MSP430FR6045:USS volume value data type question

Jung Jayden

hello. everyone

I am currently building and testing a custom board using the MSP430FR6045.

I have additional questions.
If you look at the USS Demo Source, g_ResultsOfLastMeasurement.last_volume is declared as float type.
So, when the last_volume value is over 10000, can you guarantee accuracy for spilling a small amount of water? Or have you tried such a test?

For example, if last_volume is more than 10000L and 1.234L of water flows at a flow rate of 10L/h, can you guarantee that 10001.234 can be contained in the last_volume variable?

over 2 years ago

0 Dylan OBrien over 2 years ago

TI__Genius 16050 points

Hi Jung,

I have not performed this testing myself nor am I aware of testing that has been done for this. I will add that we generally expect customers to read flow rates within a small range. There is a page describing how you can calibrate your device to read a broader range of flow rates, which I will link below. Please see the flow VFR calibration section of the page below:

https://software-dl.ti.com/msp430/msp430_public_sw/mcu/msp430/USSSWLib/USSSWLibWater/latest/exports/USSLib_02_40_00_00/docs/UserGuide/UserGuide/ch_calibration.html

Back to the question, my understanding is that a float has 34 digit precision, so I think this would be fine as long as you have set your device up to read flow rates in both of those ranges.

0 Jung Jayden over 2 years ago in reply to Dylan OBrien

Prodigy 80 points

Dylan OBrien said:
precision

Hi,

How did it come to be that the precision of the float type is 34 digits? The float type has a mantissa of 23 bits, so it only has 6 to 7 digits of precision in decimal numbers. Did I misunderstand?

0 Dylan OBrien over 2 years ago in reply to Jung Jayden

TI__Genius 16050 points

My apologies here, I did a quick search and it looks like the source that I used said that the max precision of a float is 34 digits.

The float type used in the USS library behaves the same as a float type in any other c program. Using a float to hex converter that I found online, it looks like 10000.1234 would be stored as 10000.1. So there would be some truncation, as there would be in any other c program using a single float.

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MSP430FR6045:USS volume value data type question