What is a stilling well?
In monitoring locations where turbulence, waves, or other factors might impact the accuracy of water level readings, constructing a stilling well creates an isolated chamber for a sensor, data logger, or gauge to measure water level and/or barometric pressure, and it makes it easier to deploy and retrieve your monitoring equipment. Stilling wells can be used in both aquatic and groundwater applications:
In streams, rivers, coastal areas, canals, and dam management applications to dampen transient fluctuations and protect sensors, loggers, and cables from passing debris
In wells, wetlands, vernal pools, and other areas where you need to get a water level sensor into the water table while keeping soil and silt away from the sensor
Onset’s many water level monitoring solutions, like submersible water level loggers, Bluetooth-enabled data loggers, and remote monitoring stations that provide cloud data access, can all be used to collect water level data for a wide range of applications. Any of these can often be deployed in existing wells. In many cases, however, you’ll need to build your own stilling well, which is actually quite straightforward. How you construct your stilling well will depend on:
- Whether the data logger/sensor will be inside or outside of the well
- Deployment conditions and location
- Cost
What materials do you need to build a stilling well?
Although many materials can be used to build a stilling well, PVC is an excellent solution for both ease and longevity. It withstands harsh UV exposure and outdoor conditions, reduces maintenance concerns, and makes feeding your data logger's sensor to the bottom of the well much easier!
You can build a stilling well with any diameter of PVC pipe. However, for most deployments we recommend using 2" PVC, which fits our HOBO® Well Cap and easily allows a water level logger or sensor to feed through with plenty of clearance. However, for wells being installed into the ground or shorelines, many prefer smaller diameter pipes, which make drilling the holes needed to install these much easier.
Materials Checklist for Building a Stilling Well
Enough length of PVC pipe to construct a line that reaches from the surface to the depth of water level you want to monitor. If your logger needs to be vented to the atmosphere, make sure you have enough pipe to extend above the highest potential water level so that the vent or barometric pressure sensor do not get submerged
Tools for cutting and/or drilling the PVC pipe
PVC cement for gluing the pipe sections together
Well cap or mounting bolt to suspend the logger or sensor cables
Slotted PVC pipe / well screen, or section with holes drilled into it, for the bottom end of the well. If installing the well into soil or sediment, make sure the slots are fine enough to keep soil or sediment out of the stilling well. For fine silts, you'll need slots no larger than 0.01 mm
Optional end cap for bottom of the well. This prevents materials from entering through that end and keeps the stilling well from sinking. If the well is being driven into soil or sediment, you will need a pointed end cap
Optional for open-water wells: filter fabric to wrap end section to prevent silt from entering well
Optional: a padlock to secure the well
Now that we have our materials...let's get started!
Use Slotted PVC for the Stilling Well's Bottom Section
The pre-slotted PVC drainpipe or well point section at the end of your stilling well will allow water to flow into it while keeping debris out. For stilling wells in water, you can also use a section of PVC with several 1/4" holes drilled into it. (If you're using slotted PVC to keep fine silts out, check that you can’t see any light through the slots. You may also want to wrap it in landscaping fabric for further filtration.)
When it's time to deploy your water level logger, it’s best practice to suspend the logger slightly above the bottom of this section, both to prevent the logger from getting clogged with sediment and to ensure the logger/sensor always returns to the same position after it’s been removed for offloading or maintenance.
Assemble PVC sections to reach desired length/depth
Using straight sections as much as possible, construct your line. This will run from the slotted end pipe to above the surface or ground level. (If you need to follow streambank contours, use sweeping elbow sections that will allow the logger to slide through easily). We recommend extending the well pipe casing 6 inches or so above the ground to help keep dirt/debris out, and above flood levels.
When using the HOBO® U20L logger alone, since it is completely submersible, the casing and well cap do not necessarily need to extend above the ground or above flood level. However, if you’ll also be using a barometric logger or vented logger in the same well, keep in mind that the PVC pipe will need to extend far enough above flood level to ensure that the logger or vent remains above water.
NOTE: The Bluetooth-enabled top unit of our MX2001 logger needs to be located above flood level. If the logger will be in high humidity/saturated conditions, further ventilation is also needed to prevent condensation in the logger. Doughnut-shaped grommets that come with this logger can be used to create a more ventilated/unsaturated area around the top of the logger. This section must have at least two holes for cross-ventilation. See the MX2001 User Manual for more details.
Place a well cap on your stilling well
Capping your stilling well helps keep out debris, insects, and small animals, while also keeping your logger secure. For wells supplying potable water, be sure to use an approved well cap for drinking water to prevent contamination.
Well caps are usually made of aluminum or thermoplastic, and include a vent hole to equalize any pressure difference between the inside and outside the well. Your choice of well cap will depend on whether you will be mounting the logger entirely in the well, or if it's going to be outside the well with a cabled sensor running into it. Specifically designed for these deployments, Onset's two locking well cap options are a great solution that come with these features already built-in:
Convenient tap-on installation
Pre-drilled ⅛” barometric pressure vent hole
Two well-cap configurations:
Well-Cap-01: Two attachment points support hanging of suspension cables OR convenient mounting of our wireless HOBO® MX2001 Bluetooth-enabled data logger at the top of the well
Well-cap-02: Compatible with Onset’s direct read cables, this cap has a slot for the logger's cable to pass through, and a disc for suspending the cable to the sensor
Alternatives for Stilling Well Caps
If you want to use a well cap that doesn't already have a vent, removing the O-ring (if any) from the cap or drilling a small hole in the pipe or cap should suffice for venting the pressure inside the well to the atmosphere.
An affordable, easy option, if you don’t want a specialized well cap, is to place a ¼” bolt through the top of the well that suspends the logger by its cable in the well. You can get materials for this from any local supply store:
Make sure the bolts and nuts are made from a material that won’t rust (brass bolts and nuts work well in saltwater environments)
Drill ¼” holes for the bolt to pass through the top of the pipe
Form a loop in the sensor cable large enough for your finger to hold the cable while feeding the bolt through it
Tighten the nuts down enough so that someone can’t remove them yet doesn’t damage or stress the PVC
Ultimately, each application for a stilling well is different. As long as you have considered water levels, protection from silt and debris, sufficient venting for pressure, and how far above the surface to extend your well, you will be able to build a stilling well that best suits your application.
For instructions on how to use suspension cables to hang HOBO Water Level Loggers in a well, see Wireline Mounting a Water Level Logger in a Well.
Need more support? You can always contact our tech support team.