Hi jbrazio! Use up that nasty CO2 in your tank!

1) Selection of sensor
Yup, you are right - try and find a pH sensor that gives a 0..10 V output and not a 0..20 mA or 4..20 mA current output. Then you can use 1 of the 4 special digital inputs on the LOGO! "24", "24o", "12/24RC" or "12/24RCo" as an analog input. (See the following link)
/tf/WW/en/Posts/10321

2) The actual voltage value goes into the LOGO! and it is internally represented as a value between 0..1000. So 0 V would be an internal integer value of 0, 10 V would be an internal integer value of 1000.

3) All of the function blocks in the LOGO! Soft Comfort programming 'language' that manipulate analog inpuit values, give you the chance to 'scale' this 0..1000 value into the real-world value that you are actually working with. There is a very knowledgeable participant in the forum called 'Betel' who has shared a nice EXCEL spreadsheet for calculating the 'gain' and 'offset' values to do the required scaling - whether you need to use only 1 analog block or a couple in series, you will get the 'real-world' value you require (with a small amount of deviation) (see the following link)
/tf/WW/en/Posts/11326

4) The typical values that you show could be the result of the calculation might be a problem. The LOGO! Maths Block can only work with integer values up to +32767. Your first example shows a value of 600 (six hundred).

5) the lowest value you show is 12, but it looks like the 3 decimal places after the comma are also important. It is possible to 'shift' all values up by multiplying them by 10 or 100 and then displaying them correctly by placing the comma where it is needed.

6) Then your example results could at best be displayed as
476,6 (for the 476,597 result)
47,7 (for the 47,660 result)
15,1 (for the 15,071 result)
or
12,0 (for the 12) result

7) You could not be able to display 600,00 with 2 decimal places, because the 'value' would have to be 60 000 (sixty thousand) and this is beyond the 32767 integer limit. So the best that could be done would be 600,0 or the other values as shown above. So if you are happy with only 1 decimal place, then you can proceed to the REALLY DIFFICULT STUFF!

8) The PROBLEM I CANNOT SOLVE YET is how to do the EXPONENT on the 10 either with or without decimal parts!!?!?!??!

9) There are some very clever people on this forum who could possibly find a way to do it with a counter making some sort of loop etc, but right now it is beyond me.

10) The other way I thought of is to have these exponent values 'stored' in a few multiplexers (where each can store 4 values) and depending on the outcome of the subtraction (7 - pH), a Threshold trigger could digitally select the correct 'look-up' value and then use the analog maths block to add it to the multiplication part. You would need 5 MUX's, each storing 4 look-up values, and each value selected by a combination of the En anable bit for each MUX and hte S0 & S1 selection lines

11) This is getting pretty challenging, I will see what I can do about it unless someone more knowledgable than me can step in to help?????

Cheers

Aaaaarrggh!

Hi,

jbrazio

- Apply the following formula and triger a solanoid valve if required

CO2 = 3 x [K] x 10^(7-pH)

Can the algorithm be calculated inside LOGO! and trigger a output (solanoid valve) based on the result ?

No,you cannot. Because LOGO! is impossible to exponentiate (7-pH).
But "Numeric Functions Instructions" has in S7-200.

Thanks to all the replies, currently I've created a purchase order to for a 0BA6.. I should receive it on the following weeks.

The idea is to have the LOGO! Logic module screen to display the CO2 and pH value, the pH value should come from a analog probe connected to one of the four analog inputs of the logic module. The value of CO2 should be calculated by the previous equation, with the option for the user to use the arrow keys on the logic module to set the kH parameter.

From a relevance stand point, if the decimal values of the CO2 have to be discarded, that’s not a big issue. On the other hand, for pH display it’s important to have them, because a pH of 7.6 is very different of 7.2.