offset and gain problems 4-20ma

The analogue input of the LOGO! generally reads a current signal as 0-20 mA (or a voltage signal as 0-10V) and converts it into 1000 units.
For a 4-20 mA sensor, the 4 mA is are fixed as the zero point for the measuring range. This means that only 800 units are available for the usable area, namely 200 to 1000.
So the area must be amplyfied (gain) and the zero point must be adjusted.
If the 1000 units are now amplified by a factor of 1.25, then the analog amplifier has 1250 units. Now the zero point is offset by the offset of -250, the measuring range now goes from -250 to 1000.

This results in: 

 0 mA = -250 Units
 4 mA =    0 units
 8 mA =  250 units
12 mA =  500 units
16 mA =  750 units
20 mA = 1000 units

If you want to parameterize a 4-20 mA sensor in the analog amplifier, you do not need to calculate anything, this makes the LOGO! Software automaticly.

You only have to select the sensor type 4-20 mA for the amplifier, then enter the upper (maximum) and lower (minimum) limits of the measurement range and the number of decimal places. The software then automatically calculates the parameters gain and offset independently.

Example: 4-20 mA sensor with a measurement range from -30 bar to 90 bar, displaying with one decimal place.

Type of sensor: 4-20 mA
Minimum: -300
Maximum: 900

The software calculates a gain of 1.5 and an offset of -600.

You could also calculate the gain and the offset itself.

Example again the sensor with the data:
4-20 mA, measuring range -30 to 90 bar, 1 decimal place.

This would require an extended measuring range for a sensor with 0-20 mA if a measured value of -30 bar is to be displayed at 4 mA.

Calculation of extended measuring range:
Extended measuring range = (old measurement range max - old measurement range min) *
(I new max - I new min) / (I old mxa - I old min)
Extended measuring range = (90 bar - (-30) bar) * (20 mA - 0 mA) / (20 mA - 4 mA)
Extended measuring range = 120 bar * 20 mA / 16 mA = 150 bar, with a decimal place 150.0 bar.
These 150.0 bar correspond to the LOGO! 1500 units, equivalent to a gain of 1.5.

Calculation of offset:
New exhibition area 0,0 bis 150,0 bar. Since the maximum value of the new measurement range would be 150.0 bar, the maximum would thus be converted 60.0 bar too high.

Offset = Old maximum - New maximun
Offset = 90.0 bar - 150.0 bar
Offset = - 60.0 bar

mostafasamurai

for example when sensor senses 30°C, what number do I see on LOGO?(with above settings)

in other word, when sensor senses 30°C, I want to see 30 unit on LOGO, this is possible?

best regards

 It's the same, rounding errors. Same solution as in the other case.