diff --git a/backend/lib/Interpreter/Interpreter.php b/backend/lib/Interpreter/Interpreter.php
index a6c3671..70afa2b 100644
--- a/backend/lib/Interpreter/Interpreter.php
+++ b/backend/lib/Interpreter/Interpreter.php
@@ -39,7 +39,9 @@ use Doctrine\ORM\Query;
  */
 abstract class Interpreter implements InterpreterInterface {
 	protected $channel;
-	protected $em;
+
+	/** @var Database connection */
+	protected $conn;
 
 	protected $from;
 	protected $to;
@@ -54,7 +56,9 @@ abstract class Interpreter implements InterpreterInterface {
 	 */
 	public function __construct(Model\Channel $channel, ORM\EntityManager $em, $from, $to) {
 		$this->channel = $channel;
-		$this->em = $em;
+
+		// get dbal connection from EntityManager
+		$this->conn = $em->getConnection();
 
 		$this->from = (isset($from)) ? self::parseDateTimeString($from, time() * 1000) : NULL;
 		$this->to = (isset($to)) ? self::parseDateTimeString($to, (isset($this->from)) ? $this->from : time() * 1000) : NULL;
@@ -71,9 +75,6 @@ abstract class Interpreter implements InterpreterInterface {
 	 * @return Volkszaehler\DataIterator
 	 */
 	protected function getData($tuples = NULL, $groupBy = NULL) {
-		// get dbal connection from EntityManager
-		$conn = $this->em->getConnection();
-
 		// prepare sql
 		$parameters = array(':id' => $this->channel->getId());
 
@@ -93,10 +94,10 @@ abstract class Interpreter implements InterpreterInterface {
 		}
 
 		// get total row count for grouping
-		$rowCount = $conn->fetchColumn($sql['rowCount'], $parameters, 0);
+		$rowCount = $this->conn->fetchColumn($sql['rowCount'], $parameters, 0);
 
 		// query for data
-		$stmt = $conn->executeQuery('SELECT ' . $sql['fields'] . $sql['from'] . $sql['where'] . $sql['groupBy'] . $sql['orderBy'], $parameters);
+		$stmt = $this->conn->executeQuery('SELECT ' . $sql['fields'] . $sql['from'] . $sql['where'] . $sql['groupBy'] . $sql['orderBy'], $parameters);
 
 		// return iterators
 		if ($sql['groupBy'] || is_null($tuples) || $rowCount < $tuples) {
diff --git a/backend/lib/Interpreter/SensorInterpreter.php b/backend/lib/Interpreter/SensorInterpreter.php
index e9c3f4d..cf32d1c 100644
--- a/backend/lib/Interpreter/SensorInterpreter.php
+++ b/backend/lib/Interpreter/SensorInterpreter.php
@@ -53,34 +53,33 @@ class SensorInterpreter extends Interpreter {
 	}
 
 	/**
-	 * @todo adapt to doctrine orm
-	 * @todo untested
+	 * Fetch the smallest value from database
+	 * @internal doesn't fits the SQL standard
 	 * @return array (0 => timestamp, 1 => value)
 	 */
 	public function getMin() {
-		//return $this->dbh->query('SELECT value, timestamp FROM data WHERE channel_id = ' . (int) $this->id . self::buildFilterTime($this->from, $this->to) . ' ORDER BY value ASC', 1)->current();
+		return array_map('floatval', $this->conn->fetchAssoc('SELECT value, timestamp FROM data WHERE channel_id = ' . (int) $this->channel->getId() . parent::buildDateTimeFilterSQL($this->from, $this->to) . ' ORDER BY value ASC LIMIT 1'));
 	}
 
 	/**
-	 * @todo adapt to doctrine orm
-	 * @todo untested
+	 * Fetch the greatest value from the database
+	 * @internal doesn't fits the SQL standard
 	 * @return array (0 => timestamp, 1 => value)
 	 */
 	public function getMax() {
-		//return $this->dbh->query('SELECT value, timestamp FROM data WHERE channel_id = ' . (int) $this->id . self::buildFilterTime($this->from, $this->to) . ' ORDER BY value DESC', 1)->current();
+		return array_map('floatval', $this->conn->fetchAssoc('SELECT value, timestamp FROM data WHERE channel_id = ' . (int) $this->channel->getId() . parent::buildDateTimeFilterSQL($this->from, $this->to) . ' ORDER BY value DESC LIMIT 1'));
 	}
 
 	/**
-	 * @todo adapt to doctrine orm
-	 * @todo untested
+	 * Fetch the average value from the database
+	 * @internal doesn't fits the SQL standard
 	 * @return float
 	 */
 	public function getAverage() {
-		//return $this->dbh->query('SELECT AVG(value) AS value FROM data WHERE channel_id = ' . (int) $this->id . self::buildFilterTime($this->from, $this->to))->current();
+		return (float) $this->conn->fetchColumn('SELECT AVG(value) FROM data WHERE channel_id = ' . (int) $this->channel->getId() . parent::buildDateTimeFilterSQL($this->from, $this->to));
 	}
 
 	/**
-	 * @todo to be implemented
 	 * @todo possible and/or required?
 	 * @return float
 	 */
@@ -89,4 +88,4 @@ class SensorInterpreter extends Interpreter {
 	}
 }
 
-?>
\ No newline at end of file
+?>