analog input

I have 4-20mA temperature transmitter, temperature indicator (which can read 4-20mA signal), analog input card 1762-IF4 for ML1400. Here we needed to take temperature readings at two different places, thats why 4-20mA signal coming from temperature transmitter is first given to temperature indicator then its given in series to PLC.
When temperature is 25 degress actual, then temperature indicator shows correct value, but PLC shows lower value (18 degrees). Why? I checked I/O configuration where data format is "Scaled for PID". In ladder, for SCP, input min is 0 & Input max is 16383, scaled min is 0 & scaled max is 150. scaled min & max are correct as these values are set values for that temperature transmitter which also I entered in temperature indicators setting. Everything looks correct, but then why I am getting lower temperature value? Is this due to losses in transmission? Should i use proportionally lower value for Input max ? Because ideally if input card is set as scaled for PID then for 4mA it should give 0 & for 20mA it should give 16383. Here for 25 degrees it must give 4095.75 Am I correct? I checked the actual temperature with handheld laser temperature indicator it also shows 25 degrees.
Where is the problem?

SCP here used for indication of temperature in SCADA screen.

then for 4mA it should give 0 & for 20mA it should give 16383. Here for 25 degrees it must give 4095.75 Am I correct?

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No,

25/150=.16666

.16666*16383= 2730

See scaling chart below. Credit to John Soltesz

Post your .rss file ( zip it first) if you can.

What is the actual milliamps at the input card?

Make sure that your transmitter can support the extra load with two devices in series.

If this is the same program you posted in your other thread then all the "SCP" instructions are scaled 0-100.

yes it is similar. Actually I have 4 controllers in network here as plant has 4 different sections. The one I posted in different thread is for another controller. But ladder is similar. Only quantities & ranges differs. Is the data in SCP instruction is correct? I am checking the excell file u sent.

for the 25 degrees example, the range is 150 degrees & in SCP also there is scaled max as 150

Can you answer my other two questions?

What is the actual milliamps at the input card?

Make sure that your transmitter can support the extra load with two devices in series.

Click to expand...

I dont know if my transmitter supports extra load with two devices in series. But as I am sending 24VDC to transmitter then in return I must get mA gievn by transmitter in 2 places. Why it should matter if i am checking current in one place or two places or three places? I can tell u tomorrow the exact mA of transmitter across card. But yesterday when I checked, mA was same across indicator as well as across card.

When you take the milliamp reading please post it here along with the raw data value and engineering unit value at that moment.

Why it should matter if i am checking current in one place or two places or three places?

Click to expand...

Because of the basic laws of physics. Each device added to the loop has a resistance and is a small additional load on your power supply. Your common sense probably tells you that you could not add an infinite number of loads and expect to read the correct temperature.

If the transmitter does not produce enough current to supply all loads, the current level may droop and the signal reading would be lower than expected.

Wait, you expected 25 degrees but got 18? That is one piece of evidence that supports Mickey's diagnosis.

Solutions: get a different transmitter, or get rid of one or more of the devices in the loop, or increase the power supply voltage slightly.