Python Script for Raspberry Pi + HC-SR04 sensor to measure water level in a sump pit

range_sensor.py

#!/usr/bin/python

# Measure distance to water level in sump pit using
# HC-SR04 ultrasonic ranging module
# Sensor info:
#	2cm - 4m. range with ~3mm. accuracy, 15 degree measuring angle
#	5VDC, 15mA working current
# Suggestion in spec to connect ground before 5VDC line when hot plugging
# Measure surface area no less than 0.5 m^2 (really?)

# range_sensor.py
# Prints the current date/time and measurement in cm. as CSV
# Calls another process if reading falls below threshold

import RPi.GPIO as GPIO
import time
import datetime
import os
import subprocess

# Minimum distance to water level (cm) / threshold
MIN_DISTANCE = 24.5

# Calculate median of a list
def median(lst):
	lst = sorted(lst)
	if len(lst) < 1:
		return None
	if len(lst) %2 == 1:
		return lst[((len(lst)+1)/2)-1]
	else:
		return float(sum(lst[(len(lst)/2)-1:(len(lst)/2)+1]))/2.0

# Sensor Pinout
TRIG = 23
ECHO = 24

# Set mode and data direction
GPIO.setmode(GPIO.BCM)
GPIO.setup(TRIG, GPIO.OUT)
GPIO.setup(ECHO, GPIO.IN)

# Speed of sound constant
# 343 m / sec. is rougly the speed of sound in dry air at 20C
# The speed decreases with temperature and increases a bit with more humidity
# This is a cool, damp basement, so we'll assume 341 m / sec. (15C / 59F with 60%RH)
# Sensor measures round-trip time, so we divide by 2 to get half of that distance
SPEED = 17050

distances = []
sampleCount = 0
while sampleCount < 30:
	# Distance Measurement In Progress

	# Limit sample rate to every 10ms (even though spec recommends 60ms)
	time.sleep(0.01)

	# Send 10uS pulse
	GPIO.output(TRIG, True)
	time.sleep(0.00001)
	GPIO.output(TRIG, False)

	# Waiting for ECHO to go high...
	# Max. wait 1 second using pulse_end as temp variable
	pulse_start = time.time()
	pulse_end = time.time()
	while GPIO.input(ECHO)==0 and pulse_start - pulse_end < 1:
		pulse_start = time.time()

	# Waiting for ECHO to go low... (max. wait 1 second)
	pulse_end = time.time()
	while GPIO.input(ECHO)==1 and pulse_end - pulse_start < 1:
		pulse_end = time.time()

	# Append distance measurement to samples array
	distances.append((pulse_end - pulse_start) * SPEED)
	sampleCount = sampleCount + 1

# Cleanup (set GPIOs to input, etc.)
GPIO.cleanup()
# Print date/time and median measurment in cm.
median = round(median(distances), 1)
print datetime.datetime.now().strftime("%Y-%m-%d %H:%M") + "," + str(median)
# Notify of problem if water level is high
if median < MIN_DISTANCE:
	subprocess.call(["node", os.path.dirname(os.path.realpath(__file__)) + "/email_warning.js", str(median)])

@Mirankumer - I'm confused by your question. This script already measures the amount of time that passes between HC-SR04 pulses. Thus, this script will print the median distance measurement taken in centimeters.

HC-SR04 sending data from 0 to x meters. I need to have at some point zero. Wich is normal level of water. Level of water going down because a sun. So, that is negativ level called MINUS (-1m,
-1,2m............ Everiting above zero ccalled PLUS, +0.4m, +0.5m, +0.6m,.....and so on.
+01m
+0.2m
+0.3m
+0.4m
+0.5m
0
-0.1m
-0.2m
-0.3m
-0.4m
-0.5m

Thanks

Sorry, @Mirankumer. I'm having trouble understanding your question. The output of this program will always be positive, as it measures the distance from the HC-SR04 sensor to the nearest object.