accuracy to be +/- 3.2% for ideal approach conditions.  They will pass debris down to zero flow – the flume shown here has an opening of 0.05 feet, or 5/8 inch at the flat bottom, and the opening increases with height.

Why aren’t HS flumes common in California?  I suspect that the early adoption of Parshall flumes here established the standard.  I have seen a few hundred flumes, but I had never seen an operating HL (wide, high flow), H, or HS flume, prior to my installations.

Why go to the trouble of using an HS flume, if Parshall flumes are readily

available?  A Parshall flume may be +/- 10% accurate down to perhaps 5% of its maximum flow.  Below that, the accuracy decreases.  An HS flume is +/- 10% accurate down to 1% to 2% of its maximum flow.  If the flow regime is predominantly low with occasional high flows, it is important to measure those low flows with the best possible accuracy.  Some places where low flow measurement is critical include field runoff where pollution is proportional to flow, small water rights, and dam leakage.

HS flumes are easier to construct than a Parshall, too.  The HS flume bottom is flat, and it has 3 vertical planes.  The photos of the Parshall flume here show

that it has 3 horizontal planes, and 5 vertical planes.  An HS flume takes less time to build, and can be put together fairly quickly in any farm or ranch shop.  Parshalls are complex enough that they are purchased, including design and shipping costs.

## Converting Logger Pressure to Depth & Storage/Flows

If you have a pipeline as part of your diversion, then an in-line meter with an integrated data collector can be installed.  The data files from these units are

easily readable in Excel, and the files can be sent directly to the Water Board to meet the requirements of SB 88.

What if you don’t have a pipeline?  Then your flow needs to be measured in the open ditch with a weir, flume, or orifice.  These devices measure the flow but they don’t record the data.  To continuously record data, a submersible logging instrument must be used to measure the water pressure at the bottom of the box.  These logging instruments are commonly put into stilling wells that are inside or outside the measurement device.

How are water pressure logger measurements converted to diverted flows or reservoir storage?  Why does anyone even have to have an electronic pressure logger?  Onset ComputerPMCIn-Situ, , and other manufacturers sell data loggers and water level loggers, not pressure loggers, so why is this post talking about measuring pressures at all?

Loggers record pressure, because that is the easiest physical attribute to measure.  A data logger in water does not know how deep it is, and it does not

know how much flow is going by, or how much water is being stored in a reservoir.  Pressures relate directly to static (standing) water depths, and then equations convert the depths to flows, or to reservoir storage volumes.

How is pressure converted to depth?  It’s an easy calculation – water that is one foot deep has a pressure of 0.4335 psi at the bottom.  So, if your logger measures 1.60 psi, then the calculation to get depth is 1.60 psi / 0.4335 psi per foot = 3.69 feet of depth.

Note that water level loggers can be of two types.  The least expensive are completely submersible, and do not compensate for barometric pressure.  For an idea of  the readings of barometric pressure in a measurement device, a 2 foot deep logger records a pressure of 0.8670 psi.  Atmospheric pressure at sea level is about 14.7 psi, and high in the mountains may be 12.0 psi.  Air pressure is much greater than those measured in ditches.  Usually two of these loggers are used at once, one in the water, and one out of the water measuring only air pressure.  This also eliminates the variability in pressure due to weather changes.

The second type of data logger compensates for barometric pressure at the same time water pressure is being recorded.  That way, the water and air pressure data sets do not have to be combined before conversion to depths.  These loggers were always more expensive until the Bluetooth Hobo water level logger came along; as of February 2019 I found that it is the least expensive option for a single location.

Now that you can calculate any depth, how do you convert depths to reservoir storage?  That requires an Area-Capacity curve, also known as an Elevation-Storage curve.  The points can be picked off the curve.  For example, in the curve below, a depth of 8.5 feet would correspond to an elevation of 2,802.5 feet, and a reservoir storage volume of 30 acre-feet.

An owner of a reservoir with a capacity over 10 acre-feet must collect monthly storage values.  That’s easily done by hand.  However, a reservoir with a capacity of 50 AF requires weekly measurement; over 200 AF requires daily measurement; and over 1,000 AF requires hourly measurement.  That is really tedious to do by hand.

This is where an Excel spreadsheet can make the task a whole lot easier!  The spreadsheets shown below are just for this.  The first sheet helps translate a graph into a table of elevations and storage volumes.  The second sheet translates collected pressure values into depth and storage values, for as many data points as needed.

For diversion ditches from a stream, how are pressures converted to flows?  The logger is in a stilling well, usually a pipe connected to the inside or outside wall of the weir, flume, or orifice.  It measures pressure, which is easily converted to depths.

As with reservoirs, Excel spreadsheets make the conversion process a whole lot easier.  The sheets below have the rating curve for a suppressed weir, and the second sheet converts pressure to actual water depths over the weir boards.  Even for thousands of hourly readings, the hourly flow volumes are quickly calculated and are ready to send to the Water Board:

## Maybe: Measure H2O Phone App By USBR

A watermaster recently showed me a handy phone app

to calculate flows through your measurement device.  Measure H2O  works on iphones and android phones.  I have used it for several devices – it’s quick and accurate.  Just fill in the blanks and out pops the flow amount.

However, it is currently not available when I hunt for it in the phone app store.  I don’t know why, I’ll see if I can find out.

Data needs to be collected at your diversion or reservoir at SB 88’s required intervals, whether monthly, weekly, daily or hourly.  This could help you meet the need for monthly or weekly data, if you can visit the site that  often.

## How Do You Record Diversion Data? Water Level Loggers, Value Vs. Costs

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 retired 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 download 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   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!