What is a Double-Blind Sample and Why Do I Care?

Double-blind refers to the same collected sample being divided into two samples and analyzed “blind” meaning that lab technician or off site lab does not know that within the samples being analyzed that several are duplicate samples.

When calibrating an analyzer the results can never exceed the laboratory accuracy and sample collection method.  Variances can be due to sample collection, head space in sample container, temperature and duration and type of lab method and other variables.  All can be minimized in a simple specific procedure outline for the product.

Moisture samples should be collected in a polypropylene or other suitable container with a moisture barrier lid.  Container should be full with minimal air head space.  Crush samples if possible to increase sample density.

Determine the appropriate sample size 200 – 450g, temperature and duration for weigh dry weigh lab determination of control samples.  Many labs feature automated weigh dry weigh analyzers that predict sample moisture.  The higher the temperature of the convection oven the quicker the extrapolate result, and usually a less accurate result.  While this speeds sample turn around and increases throughput its contribution to variance must be considered.  Hence, double-blind sample averaging minimizes this variation and improve calibration.

For example, using a weigh-dry-weigh gravimetric procedure with an oven temperature of 115?C for 6 minutes will yield a different result than running the sample at 80?C two or three hours.  There also may appear to be “crisping” or “sample browning” at the higher temperature that adversely impacts accuracy.  Again, this is in regards to calibration samples and not necessarily day to day production lab tests that require rapid turn around and high throughput.

Let’s view results for samples tested at 80?C, 115?C and those sent to an outside sample as double-blinds.  Outside labs can be subject to the same variation as in-house labs and double-blind samples reveal the accuracy of their results too.  The chart below shows the benefit of using double-blind samples for calibration and any subsequent validation to improve accuracy.


Double Blind Chart for Word 051116 D

What is Damping Setting?

Damping is a smoothing function to make process measurement with point-to-point variability a stable measurement that reflects the moving average or the product measurement for adjustment and control.

If you are measuring hog fuel for a boiler or ore for mining or even paper moisture for label stock, you want the measurement quickly and also representative of what is being measured over time so that the control scheme doesn’t “chase itself” and instead optimizes operation.  Instead of reacting to point-to-point measurement variation that causes over compensation, a moving average allows control based on a more representative continuous measured value.

The moving average “smoothes” the measurement to allow for real time process control of application dependent unit operations.  Damping is the expression of the “Time Constant”.  The Time Constant is the time it takes for 2/3 of a step change to be reflected in the measured value.  The Time Constant or Damping is necessary to make the measurements more useful to optimize process control.

Example:  System is operating at 2%, an upset causes the constituent (moisture, organic, active ingredient or other) to 8%.  The system upset is 6% in this example.  The damping time constant is the time for the digital display an analog output to reflect 2/3rds of this change.  If the time constant is set for 1 second, the display and outputs will display 6% at one second.  If the trend continues, it will continue to increase value.  If trend stops or “lines out” (stabilizes), it will also reflect that in the moving average.

The damping is used from 1 to 10 seconds depending on application to smooth the real time measurement into a useful process control measurement.  There are also other integration and average measurements available.