Another likely dumb question, but I measured the amount of water my lawn subsurface irrigation uses, and it comes to around 6.5 cubic ft per hour. How do I convert that to the “inches per hour” unit in the rachio app. Over what area is that inches per hour measured? Or is it cubic inches per hour (probably not as that would be a huge number)?
You have the flow (cfm or gpm works fine, just with the appropriate factor). The second part of calculating Nozzle Inches per Hour is the Area. You divide flow per unit time by area to get flow per inch.
Hmmm, I think you may have a decimal wrong (VERY easy to do with water meters, which often have a “0” written in at the end). One cubic foot is 1728 cubic inches. One gallon is 231 cubic inches. So 6.5 cubic feet per hour is 6.5 x 1728 / 231 = 48.6 gallons per hour or 0.81 gallons per minute. My zones run from 10.5 to 12.5 gallons per minute. So unless that is for a very small drip zone, it’s probably 65 cubic feet per hour or about 8.1 gpm. And to be honest, that’s probably hard to measure over a full one hour period, due to other water usage, etc.
Another way to measure flow (which should be done separately for each zone, IMHO) with a cubic foot meter is there’s usually an outer dial that makes one full revolution for one cubic foot of water. I’ve timed the number of seconds it takes to flow one cubic foot, and calculated the flow that way.
But the second part - Area - is what area of your yard or crop the zone irrigates. If it’s a 25 x 50 area, that’s 1250 square feet. If you have a Rachio 3, you can often have it calculate the area of each zone using its Yard Map feature on your phone.
So using the above numbers,
Nozzle Inches per Hour = Flow in Gallons / Time for Flow / Area in Square Feet x 96.25.
For the above that’s = 8.1 / 1 / 1250 x 96.25 = 0.62"/hour
Yep, you’re right on the missing decimal, I didn’t realize the number the dial points to needed to be tacked on the end of the numbers. See if this makes more sense:
The meter started at 158469 ccf
After 15mins it read 158532 ccf
Which is 65ccf / 15 min = 252 ccf/ hr
That’s 14101.5 ccin/hr
Area of grass = 163 sqft = 23329 sqin
So, 14101.5 / 23329 = 0.604 nozzle inches per hour.
While I thought I agreed with the result, I’m confused by the math:
158469 - 158532 = 63, not 65, but converting it to hours it does come out to 252 cf/hr.
The problem here is you said previously you measured 65 cf/hr, and now have 252 cf/hr.
Using my conversion, not yours, 252 cf/hr x 1728 / 231 / 60 = 31.4 gpm (gallons per minute). WOW, that’s a lot of flow. My flows are all in the 10-12 gpm area, so that just seems really high, unless it’s a commercial system with large pipe & tubing.
Using your calculations to convert from ccf to ccin (cubic feet to cubic inches), there would be 14101.5 / 252 = 55.95 cubic inches in a cubic foot. But there aren’t. There are 1728 cubic inches in a cubic foot.
Assuming the 65ccf/15 minutes is correct, I’d do the math like this:
The meter started at 158469 ccf
After 15mins it read 158532 ccf
Which is 63ccf / 15 min = 252 ccf/ hr
That’s 435,456 ccin/hr
Area of grass = 163 sqft = 23,472 sqin
So, 435456 / 23472 = 18.55 nozzle inches per hour, an incredibly high number
Doing the math a different way as a check:
63 ccf/15 min = 63 / 15 x 1728 / 231 = 31.42 gpm which as mentioned above seems really high.
Then 31.42 / 1 / 163sf x 96.25 = 18.55 in/hr, so that checks out.
The reason this answer is so much different than the previous calculations is previously you said you got 65 ccf/hr but now it’s almost 4 times that (63 x 4 = 252). And in my previous calculation I put in a guess of 1250 square feet, and you actually have 163. Yet your calculations just happened to almost match mine.
I’m not sure what to say. Even correcting the math results in an answer I don’t think is possible. And I hate to doubt your measurements.
Hi! I live north of the SF valley, so I feel your pain about our hot, dry summers!
