Water flow sensor calibration

I’m a new Rachio user and I installed a CST FSI-T10 water flow sensor in my main irrigation line to detect possible leaks but also to measure irrigation water consumption.

After some initial tests comparing the CST measurements reported on the Rachio App against the city water meter, by running the sprinklers for a few minutes, I noticed that the flow sensor consistently overestimated the consumption by 10-15%.

Last month I was traveling for about 3 weeks, and I wrote down the exact numbers right before I left and immediately after I returned, since the only water usage during that time was my irrigation system.

The city meter showed a water consumption of 3601 gallons, whereas the the Rachio app showed 4056 gallons.

That gives me an exact number of how much off is the flow sensor measurement compared to the city meter: 4056/3601 = 1.126 or 12.6% overestimation.

My question: How can I calibrate my Rachio controller to measure the correct consumption based on that information?

The Rachio app does provide an option to calibrate the controller but there should be a way to do that programmatically.

I would appreciate your help!

P.S. I have tested the wires for possible short-circuits using a multimeter and they seem clean.


As municipal water meters age, they tend to under-report consumption.

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Hey @vmetsis-

There is currently no way to do this via our software. Have you perhaps contacted your flow sensor provider to see if there is any calibration that can be done with the hardware? Just an idea!

McKynzee :rachio:

@sunny The installation is in house which was just build, so the water meter is no more than a few weeks old.
But even if it was the case that the city water meter was underestimating the consumption (highly unlikely), I would still want to adjust Rachio to agree with it, since I am paying based on the city meter.

@mckynzee If you are familiar with how the flow sensors work, once the hardware has been manufactured you cannot change anything on it, but you can adjust the measurements accordingly through the software.

Essentially, they way the flow sensor works is that it sends a pulse signal to the controller at a certain frequency based on the amount of water flowing through it. The bigger the flow the higher the frequency.

Then the controller estimates the Gallons per minute (GPM) based on the following formula:
FREQUENCY = (GPM / K) – Offset
*GPM = (FREQUENCY + Offset)K

‘K’ and ‘Offset’ are two constants that depend the diameter of the pipe and other factors.

These constants are entered to the program that calculates the GPM, in this case the Rachio software.

The manufacturer provides some default for K and Offset for each sensor, but they can be calibrated accordingly.

The default values for the FSI-T10-001 are K = 0.322 and Offset = 0.20.

I don’t know if that’s what your developers used, but apparently in my case these values don’t work well and they need to be adjusted.

The Rachio app ideally should allow the users to edit these values to their needs, but since you don’t provide that option to the end users, one of your developers would have to do it for me.

Can someone from the rachio team with more technical expertise help me? The price I paid for that sensor is higher than the Rachio controller itself and I would like to have accurate measurements of water consumption.


Hey @vmetsis-

We use the default values given by the manufacturer, this is why we require the user to select the type of sensor they are using. I asked about contacting the manufacturer because I would be curious as to why these defaults would not be working when you have the correct sensor selected in the app. I can speak to our development team, but I am not sure if these values can be changed on our end. I will get back to you on what they say.

McKynzee :rachio:

Just my 2 cents. Today I did a comparison between my Toro flow sensor and the water meter and the difference is only 1 to 2 gallons. :sweat_smile:

@vmetsis - question, you used 3,601 gallons in 3 weeks. How many heads you have. Just curios, I have 6 zones with a total of 12 rotors and 8 spray heads and I’m using 720 gallons for 120 minutes. My rotors have 2.0 GPH nozzles. For some reason I feel this usage is high but I don’t have anything to compare.

One could assume that the city water meter would be not be underestimating water use, but it’s not necessarily the case.

My brother and I just recently had a discussion about the accuracy of water meters after I mentioned our municipality was updating meters. He is a certified water system operator and has been the field operations manager for water systems for many years.

There are several types of water meters (positive displacement, single-jet, multi-jet, propeller, etc.) and while there are many manufacturers, only about six manufacturers supply meters to the majority of municipalities in the U.S.

A study done to determine the accuracy of new, off-the-shelf meters found that depending on the type of meter, size and flow rate, a surprising number of meters failed to meet AWWA (American Water Works Association) flow accuracy standards even though the meters are reported and advertised to meet those accuracy standards.

Only one manufacturer was able to meet the AWWA standard at all three flow rates more than 95% of the time.

None of the other 14 manufacturers tested were able to meet standards more than 85% of the time.

One manufacturer’s new meters only met the AWWA standard at all three flow rates 17% of the time.

Seven manufacturers only passed all three AWWA standard flow rates 50% of the time or less.

Another study of in-service meters found of pulled meters, some failed to meet AWWA flow accuracy standards after a short time in service or, in some cases, immediately after installation.

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@mckynzee I can try to contact CST, but I doubt they will have a way to know why the default parameters don’t work well in my case without testing that specific flow sensor, which I cannot send for testing, because it’s already buried underground. Like another user suggested, it could be the city meter that is underestimating but I have no way to verify that. The only think I can hope for is to adjust the parameters of the flow sensor to make it agree with the meter.

