Feature/fix flowmeter calculations

[gjdodd]

While using the flow meter settings on a K40 using MKS SBase I had many issues with both integer to float conversion and also the calculation used for calculating the litres per minute. while using several flow meters with various pulses per litre, the sensors I used were:
YF-S201C
FL-308T
YF-S401

I got incorrect results for all of them, or the system just halted during boot. While comparing to real world throughput of the flow, in some cases the YF-S401 reported 9.4 litres per minute (but with other leading digits dropped) when actually it was 0.7 litres per minute. In some cases, Also due to integer to float conversion I got an animation but it reported 0.0 on the display.

I have submitted this pull request to fix the issues with the calculation to convert to litres per minute, but also sort out issues I saw with the config been an integer and then needing it to be a float during the calculation.

The calculation I have is

flowrate = (flowpulses / (float)FLOWMETER_PPL) * ((1000.0f / (float)FLOWMETER_INTERVAL) * 60.0f);

which breaks down to

litres = (flowpulses / (float)FLOWMETER_PPL)
captured interval = (1000.0f / (float)FLOWMETER_INTERVAL)
convert to minutes = * 60

This gives an real world value of what was expected, I have also added several test cases to an a spreadsheet to validate the results of several pulses and intervals and all seem to match up with what was expected. If you want these converted to unit tests then please let me know and I will make it so.

To add an example

with values of

pulses 66
interval of 1000
pulses per litre of 5880

current calculation returns

66 * 60 * (1000 / (1000)) * (1000 / (5880)) = 673.47

my calculation returns 0.67

=(66 / 5880) * ((1000 / 1000) * 60) = 0.67

[descipher]

Yes, looks like I missed changing from ml per min to L per min when the display scale was changed.
If you're seeing hangs on boot it's likely to be an interrupt handling issue. We need more details on the hardware config to assess that problem. Also be aware of your pump pressure, the S401 has a max pressure of 0.8 MPa that is quite low and any pump more than 8 L/m starts overloading it.