i solved the problem ,first of all if you need to put more than critere you have to put and between,second for to get the max date for a specfic device i had to sort the result and choose the first one
max = o.GetData().Select("equipement_ref='mydevice'","time DESC")[0].ItemArray[1].ToString();
min = o.GetData().Select("equipement_ref='mydevice'", "time ASC")[0].ItemArray[1].ToString();