You can check which parameters has changes in a single trigger:

    create or replace function trg_sensor_data()
    returns trigger as 
    $$
    begin
    
      if NEW.p1 <> OLD.p1 then
          NEW.p1_tm = now();
      end if;
    
      if NEW.p2 <> OLD.p2 then
          NEW.p2_tm = now();
      end if;
    
      return NEW;
      
    end;
    $$
    language 'plpgsql';

<!-- -->
>     update sensor_data set p1 = 100;
>     
>     select * from sensor_data;
> 
> <pre>
1 rows affected
> 
> id |  p1 | p1_tm                      | p2 | p2_tm              
> -: | --: | :------------------------- | -: | :------------------
>  1 | 100 | 2020-03-17 15:08:15.877775 | 23 | 2020-01-01 11:00:00
> </pre>

<!-- -->
>     update sensor_data set p1 = 100, p2 = 200;
>     
>     select * from sensor_data;
> 
> <pre>
1 rows affected
> 
> id |  p1 | p1_tm                      |  p2 | p2_tm                   
> -: | --: | :------------------------- | --: | :-----------------------
>  1 | 100 | 2020-03-17 15:08:15.877775 | 200 | 2020-03-17 15:08:15.8801
> </pre>

*db<>fiddle [here](https://dbfiddle.uk/?rdbms=postgres_12&fiddle=2cf428c772fde5a36afc57f05b37f10e)*