便宜的空气测量仪准吗?应该买哪一台?对比测试

我2013年创立聪明空气的时候,想要买一个粒子计数器。那时我基本上只有两个选择:买一个260美元的Dylos,或者花费上千美元巨资买一个贵得要命的粒子计数器。

自从那时起,各种新的粒子计数器开始充斥市场,最便宜的仅有人民币99块钱。但是这些粒子计数器有多好用呢?

 

对粒子计数器进行测试

 

为了找到真相,我们测试了三种市场上流行的粒子计数器——Dylos DC1700,Origins Laser Egg,以及一种叫做AirVisual Node的新型粒子计数器。

Dylos

 

Dylos DC1100
Dylos DC1100

Dylos 是聪明空气自从2013年起就开始使用至今的值得信赖的粒子计数器

 

Laser Egg

Origins Laser Egg
Origins Laser Egg

Laser Egg是目前流行的,技术性更强的仪器。

 

Node

Air Visual Node
Air Visual Node

Node是更精致的版本,它有一个大屏幕,包含更丰富的信息,空气污染预报,更好的用户体验,甚至包含了二氧化碳水平监测。

 

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政府空气质量数据对比

我们把三种测试仪放在聪明空气位于东直门外大街的办公室外进行测试,这里距离农业展览馆(政府PM 2.5检测仪位于此)1.3千米。

 

Map of Smart Air Office and Agricultural Exhibition Centre

我们对这些测试仪进行了为期6天的测试。Laser Egg 和 Node 给出的是PM 2.5 微克数值,而Dylos给出的是0.5微米及以上颗粒的数量,所以我们用半官方换算公式(0.5微米及以上颗粒数值-2.5微米及以上颗粒数值)/100,将Dylos读数也换算成PM 2.5微克数值。

以下是我们在北京办公室外进行的第一项测试的结果(测试时间为72小时):

 

 

低浓度PM 2.5环境下的测试

 

接下来我们在PM2.5值很低的几天当中进行了测试。这对我们更加有用,因为一般室内(大多数人使用粒子计数器的地点)的PM 2.5浓度要低于室外水平。所以,这些数据对于测试粒子计数器在低浓度PM 2.5环境中的表现很有用。我们在天空非常清澈晴朗的情况下进行了48个小时的测试。

 

 

粗略来看上面两张图表,三个测试仪的数据跟政府官方数据的走势都很吻合。综合两个测试,我们发现AirVisual Node和Laser Egg与政府公布的PM 2.5的数据相关性达到了r = 0.98。对于不是书呆子的人来说,0.98的相关性算是基本接近完全相同了!Dylos的相关性最低,为r = 0.90,但也算是很高了(这与我们之前的测试结果吻合)。总的来说,三种测试仪测试数据与政府官方数据的相关性都非常高,这说明它们都能很好地反映政府的官方数据。

 

平均偏差

 

另一种测量精确度的方法是看这些数据与政府的官方数据的平均差距。结果证明Node与官方数据最接近:其数值与官方数据平均只相差4.8 微克/立方米。Laser Egg的数据始终与官方数据保持一定差距,二者平均相差6.5微克/立方米, Dylos与官方数据平均相差9.1微克/立方米。

也许有一点需要注意的是,Laser Egg在空气污染程度较轻的时候,PM2.5数值始终偏低。这表示存在这种可能性:Laser Egg会低估真实的室内空气质量,给出一个让人觉得空气足够安全的假象。但是尽管如此这些偏差也不能算大。

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“空气末日”测试

 

为了检验测试仪在超高浓度污染环境中的精确性,我们在面积为15平方米的密闭房间内点燃了一支香烟。这样做的目的是要看看粒子计数器在一个总体污染浓度很高的环境中(包括严重污染水平)的读数精确性如何。在一位长期吸烟爱好者和一个与我们有合作关系的北京NGO的帮助下,我们成功地让这个密闭房间中的PM 2.5浓度达到了1,000微克/立方米!

 

“空气末日”测试

 

为了完成这个测试,我们还同时使用了另一台测试仪(Sibata LD-6S)进行对比参照。这是一台工业PM 2.5 粉尘测试仪,精确性为±10%,重复性误差为±2%。因此我们使用LD-6S作为我们的参照基准。

 

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“空气末日”测试结果

 

通过以上数据,我们可以很清楚地看到Laser Egg和Dylos在污染浓度超高的情况下读数有些“跟不上”了,相比之下,Node 和LD-6S的读数保持着很高的一致性,它们都能测量出超过1,000微克/立方米的浓度值。当然,在实验之外的环境中我们基本不会遇到需要检测这样高浓度污染的情况,但是这个测试显示了Node在高污染水平下的精确性。

 

关键信息

总体来说,跟政府的测试仪相比,这三种测试仪给出的读数都相当精确。聪明空气团队认为,它们都能够较为准确地反映出家中的AQI指数。三者之中,Node得分最高,因为它在室外测试环境下与政府测试仪之间的偏差最小,并且在我们的“超级严重污染”测试中也表现出了最高的精确性。

 

可用性

既然三个测试仪的精确性都很高,现在的问题主要集中在:它们是不是方便使用?它们各自有着什么样的特点?

