How about dew point from a weather API/station?
or you can attach a moisture sensor with probe to the lawnmower.
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This, ideally from a weather station on-site.
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I found this about dew point :
"The dew point temperature indicates the temperature at which the air must be cooled in order for it to reach a relative humidity of 100% and become saturated with water vapor. When a surface is cooled beyond that point, the water in the air condenses, forming beads of liquid water – or dew.
When the dewpoint and temperature are relatively close, you will likely be waking up to a lawn covered in dew the following morning."
So, what kind of rule would you make?
If current temp is within 5 degrees of the dew point, consider dew present?
Or if current temp is below the dew point, consider dew present?
Does the amount of cloudiness matter? I also understand that dew is more likely to be present if it's a clear sky than if it's cloudy.
Interesting idea. I'll probably experiment with this to see how accurate it ends up being. Good thought.
To be honest, I don't really know how many degree you will set to have dew on your lawn.
I am in Canada West coast and dew is pretty much an everyday thing. I know I have dew last night and my temp is around 6C and dew point is around 3C.
Collectively these guys gave you most of what you need to know. Dew is coincident with the air temperature at the ground level reaching the dewpoint temp. In most areas (unless there is some local valley/mountain air current in play) the summertime temp low occurs within the hour and a half before sunrise.
I would bet you could cut between midnight and 3AM and be pretty safe without over sensor-ing the problem. However, this all does depend on where you are. Coastal weather is a whole different kettle of fish. Cold damp air masses, and rain, could come in with those evening sea breezes that are driven by the land thermals.
And about neighbors, if you are up North it is the case that a lot of folk don't have AC right? Annnnnd it might be the case that a lot of those folk sleep with open windows no? If you apply any sensor it might be a presence sensor so you know when someone runs over and grabs the thing to toss it in the local lake.
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I seem to recall that robot mowers just cut a bit at the top since they cut so often - would it be a problem in that case if the grass is wet?
Either dew point-temperature comparison or hi relative humidity should do it. Those may also correlate to rain reasonably well. You'll likely have to build in a long delay before resuming after a dew/rain event. On a calm, overcast day, it will take a longer for dew to disappear than it will with a bit of sun or breeze.
Tasmota with a bosch BME280 sensor will give dewpoint readings (along with temp, humidity & pressure) and can be imported to HE via node red or MQTT.
You might be able to use a leak detector (in a suitable enclosure) with a remote probe laid close to grass in flowerbed, etc as an actual detector.
Could you sit a leak sensor facing up near the the mower? Not sure if dew would produce enough moisture to establish a circuit or not, I've never used on myself...
Well, dew point alone doesn’t seem to predict morning dew for me. This morning there is morning dew even though the temperature was at least 10 degrees F above the dew point all night. Unless there’s some other variable to add to the rule along with dew point? Meanwhile I’ll test out a moisture probe to see if it will pick up dew.
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Is the dew point calculated from sensors right outside your home? Or a weather service?
This. Do you think dew point is hyper local to where it would deviate by as much as 10 degrees F? I guess you’re right though in the sense that perhaps the location of the dew point sensing is contributing another variable to the equation already….wonder if there’s a cheap PWS that I could give a try.
Honestly, I dunno. Sometimes temp and humidity conditions in your back yard can vary notably from nearby weather stations that supply data to cloud services, depending on geographic features, lakes etc.
To calculate dew point, I believe all you need is an outdoor temp and humidity sensor. I like Ecowitt devices, although you need their wifi bridge, which can integrate with Hubitat.
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Ah, that's true. Do you think a product like the Zooz ZSE44 would work well? It says to mount it under the eve, to avoid direct rain or direct sun. It would pair directly to HE without any gateway, so that would be nice...
I’m not familiar with that sensor specifically. But it says it’s rated for outdoor use, so probably a reasonable option (if it’s accurate). Zooz is a pretty well-liked brand among z-wave devices.
It could even be different at ground level vs. 12" off the ground. Soil moisture, wind conditions, etc. could all play significant roles.
I wonder a soil moisture sensor will be good enough by leaving it half exposed above ground.
Perhaps. I thought about testing one out, especially since I could use one for the vegetable garden anyway. I'll have to figure out which one to get, preferably z-wave.
Wrapping up this thread. Thanks to everyone who contributed. I ended up purchasing an Ecowitt Leaf Wetness Sensor and it has worked great in detecting dew on the grass. After playing around with the placement, angle, and threshold values, I now can say that I am able to reliably detect when there's dew on the grass. And, at least at this time of year, it's a lot more often than I ever knew! The HE integration for the Leaf Wetness Sensor is here.
This sensor has proven much more reliable than other methods that I tried. The easy way was to detect dew presence based on dew point. While that might work theoretically, it did not work well for me in practice, even with an outdoor Ecowitt humidity and temperature sensor.
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