When I get home, I’ll try to measure the output with my multimeter.
Getting closer! 
Can you tell me exact markings on the ACS712?
That would help to know what are the real values.
The exact markings are :
ASC712T
ELC-058
+2156
That is a 5B
The Spec sheet can be found here:
http://www.allegromicro.com/~/media/Files/Datasheets/ACS712-Datasheet.ashx
Product details here:
@ Duke Nukem - Yeah I figured. I based my formula on that model (5A). Waiting on his measurements.
@ Alicorne or @ Duke Nukem - Can you guys measure what voltage that module outputs with no current? Just using a voltmeter.
2.482 volts with nothing attached to the connectors.
Now there was a vote of confidence for the software guy, I go into the lab to get a meter and six guys run out screaming ‘he’s got a meter’, I can only imagine what would have happened if I grabbed a soldering iron too.
Here comes the puzzle.
This chip is measuring current in the range of -5A to 5A [0 to 5V]. Gadgeteer module has divider to bring that range to [0…3.3V]. So 2.482 which is very close to 2.5 is actually
0 current for [0…5V] range. So my guess is that the divider is not working on that module and it is still producing voltages in the [0…5V] range.
If you have a good current source it would be interesting to see what kind of voltage the module outputs and if it ever goes above 3V3 for higher currents.
Oh great, if I go back into the lab and grab a power supply that will completely empty the place out and no doubt result in therapy expenses for those guys. What do we want 5A?
Sure!
Ho sorry. 
Yes, the output of the module there is a tension. This voltage is proportional to the value measured by the module.
@ Alicorne - My French is very rusty. 
I found a part of the solution knowing that I measure the voltage is 230V AC.
Thus the output voltage is variable since that measures the instantaneous value.
As far proportiennel is the output of the module, AC voltage varies all the time of -3.3v to +3.3v.
The module rectifies the negative alternation.
Here is a picture to illustrate what I mean:
Are you sure you are measuring the right thing?
This chip has 0.185mV/A sensitivity. It outputs 2.5V for no current (0A). For -5A current it outputs 1.575V for +5A it outputs 3.425V.
The point is it should never output 0V.
The Gadgeteer module adds a divider to scale it down to [0…3V3] scale from [0…5V], Again it should never reach 0 or 3V3. But something is not right and that scale is off. So by measuring good know current sources we can determine the right scale and adjust the formula to be accurate. Unfortunately I don’t have that module so I am just thinking out loud.