FAQ

 

 

Frequently asked questions regarding our devices, measurement principle and applications.

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Last updated on 22.1.2024

Measuring

General questions regarding measurements with our devices.

Are there any limitations to what type of particles are measured? For instance metals, wood, exhaust fumes etc?

No, our devices measure all types of particles.


Does the device only count solid particles in the sampled air or also liquid droplets?

Our devices measure anything non-gaseous in the air, also liquid droplets.


Can the device also measure fibers or agglomerates? Will particle morphology affect the measurement value?

Particle morphology is known to affect particle charging. Fractal-like particles will obtain a higher charge than compact particles of the same mobility diameter. Fractal-like particles have a large internal surface that compact particles do not have, so the lung-deposited surface area measured with our devices remains approximately correct (to within a factor 2 for fractal particles smaller than 400nm). Particle diameter and number measurement will be affected by morphology though, as in other instruments based on diffusion charging.


There are coarse particles in my aerosol. Do I have to use a cyclone to prevent the large particles from distorting the measurement?

The answer is no: Coarse particles are a problem for the sensor primarily because of device contamination. If conditions are very dusty, a cyclone is recommended in the manual. Particles larger than about 300nm are underestimated in the LDSA measurement, and are misclassified in diameter and particle number: a 2μm particle is detected approximately as 6 300nm particles. However, the signals are usually dominated by the particles < 400nm.
If a cyclone with a 400nm cutoff was used with our devices, LDSA would be underestimated even more. The miniature pump in our devices is not strong enough to operate a cyclone with such a low cutoff.


Can your devices measure a size distribution? Yes, the Partector 2 Pro can measure a rough size distribution.

How can one convert LDSA to particle mass?

It is not possible to convert LDSA to particle mass.
Ultra-fine particles have very low mass, but a very large surface area where they interact with their surroundings. This is why alternative metrics such as particle number concentration or LDSA are often used to assess risks associated with nanoparticles. For more information, read our white paper on LDSA.


What is the measurement uncertainty of the devices?

The measurement uncertainty of our devices is different for each metric they report. The reason for the uncertainty is also different depending on the metric.

For LDSA, there is an intrinsic uncertainty when measuring it via particle charging, as the particle charge is not perfectly proportional to LDSA anyway.

For the average particle diameter and particle number concentration, the standard Partector 2 must make an assumption on the shape of the particle size distribution. If the particle size distribution is very different from the assumption (e.g. for monodisperse particles, or extremely broad size distributions), then the size and number reported by the standard Partector 2 is systematically wrong. In our experience from laboratory and ambient measurements, average particle diameter and particle number concentration agree with reference devices to ± 30 %.

The Partector 2 Pro reports an 8-channel size distribution, and makes no assumption on the particle size distribution. It is therefore more accurate than the standard Partector 2 for size distributions that differ strongly from the ones the standard Partector 2 assumes (lognormal, with geometric standard deviation 1.9). However, this higher accuracy comes at the price of a slower response time and generally more noisy signals.


Why is your sensor so expensive? There are much cheaper particle sensors available!

There are lots of nice and cheap particle sensors available on the market that work using the principle of light scattering. These sensors work well for the measurement of PM2.5, but typically cannot see particles that are smaller than approximately the wavelength of the light used. Therefore, they can typically not see particles smaller than about 300nm.

Measuring ultrafine particles is harder, and requires more complex instruments, such as our line of devices based on diffusion charging, or condensation particle counters. All these instruments - ours, and those of our competitors - are far more expensive than the cheap optical PM2.5 sensors.



Partector 2 & Partector 2 Pro

Questions specific to Partector 2 and Partector 2 Pro

Why is in the partector 2 number concentration and mean diameter limited to the size range of < 300 nm and LDSA not?

The sizing method in the Partector 2 works by measuring the electrical mobility of particles that are charged by diffusion charging. This electrical mobility becomes practically constant for particle diameters > 300nm, so sizing larger particles is impossible, and the largest diameter the Partector 2 will report is limited to 300 nm. Since the size information is necessary for the number concentration calculation, the number concentration can also only be measured properly up to 300 nm.

The LDSA measurement is a direct measurement of particle charge without any calculations (except the calibration constant) and therefore works over a wider range.


When I do a measurement with multiple Partector 2 instruments, I see that LDSA values correspond very well, but mean particle size and number concentration readings show a larger scatter - why?

The LDSA value is a direct measurement of the particle charge; whereas the particle diameter calculation works by measuring both the particle charge and the removal of a part of the particles from the air flow by an electric field. Since it is based on measuring two currents rather than one, there is a larger uncertainty due to error propagation. This also applies to the particle number measurement.


When I measure very low concentrations, the number concentration and especially the mean particle diameter become noisy. Why is this?

