# PurpleAir Classic PA-II Sensors

**URL:** <https://community.purpleair.com/t/purpleair-classic-pa-ii-sensors/6861>\
**Category:** Sensors\
**Created:** [October 6, 2023, 11:28am UTC](https://community.purpleair.com/t/purpleair-classic-pa-ii-sensors/6861 "2023-10-06T11:28:15Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Elia\_Seitz](https://sea2.discourse-cdn.com/flex020/user_avatar/community.purpleair.com/elia_seitz/32/2340_2.png) [@Elia\_Seitz](https://community.purpleair.com/u/Elia_Seitz)\
**Post date:** [October 6, 2023, 11:28am UTC](https://community.purpleair.com/t/purpleair-classic-pa-ii-sensors/6861/1 "2023-10-06T11:28:15Z")

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Hello alll

I’m currently writing a paper about the measurings, that I’ve made with my Classic PA-II Sensors. In the description of the website it says, that the Sensor has two laser counters, which get compared to get a good measurand.  
This information is a bit short. Can somebody please carry out the operation of the sensor a bit more, so I can write how this exactly works with these two laser counters?

Thank you for your help! 🙂

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**Author:** ![Kieran](https://sea2.discourse-cdn.com/flex020/user_avatar/community.purpleair.com/kieran/32/6220_2.png) [@Kieran](https://community.purpleair.com/u/Kieran)\
**Post date:** [October 6, 2023, 6:14pm UTC](https://community.purpleair.com/t/purpleair-classic-pa-ii-sensors/6861/2 "2023-10-06T18:14:19Z")

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Hi @Elia_Seitz

Classic PA-II sensors contain two PMS5003 laser counters. These counters conduct two independent measurements. When you look at the graph on the map for example, sensors with two laser counters will display 2 lines. If you look at the image below, you can see a black and a blue line.

![Screenshot 2023-09-14 at 12.52.57 PM](https://us1.discourse-cdn.com/flex020/uploads/purpleair/original/2X/4/45a82da5960f1547b6f9f9b3392e54d0dc024719.png)

These are the two different laser counters. The AQI score shown on the map is the average of these two values. Additionally, the two independent channel values can be pulled from the API using the “a” and “b” values.

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**Author:** ![Elia\_Seitz](https://sea2.discourse-cdn.com/flex020/user_avatar/community.purpleair.com/elia_seitz/32/2340_2.png) [@Elia\_Seitz](https://community.purpleair.com/u/Elia_Seitz)\
**Post date:** [October 8, 2023, 12:52pm UTC](https://community.purpleair.com/t/purpleair-classic-pa-ii-sensors/6861/3 "2023-10-08T12:52:42Z")

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Hi @Kieran  
Thank you very much for your response!  
Did I understand this right?: When you download the data, the pm2.5\_atm is the average value of the two laser counters (pm2.5\_atm\_a and pm2.5\_atm\_b).

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**Author:** ![Kieran](https://sea2.discourse-cdn.com/flex020/user_avatar/community.purpleair.com/kieran/32/6220_2.png) [@Kieran](https://community.purpleair.com/u/Kieran)\
**Post date:** [October 9, 2023, 5:59pm UTC](https://community.purpleair.com/t/purpleair-classic-pa-ii-sensors/6861/4 "2023-10-09T17:59:47Z")

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Hi @Elia_Seitz

Yes, that is correct. The same goes for the pm2.5\_cf\_1 and pm2.5\_alt categories. You can return the averaged results, or the two channels individually.