Tagged: monitor my watershed
2019-12-18 at 12:02 PM #13484
I have a use case where I need to monitor water depth and discharge in an earthen irrigation ditch, and plan to use a Mayfly and MMW to collect and report the data in real-time. I estimate the ditch is approximately 10 feet wide and 5 feet deep. We’ve done a number of Mayfly installations on streams using a CTD sensor, but in this case, we only need to measure depth (and calculate discharge, once we develop a rating curve for the site). Is a MaxBotix MaxSonar a good choice for this? If so, can you recommend which sensor model to consider? I’ll be using the ModularSensors library, and from the documentation, a TTL-based MaxSonar sounds like the best option.
Finally, any suggestions for mounting the sensor? Since this is a regulated ditch and has a screened diversion, I don’t anticipate that we’ll see a lot of floating debris nor channel movement.
Thanks for any experience you can share!
2019-12-23 at 4:40 PM #13489Sara DamianoModerator
I don’t remember exactly which models we’ve tested or which seems to be the best; maybe Shannon can pipe in.
No matter which you’re using, definitely do get the MaxTemp.
2019-12-23 at 6:15 PM #13490Shannon HicksKeymaster
We usually use the MB7389 because it has the proper filtering for looking at a single target such as water, and has TTL output, so it’s easy to read with the Mayfly. They make lots of other versions, depending on whether you want to look at snow, or people, or multiple objects at once, and with different outputs (TTL, RS232, PWM, analog, 4-20ma). And they make the MB7589 which has a heater to keep condensation from building up on the transducer, but it uses lots of power and wouldn’t be good for a battery powered station. If you use the external temperature sensor for compensation (which is usually necessary to improve performance) you have to be careful about where and how you mount that, because bad readings with the external temp sensor due to sunshine or other factors, will cause your ultrasonic sensor to over- or under-adjust the reading and cause errors. The ultrasonic sensors don’t work well in small bore stilling wells or pipes with a diameter less than about 6 inches. And the inside of the pipe must be completely smooth with no obstructions or joints, otherwise the sensor will just pick that up and never “see” the water. Even things like a spider on the wall of the pipe or some condensation will cause the sensor to pick that up instead of the water. The Maxbotix manual specifically states that using the sensor in a pipe will void the warranty and they can’t guarantee good performance. Overall, we’ve had too much trouble with reliability and accuracy of the sensors to deploy them widely. I know some other people are using other methods of ultrasonic or lidar sensors for measuring water in storm drains and ditches, but we usually use submersible pressure transducers in our applications so I have not investigated ultrasonic sensors much further. The sensors have a blanking distance of 50cm, so you’ve got to make sure the water won’t get any closer to your sensor than 50cm, or you won’t be able to measure it. The sensors have standard 3/4″ NPT threads on them, so you can find a variety of couplers, elbows, etc in the plumbing section of your hardware store that you can screw the sensor into, so we usually just bought whatever pipes and adapters we needed to put the sensor in the proper location, and then ran the wires through the pipe and out the end and over to the logger box. I don’t think I have many pictures of the installations anymore, but find the threaded pipe section (either metal or PVC) at your hardware store and I think you should be able to put together a mounting bracket for one that will fit your location.
2019-12-23 at 6:49 PM #13491
Excellent info! Thanks to you both!
2020-01-03 at 1:39 PM #13532
If you all have found that a submersible pressure transducer is a more reliable way to measure depth, we may consider that. Do you have a model that you would recommend?
2020-01-08 at 3:03 PM #13557neilhParticipant
I look at the measurement accuracy, and vented depth sensors. Especially at lower water levels vented depth sensors are I believe the most accurate.
Good measurement accuracy starts at about 0.25% of range and optionally can be of 0.1%
I’ve been using the Keller Acculevel for ranges over 10′, and characterizing the Keller Nanolevel for ranges under 10′ – both of these use a Modbus interface – RS485 physical driver and excitation voltage of about 12V.
The accuracy including temperature compensation can be selected to be 0.1% – ie for 10’range is +/-0.01′(+/3mm) – across temperature variations. That is if you have a smaller temperature range the accuracy can be better than 0.1%
https://www.instrumart.com/products/33962/keller-acculevel-submersible-level-transmitter (Start ~$527)
However if accuracy is important, you have to dig a bit deeper in to the specification, I can explain if interested.
The do have a Keller Digilevel SDI-12 which has the same spec – but haven’t seen anybody interface to it. Shouldn’t be too hard I would think, but there isn’t much value to their SDI-12 as it still requires 6V excitation.
We initially used the Keller LevelGage sensors, but in streams, they started failing after a couple of years and where replaced by Acculevel then Insitu LT500.
The hydrologists I work with have good experience with Insitu LT500 series for accuracy (0.1%FS and 0.01%FS) and stability. They are expensive (base $2K+) and are standalone, they have battery and loggers, the cables are highly engineered devices and cables. Recently I’ve had two of the pig-tail connectors fail (at$350each). There isn’t a ModularSensors library for them yet.
I’ve blogged about accuracy on sensors (but not specifically the Keller AccuLevel).
I recently got a quote for the CTD-10 (10m or 30′ – they only do the 10m now) with a fixed accuracy at 20C of 0.05% Full Scale or +-/5mm and no statement of how it varies with temperature (called temperature compensation).
UpdateJan9th: I got a reply from firstname.lastname@example.org “We typically evaluate this from 0 to 50C and we see somewhere around +/- 2 cm in that range. It does vary sensor-to-sensor and some sensors have lower sensitivity.”
So practically they are saying temperature compensation is challenging, and they could have the readings varied by ±25mm or 0.25% of scale – and they wouldn’t take the instrument back.
https://www.metergroup.com/environment/products/hydros-21-water-level-monitoring/ Quote $491 per sensor and with a 10-meter cable.
The CTD-10’s value seems to be its widely used in ModularSensors, and can be interfaced to simply with the base Mayfly, though I don’t have any experience with it.
Hope that’s useful, and not too much info!! There are a few people using Acculevels, so if you have any other questions, maybe you can describe the expected site and also the accuracy you are aiming for 🙂
2020-01-13 at 6:38 PM #13606
Hi Neil, thanks so much – the detail is appreciated! I need to check with our hydrology staff to learn their accuracy requirements. To date, our Mayfly deployments have used the CTD-10 from Meter, but we have a new site where we’re only interested in depth, and the MaxSonar would be a cost savings over the CTD-10 in that case. However, I’m hesitant to go that route now because of @shicks ‘ comment about having trouble with MaxSonar’s reliability.
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