Our sensors measure tiny electrical currents, and the electrometers used always also measure a small amount of electrical noise. If there is no signal at all any more because there are no particles, the electrometers measure only noise, and the signals produced are random. For the particle diameter this means it can be anything between 10 and 300nm, the particle number will also be random but very low, as the noise levels are low.

If you want to make meaningful measurements at low concentrations, you can (and should) increase the integration time of the device (info - config - timebase). A four times longer timebase will reduce noise level by a factor 2; at the price of a 4x lower time resolution. Note that you must do this on the device, you cannot get rid of the noise by averaging the output of a measurement with shorter timebase.


What is the difference between the Testo DiSCmini and the Partector 2 ?

Both units were developed by Martin Fierz (naneos) and colleagues. The DiSCmini was developed in 2008 at the university of applied sciences northwestern Switzerland (FHNW) and commercialized by Matter Engineering, which later got sold to Testo. The Partector 2 was developed a few years later at FHNW and naneos; and we applied many lessons learned from the DiSCmini to the new device.

The two instruments are similar in that they both use diffusion charging and sensitive electrometers; and have similar measurement ranges and accuracies. The particle sizing is done differently though: in the DiSCmini, diffusion separation is used, whereas the Partector 2 uses electrostatic separation. Electrostatic separation has the advantage that the separation can be adjusted by changing the voltage used; this allows the measurement of a particle size distribution in the Partector 2 Pro, by measuring at different precipitation voltages - this is not possible in the DiSCmini.

Another key difference is the use of induced currents in the Partector 2, which removes issues with drifting electrometer offsets.


The GS-3 cyclone you recommend is designed for a much larger flow rate. Isn't that the wrong pre-separator for this unit?

Cyclones always work as cyclones; at lower flow rates their separation cutoff just becomes larger. We recommend the GS-3 to remove large particles - it protects the Partector 2 from coarse dust / fibers. We estimate that with the lower flow, the cutoff should be around 25-30 μm. This is still helpful to protect our devices from coarse dust, premature aging and short service intervals.


Does the device measure the air pressure? Is there any kind of internal correction of the measurement value due to pressure changes?

The Partector 2 has an internal pressure sensor; its values are also recorded in the data file. The Partector 2 attempts to correct its measurements for different air pressures (and also for varying temperature).


The LDSA value changes colour after 50 μm²/cm³ and 250 μm²/cm³. Where do these limits come from? Are these official limit values?

No, this is a definition by naneos. From our experience, we measure about 10 μm²/cm³ in clean Swiss ambient air. Based on that we interpret a five times higher value as an elevated value and color it yellow. A value 25 times above normal clean air is defined as dirty air and therefore colored in red.


I measure 5 Mio. particles/cm³ but in your user manual, measurement range is only up to 1 Mio. pt/³. Is there anything wrong? Can I trust this measurement?

The true limitation of the measurement range comes from the values measured by the electrometers. Since larger particles acquire more charge, a given number concentration of small particles produces lower signals on the electrometers than the same number of larger particles. If the particles are small, the instrument can therefore measure higher concentrations. If you want to make sure that the measurements are correct, you can check the electrometer amplitudes in the data file (A1 and A2). If they reach the maximum values (2048 mV), then the device was in overrange and you can no longer trust the measurements. You can say with certainty though that the air was extremely dirty if this happens.



Partector 2 Pro

Questions specific to Partector 2 Pro

In the Partector 2 Pro, there is a value called "GSD" on the display, what's this?

GSD stands for geometric standard deviation, and is a measure of the width of the particle size distribution. More information on geometric standard deviation can be found e.g. on Wikipedia.


Size bins of Partector 2 Pro: I see different size bins e.g. "n10.00 n16.26 n26.43 .." in the data file. Is 10 nm the center of the n10 size bin and if yes, what are the upper and lower limits of this bin?

The sizes listed in the data file are the channel centers. The limits can be calculated from the logarithmic distance between two channels, e.g. log(16.26)-log(10) = 0.211, so the limit is at 10^(1+ 0.211/2) = 12.75nm.


If I sum up the counts of all the size bins of the Partector 2 Pro, I don't get the total number concentration shown on the display. Are the different channels normalized to dlogDp?

Exactly, the number concentration is reported as dN/dlogD (as is usual in aerosol science). To convert to absolute numbers for each size channel, you must mulitply the concentration by dlogD = 0.211.


Application

Here you can find some questions regarding specific applications.

Are your devices suitable for measuring ultra-fine particles at airports?

Yes! Our devices have already been used many times in such applications. Go to the scientific paper section for more information.

A note at this point: UFP concentrations at airports can change very rapidly. Concentrations can vary over several orders of magnitude depending on wind, jet blast direction etc.
Since concentrations change so rapidly, it is probably better to use a standard Partector 2 rather than a Partector 2 Pro for such measurements. Aerosol plumes at airports are often only present for a few seconds, and since the response time of the Partector 2 Pro is at best 16 seconds, it might be too slow for this application.


