Gas_680 value on Purple Air Zen never changes!

I’ve had my PurpleAir Zen up and running (indoors) for more than a day now and the gas_680 value has never changed. It is constant at 25.00.

From json?live=true:

From the device web page:
BME680: 82.00°F RH: 30.00% Dew: 47.00°F P: 1018.56 hPa AQ: 25.00 ADC: 0.04V

Note: I record all values every 30s and save to a database. I can state definitively that it never changes — even when I open the door. The other values listed above on the BME680 line do indeed change over time.

Also Note: I also have a PurpleAir Flex (outdoors) that also reports gas_680. That value does indeed change (I’m also saving every 30s and graphing this device).

What’s up with this? Do I have a defective BM688 in my new Zen?

@Amanda_Hawkins Would you be able to find out an answer for this question? I have a new Zen that has always read 25.00 for gas_680 (it never changes). Do I need a new unit?

I can confirm the same behaviour on my Zen

It’s disappointing that PurpleAir never replied to this; but the good news:

As of little after 8 am PDT this morning (March 16), gas_680 is now reporting correctly. I presume the device took an update.

Same here. Thanks for the update! (And the update.)

The VOC monitor in PurpleAir sensors is experimental. This is due to our implementation. We are finding ways to include everything required for it on the little space available (2MB) on the sensors.

We are working on changes to the firmware and hope to soon correct this issue.

Hi Ethan,

As I mentioned above, the issue has already been corrected (as of March 16).

I’ve also noticed strange behavior with the VOC readings on some recently installed Zen devices. These are two PurpleAir devices on my front porch with VERY different readings. The orange Flex line shows changes according to expected air patterns. The recently installed Zen appears to be largely unresponsive to changes compared to the Flex. Does anybody have any insights into what is going on? Do I just need to wait a while longer for it to possibly finish some kind of calibration phase? Thanks for any insights!


Here is the same kind of pattern observed at three monitors installed near to each other. I’d expect them to have very similar readings, at least with respect to the amplitude changes of the VOC’s, but the two older VOC sensors seem to be tuned very differently from the newer FLEX sensor. Eager to hear thoughts from others to help me understand what is going on here:

This may be a very strange question but I will ask. I just purchased a Purpleair Zen device and plan on doing a simple experiment. I will set it in a stable environment and then place some crumb rubber (shredded tire material) on a heated (red hot) teaspoon in relatively close proximity to the device. First I want to determine whether I can smell something coming off of the heated crumb rubber (pretty confident that I will) and also see if the VOC recording show anything. Am I crazy to try this experiment? Has anyone ever tried a similar experiment? Thanks Steven

Are there simple tests to determine with the BSE688 chips is working ‘properly.’ I got it set up this afternoon around 3:45 PM and the humidity/temperature are varying over time, but the experimental VOC (Bosch Static) IAQ) reads a constant 25. I even put a paper towel soaked with isopropyl alcohol a 3-4 inches below the sensor and the readings remained constant. It has been running for slightly over an hour now. Someplace I thought I saw a note that it took a while for the BSE688 to burn in, but not sure that is applicable. Thanks

The US EPA PM 2.5 AQI does seem to vary as one might expect.

I believe is till start working for you. Perhaps due to an update. See our experiences above.

Do you recall how long it took before it started to respond? Do you know if there is a power interruption whether the VOC take a long time to come back up?

Do you recall how long it took before it started to respond?

I installed the sensor on February 9 and gas_680 didn’t work (i.e, had the value of 25) until March 16. Your question implies that it took that long to burn in. I have no reason to believe that. I assume it was a firmware update. When I said I believe it will start working, it was because I thought the sensor would take an update.

Do you know if there is a power interruption whether the VOC take a long time to come back up?

I have power cycled the sensor with no issues (i.e., gas_680 values behaved appropriately).

One more thing. gas_680 was fine from the start on my Flex.

I have firmware 7.02. Do you know your firmware number? Since the firmware update occurred in March, I would assume since I recently purchased (a couple of days ago), I would have the latest firmware, perhaps not. I just noticed a few people on the map have firmware version 7.04 and they seem to be the one reporting VOC numbers. Do you know whether there is a way to force an update?

It has been running for 14 hrs at a constant 25.


I’m on 7.04. Unfortunately, I do not know when your sensor will update; nor do I know how to force an update.

John, My device (PurpleAir Zen) was updated last night sometime to 7.04 and it now reporting out the VOC data. Thanks for you help.