Seeming simple SCP question

Thank you parky...
Not intentionally hijacked the post..am new here.
I accidentally removed battery for few minutes from Mitsubishi FX3 PLC when power is off.now planning to install PLC back and in doubt regarding program state.

Thank you parky...
Not intentionally hijacked the post..am new here.
I accidentally removed battery for few minutes from Mitsubishi FX3 PLC when power is off.now planning to install PLC back and in doubt regarding program state.

No problem.

So the sensor is a pt100 0-150 C 4-20mA. So in order to get what they want, below 0 C and 32 F, we'll need a lower rated sensor.

If it is 4-20 ma 0-150 then you will have to re-scale the converter. as dropping below 4 ma will just throw your plc scaling out. you need to scale the converter to read say 4 ma = -50 20 ma = +150 then scale your function to low = -50 and high = 150, you may need to get a new converter as it may not trim that much. PT100's have a large range -200 to +600 where 100 ohms = 0 and if i remember correctly 100 Deg. C = 137.5. so you don't need a new probe it's the converter you need to change from 0 to 150 to say a -50 to 150.
You mention reading Deg. F in your post with some conversion I assume this would be Deg F = ((Deg. C x 9) /5) + 32. (this is old school).

All thermocouples will give readings outside of their stated "range", which is just a "usable" range where the temperature to induced voltage is "most linear".

If you are only going a few deg.C below 0, then I don't really see a problem, except that the reading will be *marginally* inaccurate.

As well as that, the sensor head will most likely NOT clamp the output signal to 4mA/20mA, they usually allow +/- 5 to 10% under and over the stated "range".

And on top of all that, most PLC analog inputs also allow a %age under and over "range" of +/- to to 10%, so that going outside the "range" produces correct values.

And don't forget that the math done in the SCP is just a "straight-line" relationship, and it will scale the input value to an output value anywhere on that straight line, even if the input value is outside the stated Low and High input parameters.

The only problem I can see is if the sensor head is configured as a 0 to 20mA signal, because you just cannot go below zero current ...


The other thing I can see is that that SCP_RD provides two output values, one as a REAL value, and one as a DINT value. But it gives the DINT value as having an implied 1 decimal place, so the 13 is really 1.3, an approximation of the 1.2817774 coming out of the REAL parameter.

Even if the head converter is 0-20 then providing it has trim pots it can be trimmed to put the 0 Deg. c at a higher value say 2ma, note although the zero & span pots work on their respective zero & span one will affect the other so it is a matter of doing zero & span a few times.
The best way is to use either a calibrated heat source i.e. a dry well calibrator or a calibrated decade resistance box (PT 100's depending on type will have typical accuracy in their data sheets so even though you use a calibrated decade resistance source the PT accuracy will depend on it's own stated accuracy).