Can your instrument be used to measure particle concentrations in industrial clean rooms?

Our lower detection limit is around 500 pt/cm³, so an order of magnitude or more higher than limit values for industrial clean rooms. We do not recommend our instruments for such applications.


Can the devices be used in high altitudes or low ambient pressure environments?

The pressure affects the device in multiple ways (charging, deposition, flow measurement) but should not have any serious effect, since we attempt to compensate for these effects on the measurement. We have used the devices at heights up to 3000m, and they seem to work fine there still. For even higher altitudes / lower absolute pressures we have no experience.


Can the devices be used to measure out of a volume that has a lower pressure than ambient e.g a low-pressure chamber?

The internal pumps can deal with an underpressure of around 20mbar. In the Partector 2 the flow is regulated as long as the pump can handle the underpressure. If you sample against a too high underpressure, then the internal pump can no longer produce the necessary flow rate, and the device will show an error.

Since the pump only has to handle the pressure difference between inlet and outlet, it is possible to measure in a low-pressure chamber simply by putting the Partector 2 in the chamber, rather than trying to sample air from that chamber.


Is it possible to detect viruses with the device?

The devices sees all particles, so it can theoretically also measure viruses. However, virus concentrations are much lower than "normal" ultrafine particle concentrations, so the Partector 2 is not the right device to measure viruses in air, as you cannot differentiate between viruses and normal background ultrafine particles.

To detect bioaerosols, a method that can specifically measure only bioaerosols is necessary, e.g. based on fluorescene.


Can your devices detect Carbon Nanotubes (CNTs)?

Yes - there was a EU project where it was used (alongside with other instruments) to detect CNTs and that worked reasonably well.

However, there are two main issues when trying to detect CNTs:

1) all physical particle detectors are unspecific, whether ours, or devices of competitors that work based on light scattering or condensation of fluids + light scattering. Therefore, you never know whether you have an issue with CNTs in the air or whether it is just something else much less harmful. Only sampling + subsequent TEM analysis can help you there.

2) If you use limits similar to those for asbestos fibres (0.01 fibres/cm³), the concentrations of interest are far lower than what we and most others can ever measure in real time. To measure such low concentrations, high volume samplers are used that sample large amounts of air through a filter, and subsequently the filter is analyzed offline with an electron microsope.


Can your devices be used for diesel particle filter testing on cars?

With the exception of HEPaC, you should not use our devices to measure engine exhaust. Engine exhaust gas can be very hot and contains a lot of humidity that usually is a problem for our devices, as water will condense in the instrument, with very unfortunate consequences (you will destroy the device and your warranty will not apply). Particle concentrations can also be extremely high in such applications and instruments will age rapidly.

Typically, tailpipe particle measurements include a dilution/drying step to reduce humidity and particle concentrations. If you do this yourself, you can safely use our devices to measure tailpipe exhaust.

Interfaces & data exchange

Here you can find some questions regarding the interfaces and data exchange with our devices.

Is it possible to send data to a PC, mobile phone or over the internet?

All our devices can send real time data over USB to a PC. If you connect our devices to a PC (Windows 10/11), the PC will recognize them as a virtual COM port. The virtual COM port acts as a serial interface, and you can read measurement values and send commands to the instruments. It is also possible to read from this virtual COM port with another software e.g. LabView or Python. LabView VI's and Python scripts to interact with our devices are available on request.

The Partector 2 can also send real time data to a mobile phone or PC over a wireless connection. For that you need to install the naneos App that can be downloaded from the Google Play Store or you use a small program for your PC. For more information on that, go to our IoT section.


Is it possible to connect your devices over a serial interface (RS232/UART)?

Yes, with limitations:
The μSD-Slot can be configured as a serial output and we have a special cable that can be plugged in the μSD-Card slot.
The serial interface has 3.3V microcontroller logic levels. For real RS232 communication, other voltage levels might be needed. It is possible to include a level shifter in the cable to allow 5V serial communication for connection with a PC.

We don't recommend this connection option, because (1) you lose the internal storage on the μSD card and (2) if you apply force to the adapter by accident, you can damage the instrument. You have to use this option at your own risk (no warranty).


Is it possible to download the measurement file from the devices over USB?

It is not possible to access the device as a "mass storage device" for downloading measurement files.

It is only possible to stream real time data to a PC or to remove the μSD-Card with the measurement files after a measurement.
If you have sent realtime data to the naneos IoT cloud, you can download it from there.


What wireless protocol do you use in the Partector 2?

We use Bluetooth low energy. In principle, you can listen to the advertisement data packages and parse them. The protocol is available on request.