Citrin 1.3 Solar : Minus temperature on the collector gives a wrong result

[Richard Haslauer]

Hello,

I use ESPHome to read the VBUs datas of a Citrin Solar 1.3. All works fine, but minus tempertures are wrong.!

I use the following formular to get the temperature :

lambda: return x[0] * 0.1f + x[1] * 25.6f;

If in this case the temperature is minus, so you get a diplay value about

65363 degrees.

When I use this formular, I get approximately the right value:

lambda: if (x[1] == 0xFF) {return (x[1] - x[0]) * -0.1f;} else {return x[0] * 0.1f + x[1] * 25.6f;}

Question: Is this the right formular?

Regards Richard Haslauer

[Edwin van den Oetelaar]

Hello  Richard,

I am just reading your code and it looks like you are trying to do a decode of 2 bytes yourself.

Looks like a Signed int from your example.

import struct

y = bytearray(2)
y[1] = 0xFF 
y[0] = 0x10
def get_temp2(x):
    t = struct.unpack('h',x)
    return t[0] * 0.1

Read up on the 'struct' module of python, this works with positive and negative numbers without issue.

Best regards and a happy new year,

Edwin van den Oetelaar

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[Richard Haslauer]

Hello Edwin,

your function works well. Do you have any idea, what it would look like in C or C++.

Thank you and also a happy new year!

Regards Richard Haslauer

[Edwin van den Oetelaar]

Sure here is the source code and how to compile this on a normal unix/linux system

// print the demo code to console

$ cat demo.c

#include <stdint.h>
#include <stdio.h>

// Union to aid in converting byte data to signed shorts
union {
    char bytes[2];
    int16_t value;
} shortUnion;

float get_temp2(char *x) {
    shortUnion.bytes[0] = x[0];
    shortUnion.bytes[1] = x[1];

    return shortUnion.value * 0.1f;
}

int main() {
    char bytes[] = {0x10, 0xFF}; // Example byte array
    printf("%f\n", get_temp2(bytes));

    return 0;
}

// compile the demo code
// creates 'a.out' executable

$ gcc demo.c 

// run the demo

$ ./a.out
-24.000000

// result seems fine

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[Richard Haslauer]

Hello Edwin,

thank you very much for your help!

Regards Richard Haslauer