Hardly suitable for measurements

When the thing arrived, I put first time batteries - no display. Then I took them back 'out, looked after if I had not perhaps been inserted backwards - was OK, put them back' pure - no display. Again 'out in the battery tester - batteries OK - again' pure - again no display. The whole thing is trying again - Eureka, it indicates: 40% with the arrow in the green zone; five minutes later 39% in the red. OK - started humidifier and waited two hours a day - 56% in the yellow (all of course at about the same temperature) ...
In the days since then apparently randomly controlled display how much% in which area include, in between sometimes percent display without arrows - so as * measuring * instrument to forget rather; a rough indication of the change in the moisture it should deliver but - I hope ...