Alphacool ES flow and temperature indicator can not detect RPM lower than 180

[abcdehc]

I'm using this senser for monitor the state of my PC.
But it did not work if RPM lower than 180, it will show "RPM:0".
But at the same time, the cooling system working good, it can keep 7950XCPU running under high load.
By the way, for silence, i keep my VPP655 pump working on 1500RPM.
So i want to know if this situation is normal? Should i exchange it?

 

[Eddy]

If the flow is too low, i.e. below e.g. 50L/h, no flow sensor will show anything. You have set your pump very low. You simply have too little flow for the sensor to measure anything.

 

[abcdehc]

If the flow is too low, i.e. below e.g. 50L/h, no flow sensor will show anything. You have set your pump very low. You simply have too little flow for the sensor to measure anything.

in the Flow measurement list:https://www.alphacool.com/download/...ghFlow_with_RPM-signal_(flow-measurement).pdf
180RPM roughly corresponds to a flow rate of 100L/H，and the change is linear, so it means that my senser is showing "0" when the flow lower than 100L/H.
It is worked fine when flow higher than 100L/H, the RPM is changing linear with a gradual decrease in RPM of pump, but it will just jump to "0" since 100L/H.
It is showing 0L/H with pump RPM 1500, and showing 100L/H with pump RPM 1550.

 

[Eddy]

Sorry... totally missunderstood your first request.
Are you sure the water is flowing through the sensor in the right direction? It is also possible that something has got stuck in the measuring turbine and it is no longer running properly. Otherwise, only replacement will help.

 

[abcdehc]

Sorry... totally missunderstood your first request.
Are you sure the water is flowing through the sensor in the right direction? It is also possible that something has got stuck in the measuring turbine and it is no longer running properly. Otherwise, only replacement will help.

Yes i'm sure the water is flowing through the sensor in the right direction. And i detach it from PC, it seems that nothing stuck it.
So if i want to replacement it, what should i say to local reseller？
I mean, it is showing the rated range of the temperature sensor "readable temperature range is between -40 and 125°C" in product introduction page, but it doesnt show the rated range of flow sensor.
Is the rated range of RPM from 35 to 1440? Just like the data in Flow measurement list?
If so, then I can replacement the product with the local reseller on the grounds that the product does not meet the rated standard.