内部如果Arduino代码不能正常工作

Question

项目使用了土壤湿度传感器和超声波传感器。根据水分含量，它将决定是否浇水。同时使用超声波传感器，它将检查水箱的水位。如果水位不足，则无法启动电机。

int val; //stores the value received from Soil moisture sensor.
const int trigPin = 9; //pin for ultra sonic out
const int echoPin = 10; //pin for ultra sonic in
float duration, distance; //stores the values received from ultra sonic sensor.

void setup() {
  pinMode(13,OUTPUT); //Set pin 13 as OUTPUT pin, to send signal to relay
  pinMode(8,INPUT); //Set pin 8 as input pin, to receive data from Soil moisture sensor.
  pinMode(trigPin, OUTPUT); //Set pin 9 as OUTPUT pin, to send signal from ultra sonic sensor
  pinMode(echoPin, INPUT); //Set pin 10 as input pin, to receive data from ultra sonic sensor.
  Serial.begin(9600); //start serial monitor
}

void loop() { 
  val = digitalRead(8); //Read data from soil moisture sensor  
  Serial.print("Moisture: "); //print
  Serial.println(val); //print
  digitalWrite(trigPin, LOW); //set ultra sonic sensor to low
  delayMicroseconds(2); //delay
  digitalWrite(trigPin, HIGH); //set ultra sonic sensor to high
  delayMicroseconds(10); //delay
  digitalWrite(trigPin, LOW); //set ultra sonic sensor to low
  duration = pulseIn(echoPin, HIGH); //get the duration
  distance = (duration*0.0343)/2; //calculate the distance
  Serial.print("Distance: "); //print
  Serial.println(distance); //print
  if(val == LOW) {
    digitalWrite(13,LOW); //if soil moisture sensor provides LOW value send LOW value to relay
  } else {
    if(distance < 20.00){
        digitalWrite(13,LOW); //if distance is lesser than the limit, send LOW value to relay
    } else {
        digitalWrite(13,HIGH); //if distance is greater than the limit, send HIGH value to relay
    }
  }
  delay(1000); //Wait for few second and then continue the loop.
}

问题是，根据土壤湿度传感器，它能够正常工作。但是超声波传感器的内部if if不起作用。即使计算距离的值总是电机开启而不是水位。如果水土壤湿度传感器相关，水位大于20.00，否则也不起作用。