Null values at constant intervals

[fu-hsi]

Looks familiar?

    id                   dt     pm1    pm25    pm10  
------  -------------------  ------  ------  --------
134853  2018-11-24 13:22:59  (NULL)  (NULL)    (NULL)
134852  2018-11-24 13:20:13      16      30        36
134851  2018-11-24 13:17:27      14      30        36
134850  2018-11-24 13:14:42      12      26        32
134849  2018-11-24 13:11:58      10      24        24
134848  2018-11-24 13:09:12       8      20        20
134847  2018-11-24 13:06:28       8      20        20
134846  2018-11-24 13:03:42       2      16        16
134845  2018-11-24 13:00:58  (NULL)  (NULL)    (NULL)
134844  2018-11-24 12:58:12      14      24        34
134843  2018-11-24 12:55:27      12      22        34
134842  2018-11-24 12:52:43      10      22        24
134841  2018-11-24 12:49:58      10      18        18
134840  2018-11-24 12:47:12       6      16        16
134839  2018-11-24 12:44:28       4      16        16
134838  2018-11-24 12:41:42       0       6         6
134837  2018-11-24 12:38:58  (NULL)  (NULL)    (NULL)

In passive mode I receive NULL values from PMS 7003 sensor with constant intervals.
I have not received any complaints from others, these are my observations only.

Simple solution without modify library:
Increase default timeout value for readUntil().

E.g.

pms.readUntil(data, 2000);

In next release I will use flush() after all commands to the sensor to be sure that the whole message has arrived.

[YiannisBourkelis]

@fu-hsi yes I have the same issue. I am also getting NULL in passive mode. I will try increasing the timeout as you suggest and let you know.

[coelner]

As noted by adafruit (https://cdn-learn.adafruit.com/downloads/pdf/pm25-air-quality-sensor.pdf , pg.13) there is a delay in the refresh rate abound 2.3 seconds max when there is no change in the particle counting, but it can speed up to 200-800ms refresh rate.

[fu-hsi]

As noted by adafruit (https://cdn-learn.adafruit.com/downloads/pdf/pm25-air-quality-sensor.pdf , pg.13) there is a delay in the refresh rate abound 2.3 seconds max when there is no change in the particle counting, but it can speed up to 200-800ms refresh rate.

It's referred to the Active mode (default). From official documentation:

There are two options for digital output: passive and active. Default mode
is active after power up. In this mode sensor would send serial data to the
host automatically .The active mode is divided into two sub-modes: stable
mode and fast mode. If the concentration change is small the sensor
would run at stable mode with the real interval of 2.3s.And if the change is
big the sensor would be changed to fast mode automatically with the
interval of 200~800ms, the higher of the concentration, the shorter of the
interval.

But in our example (Passive mode) response time is less than 1s, but I recommend setting to 2s.