graphic showing one hundred correlation function decay times as suggested run duration for the measurement. Image by Ulf Nobbmann. What is the extended measurement duration option?

Every once in a while questions about the advanced settings in the classic  Zetasizer Nano software come to us. Here is the exact question from a customer asking why to extend the correlation measurement duration:

“When I perform DLS measurements, I see an option for measuring large particles. What’s the physics, math, or mechanism that changes when that box is checked? What expected size range should trigger me to use that box’s feature?”

Since this is an advanced topic, we will share the answer here. First the caveat: you don’t need to alter the setting for the vast majority of applications scenarios. If in doubt, stick with the automatic settings.

How to access the extend duration option?

The ‘extend correlation measurement duration‘ for large particles in an option in the Advanced Measurement settings of the size measurement type. The online help indicates: “Enable this option to automatically extend the measurement time when large particles are present.”

Zetasizer Nano software, advanced option for extended duration for large particles, screenshot by Ulf Nobbmann

By default, the correlation measurement run duration of each correlation function is 10 seconds. This typically leads to excellent statistics to achieve a complete decay to baseline in the correlation function. Very large particle diffuse more slowly. In rare cases it may be advantageous to extend the correlation measurement duration of each correlation function to be longer. This ensures that the full decay is captured.

Enabling extended duration with multiplier

Setting this option to ‘Yes‘ will automatically extend the duration of each subrun, based on the decay time of the correlation function. You can enter a specific multiplier. For example, if the relaxation time is 1 msec, then a relaxation time multiplier of 100000 would extend the duration to 100000 x 1msec = 100 sec.

run duration relaxation time multiplier to extend the automatic run time for very large particles, shown in the Zetasizer Nano software

So instead of 10-second subruns, the software would then take 100 second subruns. This option is very rarely if ever used. The only scenarios where it might be of use would be:

  • very large particles (several microns) in
  • high viscosity samples (several 10’s cP) or when measuring at
  • small (forward) scattering angle.

The same result can be obtained by manually selecting run duration. In that case, the ‘Option for Extended duration‘ for large particles is not available.

How do I know if this is relevant for my sample?

This is an advanced option because it does not affect the vast majority of samples. Take a look at your correlation function and note the relaxation time. In the example below, display your correlation function in the software. Then hover with the cursor over a place where the correlation function has decayed to half the value of the intercept. The software shows the x,y values in the popup. There are the correlation function and the time value. In this case, the correlation function is 0.452 at a decay time of 101 microseconds.

Correlation function with the half decay point indicated for indentifying the decay or relaxation time of the function

If we want to have at least 100000 relaxation times in the correlation function, we calculate: 100,000 x 101 µsec = 1 E5 x 101 E-6 sec = 10.1 sec . So in the above example, enabling the extended run time would have no influence at all, since the default time is 10 sec.


If you have any questions, please email me at Thanks! While opinions expressed are generally those of the author, our editorial team may have modified some parts.