If you really want to know the nozzle flow rate, and can’t get to the water meter (which I also feel for, given that mine is located in my neighbor’s yard and the cover regularly gets buried under bermuda grass), you could try a catch cup test. The easiest way of doing that is ordering some catch cups on line and then using the manufacturer’s handy website calculator to determine the answers. That will give you at least a ballpark figure on what the nozzle flow rate is, although the placement of the cups can affect that.
Another option, which isn’t as good as a catch cup or meter read calculation, is to find out what kind of sprinklers/nozzles you have, and use the manufacturers spec sheets. This isn’t perfect by any means, but will certainly dial you in better than a wild guess…
Hi Richard,
Sorry for the typo and unit mix-up, agreed should be 63 cf / 15m. I also found a mistake in my spreadsheet and now agree the nozzle in / hr appears to be 18.552. I’m m aking a meal of this.
Also agree this seems ridiculously high. I must be reading the meter wrong, and infact just noticed that the dial numbers are tenths (didn’t see the tiny decimal points). Here’s a picture of my water meter, It’s a Badger Water Model 25: https://www.dropbox.com/s/2qpyc411z1sm6hi/2021-05-12%2011.37.07.jpg?dl=0
So, here’s how it read before and after, the square brackets show the black numbers, and the angled bracket shows where the pointer was on the dial:
Start: 0158 [46] <.92>
Finish: 0158 [53] <.20>
So I think my original placing of the decimal was correct, and the flow rate is actually 25.2 cuft/hr
This would come out to Nozzle inches per hour of 1.855, which seems more reasonable. Right?
It’s actually a subsurface drip system for a lawn, but thanks for the info
Sorry, missed that part in this thread…
Sometimes I wish I had something like that! Assume you have a Netafim type of product? Most products like that can vary a ton based on a lot of aspects of the installation, but your findings seem doable, based off a spec sheet from Netafim…
Yes, it’s Netafim Techline, 12" pitch, 0.6GPH emitters. It seems to be working well, the new sod’s taken well, hoping it’ll be more efficient overall than spray. It definitely isn’t even in distribution though, more comes out near the supply line, which makes sense given the pressure drop over the system. I had hoped the nozzle flow would be so low it wouldn’t be an issue.
Let’s see, your flow is actually 1/10th of what we thought last (as you say, you were right, originally), so total flow for the zone would be 3.14 gpm rather than 31.4 gpm, which is low for a normal zone, but not for a small subsurface drip system. And the Nozzle Inches per Hour comes out to, as you say, 1.855"/hr, rather than the impossible 18.55"/hr.
Just wish all this part was in a separate thread; it’s had enough keeping these straight.
Just wish all this part was in a separate thread; it’s had enough keeping these straight.
Indeed, seems to be the way these forums go though, and why it’s so hard to find the answers sometimes. Thanks for the help. Fundamentally it’s all super simple, but it’s made so confusing by inconsistent, vague, and often downright dumb units the industry seems to use.
Absolutely true. Between variation of units, and to me some very confusing terminology (much of it standard, though), it makes it harder than it should be. And to confound that, explanations of how it all works, valid ones, make it more confusing. Rachio has some good examples on their web site of how the system works, and that helps some.
That chart is from the Techline series, since that is most widely used in residential landscaping…These systems can be amazing, but really do need to be set up right to function well. You are technically a tad under double what you “should” be for .6gph, but not knowing your spacing, pressure, etc. it still could be pretty accurate.
It is a cool product. I had one of my customers trying to get me to redo my lawn with it, but I didn’t want to spend the money to redo my lawn when I had a perfectly good sprinkler system in place (just needed new nozzles). If I was starting from scratch though, I’d really take a hard look at it.
Final twist to the story of this thrilling ride through irrigation land, it turns out the Rachio app under estimated the lawn area based on aerial photograph by 50%, I measured the space and found it to be around 370 sqft, so it turns out I’m at 0.77 nozzle inches per hr, which is in the right range according to that table, right? Now I understand what those numbers mean too.
Almost dead on! Isn’t it fun! 
That’s kind of disturbing. Wonder what the problem is/was? I found mine to be pretty close.
Anyhow, glad you’ve nailed it down and makes sense too! Best of both worlds!