GUI Tkinter: start button is working, stop button is not working

I was trying to use the "Start" and "Stop" button in GUI using Tkinter. I am using Raspberry Pi 3 and python 2.7 version, start button is working properly, stop button is not working. Someone, please suggest some solution. Thanks Mehnaz

from __future__ import print_function, division
import os, sys, inspect, platform
import time
import serial # import Serial Library
import numpy  # Import numpy
import csv    # import csv module
import os
import scipy
#import Figtodat
#from images2gif import writeGif
#matplotlib.use('Agg')
import matplotlib.pyplot as plt #import matplotlib library
from drawnow import *
from collections import OrderedDict
#from detect_peaks import detect_peaks
from Tkinter import *

os.getcwd()

ruing = True


def Data_Acquisition():

    def scaing():
        if ruing:
                    ser = serial.Serial('/dev/ttyUSB1',9600, 8, 'N', 1, timeout = 1)
        # Open ve_direct.csv
        file_data = open(Patient_Name+'.csv', 'w')
        print "Reading data and writing to ve_data.csv"
        # Listen for the input, exit if nothing received in timeout period
        output = " "
        while True:
        output = " "
        while True:
          while output != "":
            output = ser.readline()
            file_data.write(output)
            print output,
          output = " "
        # close file
        print ("Stopped writing to ve_data.csv")
        file_data.close()

    def start():
        global ruing
        ruing = True


    def stop():
        global ruing
        ruing = False


    root = Tk()
    root.title('Elbonix')
    root.geometry("800x480")

    app = Frame(root)
    app.grid()

    start = Button(app,text="Start Scan", command=scaing)
    stop = Button(root,text="Stop", command=stop)

    start.grid()
    stop.grid()

    root.mainloop()
    ser.close()
    file_data.close()
    root.mainloop()

def Enter_Patient_Name():
#print("First Name: %s" % e1.get())

    master = Tk()
    master.title('Elbonix')
    Label(master, text="Enter Patient's Name" ,font=("Helvetica", 10, "bold italic"), fg="blue").grid(row=0)
    Label(master, text="Ex- LastnameFirstnameDate", font =("Helvetica", 10, "bold italic"), fg="dark grey").grid(row=1)

    e1 = Entry(master)

    e1.grid(row=2, column=0)

    def Continue_scaing():
        global Patient_Name
        Patient_Name = e1.get()
        Data_Acquisition()

    Button(master, text='Continue', font=("Helvetica", 10, "bold italic"), fg="white",bg="lime green",command=Continue_scaing).grid(row=3, column=0, sticky=W, pady=4)
    Button(master, text='Go back',font=("Helvetica", 10, "bold italic"), fg="white", bg='red' ,command=homeblock).grid(row=3, column=1, sticky=W, pady=4)
    print(e1.get())
    master.mainloop()