4 Point Calibration

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I am working with an SRI 8610C GC, and attempting to calibrate the Total Gas. It's output is not linear and they have suggested I do a 4 point calibration, that has me stumped. I have the channel set up to read the voltage from the machine and can set the variables (0 = x mv, 100 = x mv, etc). But am at a loss as to what to do next. Any help is greatly appreciated.

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4 point cal and its non-linear? Did they tell you what form the response is in? A polynomial for example? I would assume its a 3rd order polynomial since they are suggesting a 4 point cal. A linear sensor would be a 1st order polynomial and requires a 2 point cal. Anyhow, the rest is just linear algebra. You need to set DAQFactory up to display voltage, which it sounds like you have, and you need to feed the sensor 4 different readings, preferably relatively evenly distributed across the range, so if the sensor is a 0-100ppm sensor, maybe do 5, 30, 70 and 95ppm. You'll need to know precisely what the actual ppm is. The polynomial is going to be something like:

PPM = A*V^3 + B*V^2 + C*V + D

where PPM is the actual gas measurement, V is the voltage you read, and A, B, C, and D are the 4 constants you want. If you do this at 4 different locations, you end up with a matrix, and some basic linear algebra will allow you to reduce out the actual values for A, B, C, and D. I'll leave the explanation on how to solve linear algebraic equations to the rest of the Internet.

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Very interested in this post MKGrier...

Did you make a custom LawsonLabs driver, or are you just reading in channel 0 from the Lawson board to a Labjack per-se??

We've created a custom driver, but it doesn't have all the bells and whistled scripted in, just the basic analog in channels, 1 digital out (valve), and 1 analog out (Column Temp)...

Have you ever calibrated the GC with their PeakSimple??

We calibrate with 5% C1-C5, with 3 calibration runs...

Seeing the "gas curve" is relatively simple, but cutting the trace up into 5 sections, calcing the area/ect, and finally seeing ppm is the tough part (for me atleast) ...

Will be watching this post as getting our 8610GC in DF has stumped me for some time now.

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• 1 month later...

Any update on this??

Just curious!

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• 2 weeks later...

Just Curious, but why 4 point! If you can do a 2 point, doing 2 more isnt that much more work, but why! GCs are a rather simple calculation to TG, and if its nonlinear, why dont they suggest to change the GC detector to one that is linear. In any case, how do you know its nonlinear to begin with? Because they tell you? Think about that.

Just thinking, but even GCs have their faults in reading total gas. Some think that GCs read TG more acurately than a TG detector, but that is not the case, in my experience.

Cheers-JohnyQuick.