4-20ma question

squints · May 24, 2014

Hi all,

I have an automation direct pressure transmitter (QPSH-AP-42). It has a range of -14.5 psi to 145 psi, with the output at -14.5 psi being 4ma and the output at 145psi being 20ma. At 0 psi it outputs 5.45ma. My question is how do you scale this thing so it will read 0 psi when the pressure is 0 I have an automation direct 205 PLC with a DL250-1 processor. So far I have a value of 371 BCD at 0 psi, and I have not been able to figure out what number to use to scale it correctly. I pulled a vacuum on the transmitter and at -14.5 psi I get 0 BCD. Any help would greatly be appreciated!

benkouiderlak · May 24, 2014

enter in Analog Input configuration and replace 4-20mA by 0-20mA, you will find at 00 Psi (1E8)BCD

Lancie1 · May 24, 2014

You should be looking at the DL-205 manual for Analog Current Input Scaling. I do not have the DL-205 manual, but the scaling equations and math instructions will be similar to these pages from the Direct Logic DL-06 Analog Input Module. If the DL-205 analog module has the same max count range of 4095, then the scaling will be the same.

squints · May 24, 2014

I have the manual for the 205 and you are right the 2 PLC's are identical, but I just cannot figure out what I'm doing wrong here. The resolution is 4096 (0 to 4095) if I set the pressure to 65.3 psi (which is half of the range and 12ma), I get 65 out of the PLC. When I drop the pressure back to 0 I get a value of 13 out of the PLC. My high engineering units are 160 and my low engineering unit is 0. Am I correct that the range is 160?

squints · May 24, 2014

The output load resistance for the transmitter is 400 ohms and the card has a 250 ohm resistance built in to it. I added a 150 ohm resistor in series with the transmitter output.

Bit_Bucket_07 · May 24, 2014

squints said:
I have the manual for the 205 and you are right the 2 PLC's are identical, but I just cannot figure out what I'm doing wrong here. The resolution is 4096 (0 to 4095) if I set the pressure to 65.3 psi (which is half of the range and 12ma), I get 65 out of the PLC. When I drop the pressure back to 0 I get a value of 13 out of the PLC. My high engineering units are 160 and my low engineering unit is 0. Am I correct that the range is 160?

I would have expected 62.25 PSI at 12 mA.

0 to 4095 counts = -14.5 to 145 PSI
Span = 4095 counts
Range = 159.5 PSI
Zero Offset = 14.5 PSI

Value in EU = (Input/Span * Range) - Zero Offset

At half scale, Input = 2047.5
2047.5 / 4095 = 0.5
0.5 * 159.5 = 79.75
79.75 - 14.5 = 62.25 PSI

squints · May 24, 2014

Bit_bucket,

Thanks you are 100% correct! Once I subtracted the out the negative part of the scale it is right. Thanks for your help. I knew I was over thinking this one.

Tark · May 24, 2014

You'll also want to check that the analog value is greater than K371 (or whatever 0psi is) before doing the scale instruction. On a separate rung perform some logic if the analog value is less than K371 to indicate in the program that the analog value is less than 0psi.

keithkyll · May 24, 2014

squints said:
The output load resistance for the transmitter is 400 ohms and the card has a 250 ohm resistance built in to it. I added a 150 ohm resistor in series with the transmitter output.

Remove the 150 Ohm resistor. Ignore the 400 Ohm spec.
Internal to the analog card, it uses the 250 Ohm resistor to convert 4-20 mA to 1-5 Volts. Some cards use 500 Ohm to convert 4-20 mA to 2-10 Volts.
The 0 to 1 or 2 Volts is used for fault detection. If the loop is open, you have about zero Volts (zero current).

The internal resistors really don't matter to you. Just connect it and scale it.

squints · May 24, 2014

Tark,

In this case I don't need the -psi only the positive. Thanks for all the replies! I learned quite a bit from ya'll!

Lancie1 · May 25, 2014

The moral of this tale is "use the scaling equation, all of it". Do not try to use shortcuts and intuitive math. Use the real thing.

Value in EU = (Input/Span * Range) - Zero Offset

Tark · May 27, 2014

squints said:
Tark,
In this case I don't need the -psi only the positive. Thanks for all the replies! I learned quite a bit from ya'll!

Yes I understand that you're looking for only positive values. You might want to see what happens to your scaled value when a raw value less than 371 is received. Hint hint, your scaled value won't be 0. This is why I suggested that you make sure your value is within range before scaling, and if it's not in range throw an error.

Subtraction in BCD that yields a value less than 0 will give you unexpected results.

4-20ma question

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