measuring voltage above two sensors and below two sensors from edge liquid sensor

[sujeep]

Sir,

      (1). Our CPU is connected to PXI6259(DAQ card),
this PXI6259 is connected to SCC-68 through SHC68-68-EPM cable,
Then to activate sensors in our PCB, we made use of digital signals(port 0, line 0 to 7)from Scc68,
 the activated sensors voltage is connected to (SCC 68) ai3(positive Voltage input)
and ai 11(negative volgate) in a differential mode, this voltage if we reads,
its not giving accurate values as we measured just before reading the software program in our system,it's giving two digits accuracy after the decimal point  ,it's fluctuating.

(2) In our project we are having 140 sensors(0 to 139,liquid will starts filling from 139 in decrementing order), this sensors are used to measured the level of liquid Nitrogen with the help of voltages  of that sensors, when this sensors are in liquid, the sensors will give above 1.046v(threshold voltage), at the edge of liquid the particular sensor will give nearer to threshold voltage, above the liquid level  all the  ensors will give less than 1.046v. When ever the liquid level ends, the program has to measure above two sensors voltage,as well as below two sensors voltage,as measured like liquid level sensors for  conclusion of whether the sensors is in liquid or vapour. If the bottom most (139the sensor) sensor itself  is not in liquid no need to measure the voltages up and below the sensors, it has to recheck the same sensor . 
 
I have attached the programs.Plz see to it and reply me. I am awaiting for your reply.
 

[ryan_d]

Hi sujeep,

From what I understand you are not getting an accurate reading by measuring with a voltmeter at the SCC-68 terminals or in the Measurement & Automation Explorer (MAX). If this is not the case, please post back with a correction. I think this could be a power or grounding problem.

What do you mean by “to activate sensors in our PCB, we made use of digital signals”? Do you mean that you are using the voltage from the digital signals as an excitation voltage for you sensors? If that is the case, then I recommend an alternative means of excitation. The output of a digital line will not necessarily be exactly at 5 V. Thus, if you are using a digital voltage as an excitation, you will not have reliable excitation.

From what I understand, your sensors are floating signal sources. When measuring a floating signal, we have to ground the signal appropriately. Table 1 found at the bottom of this page shows how to properly ground a floating signal source. I would recommend, however, that you change your measurement type to Referenced Single Ended (RSE) (called Single-Ended Ground Reference in Table 1) since it is the easiest to configure. I recommend testing this configuration on a single sensor before wiring your entire system.

Let me know if this helps. If not, please post back with any information that is missing from above and additionally:

1. Has this application worked before?
2. Have you been able to get a reliable reading from these sensors before? What values are you expecting and what are you reading? Please feel free to post screen shots, as this will eliminate any ambiguity for your readings.
3. How much are the values fluctuating?

Have a great day!