@fpuig I have 5 zones but I only run them for 7-8 minutes each, every 2-3 days. I’m probably under-watering but I just want my grass to make it through the summer without dying.

@sunny That’s useful information. I would be interested to know how much off can the city meters be. I would assume they would allow a 2-3% margin of error, but >10% would not be reasonable. In any case, as I mentioned, since I am paying based on the city meter, I want my flow sensor to agree with it.

Hey @vmetsis-

I spoke to the team and I believe we will be able to adjust your values. I can completely understand wanting to match the meter since that’s ultimately what you pay!! Have you recalculated your values to adjust for the amount your reading is off?


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Hi @mckynzee,

If my calculations are correct, we need to set K=0.2859

Let’s try that and I will make some new measurements to see if it worked.


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I’m having th same issue with my CST FSI-T10 except Rachio is showing that my water useage is ~15% lower than what my water meter is showing, which I am fairly sure is not correct. Based on the statement above, I hope you all didn’t/won’t change the factory k value for everyone that is using the settings for that particular sensor on your application… Can your software developers please provide a “custom” flow sensor selection in the drop down menu with the ability to edit/adjust the K-value and offset numbers so that your customers can calibrate their own system? In contrast, the Weathermatic Smart Link systems provide the ability to adjust “pulse per gallon (PPG)”, which may be the frequency in the equation, with a “custom” selection in the flow sensor menu to calibrate the CST FSI-T10. This may be a better approach (or maybe not) to custom calibration of flow sensors that have a discrepancy with water meters. Regardless, the ability for the user to easily edit these values in the application is definitely needed.


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This situation begs the question–which is accurate–the flow sensor or the water meter?

I’m betting that the water meter is the issue. Compared to the flow sensor, one meter may be under-reporting, one over-reporting.

There are so many variables affecting the water meter accuracy such as type of meter, proper installation, flow rates and maintenance. Are the methods of measuring flow rates the same or is an electromagnetic meter being compared to a displacement or velocity type?

Water systems deal with non-revenue water–water that is lost before it reaches the customer meter or that lost due to leaks or line breaks or as a result of meter inaccuracies. Reducing NRW is addressed differently by each system depending on economic factors of that system. NRW losses are often offset by fees to the customer. Billing statements usually have a “base fee”, but may have “recycling fee”, “water quality fee” or “water rights acquisition fee”, or whatever they want to call it. (Those terms/fees are from my water statements.)

Unless the customer complains or the NRW exceeds the system tolerance level, meters may not be replaced for 10 - 15 years.

It is understandable to want the flow sensor to agree with the water meter, but that could require continuous adjustments as many water meters decline in accuracy with age.

Agreed, @Lee149, allowing a “custom” flow sensor option so that the user can make adjustments seems like a better way to address this.

I hope you all didn’t/won’t change the factory k value for everyone that is using the settings for that particular sensor on your application

I assume they are wise enough not to change the K setting for everyone, but only that particular controller.

Can your software developers please provide a “custom” flow sensor selection in the drop down menu with the ability to edit/adjust the K-value and offset numbers so that your customers can calibrate their own system?

That would be the best long term solution. Otherwise, as more people start using flow sensors they will be getting more and more adjustment requests.

I agree that in the long term this would be the most effective solution! I will get this feedback to our team.

We did not change these values for everyone. @vmetsis We did get yours changed over to your new values! Please just most sure to not change any settings around your flow sensor, or we will have to manually change it again. Let me know if the adjustment keeps your readings more accurate.

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@mckynzee thanks! I will take some new measurements at first chance and see if it worked.

OK, I just took a new measurement. Here is what I found.

City meter usage report: 239 gallons
Rachio controller usage report: 240 gallons.

Now, I think that’s as close as it can get considering the rounding errors.

I’m happy again. :smile:

@vmetsis We aim to please!! Woohoo :tada:

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My situation is slightly different but related. I installed a CST T10 model. However we have 3/4 inch pipes so I put adapters in each end To fit our pipes. It seems to me that I should have to adjust my values as well or each pulse will assume the amount of water that fits in a 1 inch pipe which is obviously more than 3/4.

I chose the CST because it handles much lower flow rates than even the Toro which has as 3/4 model.

Am I missing something? Suggestions?

@zalusky Your flow sensor still has a diameter of 1-inch, so theoretically, the water flow slows down while passing through the flow sensor and then speeds up again when entering the 3/4 pipe. Kind of like the wide and narrow areas of a river bank.

Practically, if the adapters are connected right at the two ends of the flow sensor, the changing diameter will cause turbulence to the water inside the sensor cavity, which may affect the accuracy of your readings.

Ideally, you should have connected the flow sensor ends to 1-inch diameter pipes (about 10-inches long in the entering end and 5-inches long in the exiting end) and then connect the adapters. That would have ensured proper water flow inside the sensor.

If it is too late, to change your connection, just take some measurements and see how accurate your readings are compared to the city meter.

It is too late as things are glued together. All my circuits are drip so the flow rates are not as fast as full sprays. I do plan to do some sort of calibration test to see reality vs flow sensor measurement. I will post here once I have some data.

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