One Water Saving Sensor Too Many? - A Dialogue

By Michael Gutierrez

Over the summer an area utility came to us with a technology problem. Due to an incentive-based program, soil moisture sensors were being installed on new housing development irrigation systems. This is just the kind of best practice we love to hear about. In this instance, however, the installers elected to add rain sensors as well. Wiring issues ensued.

This month the Dukes research group is conducting its 3^rd smart water technology workshop of 2016. To mark the occasion, water-use expert Dr. Michael Dukes agreed to a discussion on the above “two sensors” situation so we can all better understand the installers’ intentions and what went wrong. This is also a preview of some of the topics that will be addressed in next week’s training.  

Dr. Michael Dukes instructs during a smart water technology training earlier this year

MG: Rain sensors v. soil moisture sensors – which is more effective?

MD: Soil moisture sensors are definitely more effective than rain sensors. We’ve shown that time and time again. They’ll reduce irrigation two to three times more under the same conditions. And longevity is better in soil moisture sensors. Despite this, rain sensors have been around for a long time and they are in everyone’s way of thinking – everyone being contractors, practitioners, utility people. To us, being close to the research it makes sense just to use a soil moisture sensor. “Why would you consider a rain sensor based on all this research?” Well, for practitioners one of the things that is on top of their mind is seeing an irrigation system run when it’s raining. A rain sensor would stop that. Their next thought is “why not add a soil moisture sensor. Wouldn’t my results be even better?”

A soil moisture sensor properly installed in an undisturbed soil profile

MG: As people that specialize in encouraging best practices, we like seeing practitioners install water saving devices. In reality, one well-installed, functioning device on a system is a rare thing. Two devices is something I’m still trying to wrap my mind around.

MD: In this instance, the only reason is to get the instant shut-off that a rain sensor would provide during a rainfall event. But think about it this way: we only have a limited amount of research on testing a soil moisture sensor with a rain sensor together and in that limited study there was a benefit. It wasn’t a great deal, but there was a benefit. But think about the conditions where you get a benefit: there has to be irrigation during a rain event and if you only have several irrigation events scheduled per week that means everything has to line up perfectly. I’m not sure how likely that is. The benefits to adding a rain sensor is probably marginal because we already know they require more maintenance. You’re adding something that needs more maintenance for some perceived benefit which is not all that great.

Rain sensor: going the extra mile and getting it right

MG: This is what I was thinking. Installers may not be so familiar with the research, but they know rain sensors. What it’s like to work with these devices. Why…

MD: There’s another angle on this though: the soil sensor technology we’re discussing in this instance is slower in reacting to moisture. So the addition of a rain sensor may actually help address this. It’s not a terrible idea.

MG: So intentionally or not, installers in this instance may have compensated for a short-coming of one device by adding another.

MD: That’s exactly right.

MG: I just stumble on the idea of willingly pairing two devices, when you know one will deteriorate before the other.

MD: Perception is reality. Perception is a big deal. We’re in the process of publishing a paper on the OCU research. And surveying homeowners participating in that study, their perception of water savings was more important than the actual water savings in their attitude of whether they were going to continue using smart irrigation technology in the future.

The Orange County Utilities (OCU) study also includes weather-based irrigation controllers (WBICs)

MD: The important part here is: having a rain sensor to prevent a system from running when it’s raining, that goes a lot toward perception. Having a system with only a soil moisture sensor is probably not a real problem in the long run, day in and day out over the year. But let’s say a developer sees a system watering in the rain one time. That may mean the difference between these new homes continuing to get this technology or not getting it.

MG: The only reason we’re actually aware of this "two sensors on one timer" practice is that something went wrong. Can you talk about this?

MD: What happened was the timers in this instance have a rain sensor port and the installers wired both sensors to that port. They intended an either/or scenario to interrupt irrigation. But when both are wired in this way they are in parallel, so both have to trigger to interrupt irrigation. They needed to be wired in series, so either/or would break the circuit.

When the soil moisture sensor can wire into the rain sensor port, a series connection is necessary (courtesy: Francis Galdo)

MG: In summary, if you insist on using two devices – rain sensor and soil moisture sensor – with your timer….

MD: Know your wiring.

Catch the Dukes research group in Tarpon Springs next week for more in depth insight