 

Dylos (1800元)

Dylos在这个比较中真的不怎么给力。它没有手机连接线,而且下载数据也是个痛苦的事儿——这还是在你已经有一根“老学院派式的”针型数据线的前提下。

 

Laser Egg (499元)

Laser Egg是一款入门级的粒子测试仪。它有简单的界面并且能给出非常精确的读数。它没有那么多功能,但是在它的专攻领域——PM2.5检测中表现很好。

 

AirVisual Node (988元)

就视觉上来看,Node功能最强大。对初级入门者来说,它能测量二氧化碳浓度,温度,还有湿度。这些功能使得它更像是一个综合的“环境监测器”,而不仅仅是个粒子计数器。如果你所处的空间不大又同时有好多人在里面,那么二氧化碳浓度能够显示你所在的空间是否太小了。如果你的房间内因为新装修过或者有新家具而存在室内空气污染源(挥发性有机化合物),高浓度的二氧化碳能够表明这些室内污染物正在累积。Node的用户设计能显示过去24个小时中室内和室外空气AQI和二氧化碳指数,也能预测未来几天的情况,还有关于何时打开窗户或者何时戴口罩的建议。通过对Node一段时间的使用,我们发现这些功能在我们的办公室内还是很有用的。

 


通过了测试以后,我们将会在聪明空气淘宝商店中开始发售AirVisual Node。无论是在家中使用还是用作实验目的(如果你像我们一样也是个书呆子),我们愿意为需要一个靠谱测试仪的人提供多一个选择。去看看吧!

接下来的几个月里,我们希望能对更多空气测试仪(粒子计数器)进行测试,并且进行一些花费更高的测试。这只是一个开始!如果通过我们的独立测试确认了更多靠谱的测试仪,我们有可能也会把它们放到我们的淘宝货架上来。

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Are Particle Counters and Government Machines the Same?

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

My particle counter is a beast–I’ve loved it. But I’ve always wondered how the counts of laser particle counters like mine:

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…compare to the measurements of the huge stationary air quality monitoring stations that governments use, like this one in New Zealand:

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If you look at the US Embassy’s Twitter feed, you’ll notice that its raw numbers are “concentration,” which it explains are micograms per cubic meter (µg/m³). The way government (BAM) machines work is that they use a source of carbon 14 that emits beta particles and then measure how many of those beta particles make it through to a detector. They then use those numbers to estimate the weight of those particles (micrograms).

In contrast, laser particle counters like mine use a laser and a photo diode sensor to estimate the number of particles in the air. I don’t see why the weight should be any more important than the number–they’re both telling you how much particulate pollution is in the air.

As an analogy, if we want to understand the crowd at a basketball game, we could count the number of people, or we could weigh those people. Of course, the more people, the heavier the total weight. And of course the two numbers won’t correlate perfectly if we have more heavy people on some days and more children on other days. But the weight and the total number should correlate highly.

The other major difference is that laser particle counters give the number of particles at that particle size and above. Government machines give the number of particles at that size and below.

To see how the two numbers compare, I put my particle counter outside my second-story window 70 times (that’s nerd dedication!) and compared my numbers to the US Embassy’s Twitter feed at the same time. Here’s what they look like:

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They correlate at = .89, meaning the two numbers are very strongly related (remember the highest possible correlation is 1). That high correlation is especially impressive given that my house is near Gulou, and the US Embassy is out in Liangmaqiao–about 7 kilometers away.

The difference between the readings was particularly noticeable on days where a strong wind moved through Beijing. I noticed several times that my particle counts would drop before the embassy’s counts as the wind moved in from the west (where my house was) to the east (where the embassy is). (Remember, Beijing’s air gets a lot cleaner when we get winds from the west.)

If we want to get a rough conversion between the numbers, we can remove a few of the outliers and compute a regression line:

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For example, a government concentration of 100 micrograms (four times the WHO limit!) is approximately 25,000 on the Dylos particle counter:

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And the 24-hour WHO standard of 25 µg/m³ is about 3,000-4,000 on the Dylos.

During my home tests (before turning my filter on), the air inside my home was very often above 3,000 (even though it was still much cleaner than outside).

Conclusion: My particle counter is giving measurements that are highly related to the much larger air monitoring stations. The scale is different, but the two can be roughly converted.

 

 

QQ20160707-5

How to Make a DIY Air Purifier

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

Based on my research into how different filters work, I concluded that a HEPA filter is all you really need to fight particulate air pollution in China.
Given the fact fancy air purifiers run for 8,000 RMB here in Beijing, and I’m only here for a year, I thought I’d try making my own.
I’ll give details on how well it works later, but here I’ll explain how I made it:

  1. Buy a fan. It’s important that it has a flat front so that you can strap the HEPA filter to it.
    Fans with flat fronts usually have a recessed motor unit, so look for that. Here’s the one I bought for 58 RMB:

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  1. Buy a HEPA filter. I bought this one for 108 RMB, and I’ve tested it with my particle counter, so I know it works:

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  1. Pull the grating off the front of the fan. It helps if you have pliers for this. Then turn the power setting to 3 and pull the knob off.
    The knob gets in the way of the HEPA. Without the knob, I turn the fan on and off by plugging and unplugging it.

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  1. Use string to strap the HEPA filter onto the front of the fan, and you’re done!
    The metal bar in the middle will stick through the filter a little bit. You could saw the metal bar off, but my tests show it doesn’t seem to make a difference.

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Total cost, 166 RMB (US$27). Compare that to 8,000 RMB ($1,300) for the fancy purifiers here in Beijing.Your next question is probably, does it work? See a live test here.

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Dylos DC1100 Pro Air Quality Monitor (Particle Counter)

对不起,此内容只适用于美式英文。 For the sake of viewer convenience, the content is shown below in the alternative language. You may click the link to switch the active language.

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The beast. The Dylos DC1100 Pro Air Quality Monitor. This is what I use to test whether the filters work. It gives readings of the number of particles 2.5 micrometers and above, as well as .5 micrometers and above per .01 cubic foot. They’re available through Amazon for $261.