Danke für eure Tipps und Hinweise,

hier nun der überarbeitete Code:
Code:
import time
import RPi.GPIO as GPIO

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)

GPIO_TRIGGER = 14
GPIO_ECHO = 15

adjustment = 7

print "Ultraschallmessung mit SRF04"

while True:

        GPIO.setup(GPIO_TRIGGER,GPIO.OUT)
        GPIO.setup(GPIO_ECHO,GPIO.IN)

        GPIO.output(GPIO_TRIGGER, False)

        time.sleep(0.5)

        GPIO.output(GPIO_TRIGGER, True)
        time.sleep(0.00001)
        start = time.time()
        GPIO.output(GPIO_TRIGGER, False)

        while GPIO.input(GPIO_ECHO)==0:
          pass

        start = time.time()

        while GPIO.input(GPIO_ECHO)==1:
          pass

        stop = time.time()

        elapsed = stop-start
        distance = elapsed * 23000
        distance = distance - adjustment

        print "Abstand: %.1f cm" % distance
        time.sleep(1)

        GPIO.cleanup()