echo '<tr>
                <td>' . $row_id . '</td>
                <td>' . $row_sensor . '</td>
                <td>' . $row_location . '</td>
                <td>' . $row_value1 . '</td>
                <td>' . $row_value2 . '</td>
                <td>' . $row_value3 . '</td>
                <td>' . $row_reading_time . '</td>
              </tr>';