I am using Labview 5.0 to write a program to measure the pH value for my experiment, the obtained values should be constant. Why the data from AIRead is fluctuating instead of constant values?

[Jasmine]

Is this caused by the hardware (Intelligent Instrumentation:
PCI-20428W), the software (LabView 5.0) or the noise?

[Enrique]

Which "AI Read.vi" are you using? You should use the LabVIEW libraries provided by the vendor (Intelligent Instrumentation). Visit their website:

http://www.instrument.com/pci/

or call them for support.

In case you have the adequate libraries, make sure you are actually reading from the corresponding port by input a well known signal and trying to read it in LabVIEW. If the result is fine, the next suspect may be noise or that you require to do some signal conditioning. At ni.com there is a good reference on how to deal with those issues:

Signal Conditioning Tutorial
http://www.ni.com/sigcon/tutorial.htm

Noise
http://zone.ni.com/devzone/conceptd.nsf/webmain/8DE42E13BD089D8B86256816006545CD?OpenDocument

Noise Reduction
http://zone.ni.com/devzone/
conceptd.nsf/webmain/99B4323FAB04FFD686256811005025F2?OpenDocument

Also Jasmine, somebody asked a question about a pH vi in LabVIEW in the sensor section. It seems that you might be able to help that person. The link is:

http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RPAGEID=135&HOID=5065000000080000004E1E0000&UCATEGORY_0=_39_%24_12_&UCATEGORY_S=0

Best regards;
Vargas

[Enrique]

The following is a more practical document about noise (including shielding):

Field Wiring and Noise Considerations for Analog Signals
http://digital.ni.com/appnotes.nsf/b0be082ea11554328625648f006a132a/9f54054461a672bb862566320069b888?OpenDocument

[shoneill]

Hallo Jasmine,

How are you measuring the pH? Are you connected to a meter over a serial
(or other data) connection or have you connected the pH electrode directly
to your hardware (I`m not familiar with the PCI-20428W)?

Depending on the type of pH-electrode, you may experience high level of
interference, which of course Enrique`s mail already dealt with.

If you try to attach a glass pH-electrode directly to a card, you need to
watch the input impedence of the card (or use an amplifier) as a glass
pH-electrode can have an impedance of over 400 Megaohms...

Shane

Jasmine schrieb in Nachricht
<506500000008000000B01E0000-986697009000@quiq.com>...
>I am using Labview 5.0 to write a program to measure the pH value for
>my experiment, the obtained values should be constant.
Why the data
>from AIRead is fluctuating instead of constant values?
>Is this caused by the hardware (Intelligent Instrumentation:
>PCI-20428W), the software (LabView 5.0) or the noise?
>
>

[Biomedical]

Hi jasmine
you wanna measure a DC value but the signal you acquire from your DAQ card is flactuating.
no problem ,for measuring a DC signal you don't need to a big sampling rate ,in fact to reduce AC noise by software you can choose a great sample rate from A/D and buffer it 1 second. for example with 1 KHz sampling rate you should buffer 1000 samples then measure DC value of this 1 dimensional array with a DC estimator block in this address: Functions pallete/Analysis/Measurement/ AC & DC Estimator .vi
then output of this block is your desired PH value without AC noise and your real sampling rate is 1 second that is enough for measuring a DC signal.

I hope it will help you

but if you think i can help you more about this
don't hesitate to email me.

ka
sra_ravanbakhsh@hotmail.com

[chutla]

Jasmine:

The first thing one should always do when confronted by this problem, is to do an independent check of the data. Do you KNOW for certain that the measured PH values are constant? Your sensor might be bad...try measuring the voltage to the input of the DAQ card with a DVM or some other independent means (DC coupled oscilloscope or the like) I assume you PH probe works by measuring electrical resistance.