How to scale current output?

[KowdTek]

Hello everyone!

I have a pressure transducer that has 4-20mA output, I'm wondering if someone could tell me how to scale current. I know I can put resistor accross the output to produce voltage but I want to do it in current.

Here's my hardwares:

Cole Parmer - Pressure Transducer

Power Input: 24vdc

Output: 4-20mA

Range: 0-100 Psi

PCI-6024E (I also have PCI-6259)

SCB-68

Thanks!

[JeffOverton]

if 4 mA corresponds to 0 psi, and 20 mA corresponds to 100 psi, then the 16 mA range covers 100 psi.  so 100psi/16mA = 6.25 psi/mA.

Given 6.25 psi/mA, and 4 mA = 0 psi, it's a y=mx + b kind of thing to find the offset.  0 = 6.25*4 + b, so b= -25.

so your scaling factor will be PSI output = 6.25 (mA output) - 25

edit - a sanity check shows that (6.25*20) - 25 = 100, so you know you're on the right track

Message Edited by JeffOverton on 04-24-2008 01:41 PM

[KowdTek]

Thanks for a quick reply!!!

[KowdTek]

Unfortunately, my card (6259) cannot read current so I end up putting 249 ohms resistor across the output to produce 1-5vdc then set the daq assistant to read voltage. The transduders range is 0-100psi (@1-5vdc) and I set the scaling parameters to 25 for slope and -26 for y-int. My question is why the reading stops at 29psi? Note that I had it hooked up to a pressure regulator with digital meter for comparison, also the output is 5vdc which supposed to read 100psi.

Thanks!

Message Edited by FlipTek on 04-29-2008 06:24 PM

[JeffOverton]

Well, your offset should be 25, not 26, but the rest of it is a lot more of a gray area.  It could be hardware, you could have scaling enabled somewhere in the daq software, your measurements might be affecting the readings, etc.  Sorry I can't help you more, but I'm not that familiar with NIDaq or the daq hardware.

[KowdTek]

In addition, I've observed that the same application works when I used PCI-6024. Why it doesn't read the full scale when I'm using

PCI-6259. I checked the test panel and it reads 0-5V but somehow stops at some points (29psi) when I run the vi.

Please let me know if there's anything I should do to resolve this issue.

Thanks!

[samantham]

Hello,

When using the PCI-6024E and the PCI-6259, are you using the same code and task configurations.  I am specifically interested to clarify that both are using the same scale and mode (single ended or differential).  I am unsure why you are limited to 29 psi on the 6259, which is related to 2.16 V as an analog input and 8.7 mA for the sensor output.  Can you verify that the signal is in fact higher than this at the time you are measuring 29 psi?

[KowdTek]

Hello Samantha!

I am using the same code and task config so as the scale and mode. As I apply more pressure  the signal goes up but the actual reading stays at 29psi.

Hoping that you could help me figure this out.

Thanks!

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samantham wrote:

Hello,

When using the PCI-6024E and the PCI-6259, are you using the same code and task configurations.  I am specifically interested to clarify that both are using the same scale and mode (single ended or differential).  I am unsure why you are limited to 29 psi on the 6259, which is related to 2.16 V as an analog input and 8.7 mA for the sensor output.  Can you verify that the signal is in fact higher than this at the time you are measuring 29 psi?

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[samantham]

Hello,

Lets try testing the 6259 by creating an analog output 5 V and wiring it into an analog input channel.  Another good test will be to send a 0-5 V sine wave into an analog input to check its response.  All this testing in independent of the code and configurations you are already using. 

Then, I would suggest testing an analog input with your code but where the signal is coming from an analog output from the 6259 rather than the sensor.  I understand your code is intended to test current. So this last test would be without the 249 ohm resistor.

Let me know if the 6259 is working with these tests.

[KowdTek]

Both test works just fine. I also test the input receiving a signal out of regular power supply.

This 6259 is realy flakey to me because we have another PC that has the same daq card and when I tried the code it works.

It's just too expensive to act up like this.

Thanks for your time Sam!

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samantham wrote:

Hello,

Lets try testing the 6259 by creating an analog output 5 V and wiring it into an analog input channel.  Another good test will be to send a 0-5 V sine wave into an analog input to check its response.  All this testing in independent of the code and configurations you are already using. 

Then, I would suggest testing an analog input with your code but where the signal is coming from an analog output from the 6259 rather than the sensor.  I understand your code is intended to test current. So this last test would be without the 249 ohm resistor.

Let me know if the 6259 is working with these tests.

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