By Donna Sandfox
Omron Electronic Components, LLC
A new highly portable mechatronic system to measure harmful pollutant relies significantly on a MEMS flow sensor
Figure 1. Stationary Aethalometers are used throughout the world, but have been too heavy to be truly portable until now.
Carbon dioxide is well known as a major contributor to global warming, and there are many ways to detect and measure it. But it is not the only culprit. Scientist have found that the second most significant contributor is soot, or black carbon. Not only does black carbon contribute to environmental degradation, but these tiny particles also cut short the lives of seniors and sicken children. A recent economic impact study in California’s San Joaquin Valley (The Benefits of Meeting Federal Clean Air Standards in the South Coast and San Joaquin Valley Air Basins, November 2008) has identified the cost of air pollution and estimated it at more than $1,600 per person per year.
Black carbon doesn’t stay in the atmosphere as long as carbon dioxide, so controlling it has the potential to achieve major benefits in the short -term. Some of the major emitters of black carbon are diesel engines plus wood- and coal- burning fires. However, to analytically determine the source of black carbon and recommend effective changes to correct the problem, scientists require instruments capable of measuring black carbon in the field.
Manufactured by Magee Scientific of Berkeley, CA, the Aethalometer, is an instrument that uses optical analysis to determine the mass concentration of black- carbon particles collected from an air stream passing through a filter. However, until recently, these instruments were too large and bulky to be easily moved to a suspected point of origination for black carbon; the smallest device (the AE42) weighed approximately 25 lbs and measured 11 x 12 x 8 in. The instruments collect data from installations located around the world (Figure 1), but these only give scientists local samplings.
To get a complete picture of the black-carbon problem, scientists required a very small portable Aethalometer to easily determine black- carbon readings in almost any location. A reduction in size required some clever engineering and component sourcing.
Figure 2. The AE51 Aethalometer’s designers took advantage of the flow sensor’s port placement by designing the manifold to interface to them directly without tubing.
Aethalometers function by measuring the amount of particulate deposited on a fiber filter by a specific amount of air passing through the filter for a predetermined amount of time. This mechatronic system needed to incorporate mechanics, electronics, and computing in one compact package. One of the major size reduction obstacles to overcome was finding a small, lightweight, highly accurate flow sensor with low power consumption. Having worked with Omron in the past, the engineers from Magee Scientific again called on Omron for a solution to their requirements, and the company recommended its D6F-P MEMS mass flow sensor for gathering the required air samples.
Figure 3. D6F-P flow sensors are individually calibrated before shipping to deliver excellent repeatability results.
Size and power constraints
The body of the D6F-P measures just 10 mm high by 23.3 mm wide by 27.2 mm deep, and with a weight of just 8.4 grams, it fell within the size and weight restraints set forth by Magee. Designed for easy installation, the D6F-P has both the input and output ports on the same side which facilitates the connection of tubing.
Magee engineers made clever use of this feature, designing their new AE51 Aethalometer so that the sensor ports would mate directly to their manifold, without the need for tubing (Figure 2). Since this miniature Aethalometer was to be battery powered, current consumption was a concern. The D6F-P proved to be very efficient, drawing a maximum of only 15 mA while operating on 5 Vdc.
Accuracy and repeatability
The AE51 relies on calculating the exact amount of air, driven by a blower incorporated in the device for a given time. Therefore the flow sensor would have to be very accurate. The D6F-P’s flow range/ pressure range of +1.0SLM (+0.84 in H2O) with an accuracy of ±5% F.S. maximum and ±2% F.S.
typical would deliver the precise flow readings Magee required to obtain reliable measurements.
Additionally, since the sensors are individually pre-calibrated at the factory for high repeatability, Magee Scientific’s finished device adjustment and test time was kept to a minimum (Figure.3). Durability was also a concern since the AE51 would have to take multiple readings, but the sensor’s MEMS technology has been proven to deliver a long life with excellent repeatability.
Figure 4. A patented dust segregation system with dual centrifugal separators ensures that the sensing chip remains clean.
In the real world
Since the AE51 is designed to measure black- carbon particulate in areas of known high concentration rates, the sensor had to be reliable in these dirty, real- world environments. Measurements would need to be taken at busy traffic intersections, bus stops, industrial sites, and coal-burning power plants.
The AE51 would also be used in remote areas of the world where use of wood fires to cook and heat is common. Although the filter used to measure the density of the black carbon is in front of the sensor’s inlet, if any particles that got past were to effect sensor operation, measurement accuracy would be compromised.
Figure 5. The reduced size of the hand-held AE51 is obvious when compared to the rack mount AE22 Aethalometer behind it.
To prevent that occurrence, the D6F-P design uses a patented dust segregation system (DSS). The DSS in the flow path incorporates dual centrifugal chambers, in which particulate matter follows in the outer path away from the MEMS sensor chip regardless of the flow direction. Thus there is practically no degradation in sensor performance over the lifetime of the system.
Keeping the MEMS sensor chip clean lets Magee guarantee a long life for their Aethalometer without worry about black-carbon build- up harming the device’s performance (Figure 4).
The A51 Aethalometer (Figure 5) is so small that it can be strapped to a user’s belt, enabling the user to become the instrument’s legs and freeing the user to do other work while the meter is gathering information. It can also be tethered to weather balloons for upper atmosphere readings. Another potential application would allow the device to be carried by those whose health might be affected most by inhaling large amounts of black carbon. The AE51 would alert them to areas that have high concentrations of this toxic material.
Omron Electronic Components, LLC