镭豆只在北京才精确吗?

 

几个月以前,Smart Air测了三个空气测试仪:镭豆,Node和Dylos,跟官方PM2.5数据做了比较。测试结果比我预期的要好!三个机型的相关度都达到了= .90或者更高(= 1是最高),也就是说三种机器都很准。

 

 

不过,我有点怀疑这个结果。为什么?因为镭豆和Node两台机器的PM2.5计算公式是在北京校准过的,所以在北京的最准。然而,它们在其他地方的表现又怎么样呢?

 

为什么你的空气测试仪在上海可能就没那么精准了

很多环境原因可能使颗粒测试仪在北京以外的地区降低精准度。下面举两个例子:

  1. 湿度会影响颗粒测试仪的读数,而北京的湿度比较低。
  2. 有的城市会有更多大的颗粒物,有的城市会有更多小的颗粒物。比如北京会有更多从河北的钢铁工厂飘过来的颗粒,上海会有更多汽车尾气排放污染物颗粒。如果两种污染物颗粒源不同,大小也可能有差异,那么测试仪器的计算方法在将颗粒物转化成微克计算的时候精确度也会下降。

 

镭豆广州测试

Smart Air的DIY好朋友Nikki帮我在广州做了镭豆测试,跟当地美国领事馆官方数据做了比较。

 

 

Nikki在她的公寓得到了147个数据。她的公寓离美国领事馆2.77公里。这段距离也许可以造成读数上的差异了。但是在北京,即使我的测试在距离美国大使馆7公里的地方,显示数据也和距离大使馆200米的地方差不多。

 

结果

镭豆测试数据和美国领事馆官方数据的相关性还是很高(= .88),但比北京的测试结果(= .98)低。镭豆在广州一直测出过高的PM2.5:

 

 

在北京, 镭豆的平均差异是6.5微克。在广州,它的平均差异是23.53微克。

 

数据不精确是湿度造成的吗?

为了查出湿度是否是数据不准确的元凶,我分析了湿度的影响。这几天湿度有很大的变化,从36%到89%,所以可以看湿度高和湿度低的结果。

 

在湿度低的时候,镭豆的平均差异是18微克(左侧)。湿度高的时候,平均差异是27微克(右侧):

 

 

加入湿度的数据,可以增加镭豆读数的准确度,从76.8%提高到79.4%(R squared)。

下图看起来有点复杂,但我慢慢解释。首先,如果镭豆完全精准,就意味着镭豆上显示的1毫克等同于美国领事馆官方数据的1毫克(蓝线):

 

当湿度较低时(< 50%),两个数据会有一点不同,1镭豆微克 = 0.72实际微克(中间那条线)。当湿度很高的时时候(> 50%),两个数据会偏差更大,1镭豆微克 = 0.60实际微克(右边那条线)。

 

结论

  1. 镭豆的精确度在广州比在北京要低。
  2. 湿度能解释至少一部分的这个差异。
  3. 如果你在潮湿的环境里,实际的PM2.5应该比你在镭豆看到的数据要低。

 

其他测试仪也有这个问题吗?

我也测过另一款空气测试仪,叫Dylos。结果发现Dylos在高湿度环境下也会降低精确度, 美国Drexel大学的研究者也有发现了同样的现象。

 

 

我从来没有看过能把湿度加入测算考量的机器。所以我觉得这个不仅仅是镭豆的问题,也并非在点名批评镭豆。总体来说,数据测试结果还是说明镭豆是花600块人民币能买到的不错的测试仪,所以我才决定开始提供这个测试仪

 

镭豆正在试图解决这个问题

我和镭豆的制作者Liam聊过数据的问题。他说将要面市的镭豆2会把湿度因素放入固件升级中:

 

“过去两年里我们都在进行改进,并且有信心让人们得到比市面上任何其他产品都更加精准的测试结果。我们对硬件做了一些改进,包括温度和相对湿度的传感(经过很好的校准),也在镭豆内部空气入口附近加入了一个加热电路板更好的控制相对湿度。通过加热空气入口(大部分的政府官方测试仪也是采用这个设计),相对湿度会被降低,确保镭豆内部湿度不会超过80%。研究表明,湿度对颗粒测算在湿度超过80%的时候会有显著的影响。”

 

这些改进会让镭豆在更多城市获得更高的精确度,不过我还是会在新的镭豆面市以后进行跟踪测试。

 

公开数据和修正公式

按照惯例,我还是会把原始数据和更多测试信息贴出来给书呆子们。还有:

  • 我算的转化公式,提供给住在潮湿环境里的朋友。
  • 真的是湿度的原因吗?除了湿度之外有可能有更多原因
  • 我解释了为什么用这些数据可能低估了湿度的影响!

祝你测得开心!

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Jon

Can someone tell me which countries AQI standards is the laser egg2 plus set to? US?? China?? I mean (straight out of the box), without downloading any apps . Specifically the PM 2.5 . Thank you

Good question! I had to go look that up myself! It seems it will be set to the US AQI. One way to find out is to look in the bottom right hand side of the screen, saying either ‘AQIUS‘ or ‘AQICN‘. If you’re not sure, you can always switch to the ‘PM 2.5’ screen, and convert the raw PM2.5 concentration to whichever AQI scale you’d like.

SAURAV

I have a query.

Will the accuracy of the laser egg counter reduce with time? I mean if we keep using it, would it still show the readings with the same accuracy say 2-3 years down the line?
If no, is there a way to recalibrate it?
Saurav

Great question Saurav! Every single particle counter out there today experiences what’s called ‘drift’: the ability of the sensor to read the correct values will ‘drift’ and change over time. This means that its accuracy may well be reduced after 2-3 years. In this case, the device may need re-calibrating. I’m not sure if the Laser Egg team offers a re-calibration service (and if they do, how much it costs) so I suggest you get in touch with them directly in the country where you are

Matias

Hi there, Thanks for the clear explanation. I hope I knew this before buying my laser egg. I am curr… Read more »

Tomek

I would like to better understand formula mentioned in the article.
I use Laser Egg in quite humid climate but I’m not sure how to implement your formula?

Let’s say Egg shows 100µg PM2.5 and it’s 75% humidity around.
How should I obtain right PM2.5 number? How to do the math?

Thank you for help!

In this case, using the formula you’d do this:
PM2.5 (corrected) = (100 x 0.981) + (75 x 0.052) + (100 x 75 x – 0.006). This would give you a total ‘corrected’ PM2.5 of 57µg/m3. Remember however that this can only be used as a rule of thumb, it’s based on one Laser Egg so is unlikely to be 100% accurate.

Noam

I don’t understand this answer from Liam who made the LaserEgg (I assume this answer was given in the last couple of days, since this article is from yesterday):

“We’ve been working on this for almost two years and can say with confidence that the results people will be seeing are waaaay past anything else out there.”

What does he mean by “will be seeing”? the Laseregg 2 is already sold on the market for several months now, why is he talking like it will be released in the future? is he talking about the Laseregg 3 maybe?

Hey Noam, well spotted! Actually this article was written up and Liam gave us his response before the Laser Egg 2 was released. Liam’s answer was given back during the summer, so he was definitely referring to the Laser Egg 2!

Noam

Ok, now it makes sense 🙂