pH Probe SCP Instruction

Hello,

I am replacing a pH probe that was determined to be bad in a waste water treatment system. I am replacing it with a 2-wire Aqua Metrix 4-20mA probe; Manual Here. The original probe was also a 2-wire but the part number has long been washed away with no way of completely identifying it. This probe is wired into a 1769-IF8 analog input card Manual Here on my Compact Logix controller bank. The input channel it is using is set to 4-20mA and is also set to read Raw/Proportional with a 60 Hz filter.

The logic already has an SCP instruction taking that analog signal and scaling to a min value of 4 and a max value of 10. (Attached Picture).

My questions are; What should my resolution be for this device? Why wouldn't the pH min/max scale already be set to 0-14 instead of 4-10? According to the 1769-IF8 literature my input min should be -32767 and my input max should be +32767. When I change each scale to what I think they should be I get 8+ pH output with the sensor submerged in a beaker filled with city water. Now the city water could very well be +8 pH, but I don't have any buffer solution to test the other pH values to say yes or no on it. The overall problem is we are out of compliance with our readings sent back to the city and I don't currently have an answer for them.

Any help here would be greatly appreciated since analog resolutions always throw me for a loop...

Thanks

First thing you need to find is a way of calibrating your results.

If you don't know the output is correct you've no idea if your set up is correct.

ph8.1 isn't uncommon for water, especially in areas where the water is considered "hard"

Rob... said:

First thing you need to find is a way of calibrating your results.

If you don't know the output is correct you've no idea if your set up is correct.

ph8.1 isn't uncommon for water, especially in areas where the water is considered "hard"

Click to expand...

Yes, we are in a area with hard water normally. I am also working on getting buffer testing kits currently to simulate the acid and base levels. I would just like to know if the resolution in the image is correct so I can eliminate that.

You can scale the AI to whatever you want. So I would check what that is currently set to.

Normally I set 4ma to 4000 and 20ma to 20000 unless there's a specific reason not to. I find it easier for general troubleshooting.

Then look in the manual for your probe an see what the output range is, like you say could be 4ma = 0ph an 20ma is 14ph, but worth checking as that's a huge range dependent on how accurate the probe promises to output.

It looks to me as if someone has tweaked the Input_Min, Input_Max, Scaled_Min and Scaled_Max values in order to change the range and/or calibration of the old instrument.

All Aquametrics 65-series sensors are calibrated to output 4-20mA signal over the entire measuring range of 0 to 14.0 pH. The 65 series sensors have non-adjustable outputs.

The existing AI card range of 4 to 10 is not suitable for a probe output range of 0-14.0 pH range.

I think I know what the problem is. Some lines of logic right below what I was looking at indicate it. When calibrating you would submerge the probe in a 4.0 pH solution and a 10.0 pH solution and there is a push button to set the low pH ( 4 ) and the high pH ( 10 ). That is where the input_min and input_max values are derived from. I will just have to wait until I get my buffer solution kits.

Thanks for the help everyone.

Multiplying by unity

The -IF8 manual has this table, which suggests the "full range" of 3.2-21mA (-5%/106.25% of 4-20mA, or -1/20 to 1+1/16)) yields Raw/Proportional Data values of -32767 to +32767 counts (or DN, or whatever you want to call them).

Assuming that is correct, and that pH is linear from 0-14pH for 4-20mA, then

• an SCP Input_Min to Input_Max of -32767 to 32767 converts to a Scaled_Min to Scaled_Max of -0.7 to +14.875pH,
• or the same relationship, expressed differently, scales an Input_Min to Input_Max of -29821.65* to +29085.31** to a Scaled_Min to Scaled_Max of 0 to 14pH.

* = -32767 + (+32767 - -32767)*(4-3.2)/(21-3.2)
** = 32767 - (+32767 - -32767)*(21-20)/(21-3.2)

Ryan_Flowers said:

I think I know what the problem is. Some lines of logic right below what I was looking at indicate it. When calibrating you would submerge the probe in a 4.0 pH solution and a 10.0 pH solution and there is a push button to set the low pH ( 4 ) and the high pH ( 10 ). That is where the input_min and input_max values are derived from. I will just have to wait until I get my buffer solution kits.

Thanks for the help everyone.

Click to expand...

Ah nice, I was wondering if it was summat like that.