These are nice small cheap devices for making liquid level sensors. It involves using 2 low resistance resistors for generating heat and a thermistor for measuring the temperature. Its they are pretty cheap to make its is an involved process. This recipe explains both how to make them and a way to control them without burning them out buy using a mosfet controller and temperature sensor circuit.

#include <Servo.h>
Servo myservo;  // create servo object to control a servo

#include <Wire.h>
#include <SoftwareSerial.h>


#define THERMISTOR_PIN A0

int valveservo = 11;

int washpin = 10; 
int drypin = 9; 
int pcvpin = 6; 
int heatpin = 5; 
int turnon5vpin = 5;
int tempsensor = A0;
//int levelsensor = A1;
int washval = 255;
int dryval = 255;
int pcvval = 255;

String command;
float currpos;
int fillflag = 1;
int levelval = 50;
int heatval = 50;
int htcnt = 0;
int pumpdelayct = 0;
int pumpdelay = 0;
int pumponflag = 0;
int levelstreamon = 0;


#define NUMTEMPS 20
short temptable[NUMTEMPS][2] = {
   {1, 841},
   {54, 255},
   {107, 209},
   {160, 184},
   {213, 166},
   {266, 153},
   {319, 142},
   {372, 132},
   {425, 124},
   {478, 116},
   {531, 108},
   {584, 101},
   {637, 93},
   {690, 86},
   {743, 78},
   {796, 70},
   {849, 61},
   {902, 50},
   {955, 34},
   {1008, 3}
};


void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  myservo.attach(valveservo);
  analogWrite(pcvpin, 0); 
  analogWrite(washpin,0);
  analogWrite(drypin, 0);
  analogWrite(heatpin, 0);
  analogWrite(turnon5vpin, 0);
  currpos = 0;
}

void loop() {
   int rawvalue = analogRead(THERMISTOR_PIN);
   int celsius = read_temp();
   int fahrenheit = (((celsius * 9) / 5) + 32);
  // put your main code here, to run repeatedly:
   if ((celsius > levelval) and (fillflag == 0)){
    htcnt = htcnt + 1;
    if (htcnt > 10) {
     analogWrite(pcvpin, 255); 
     pumponflag = 1;
     delay(pumpdelay); 
     pumpdelayct = 0;
   }
  } 
  else if ((celsius < (levelval-5) and (pumponflag == 1) and (fillflag == 0))) {
    if (pumpdelayct == pumpdelay){
     analogWrite(pcvpin, 0);
     pumponflag = 0;
    }
    pumpdelayct = pumpdelayct + 1;
  }
   else if ((analogRead(tempsensor) > levelval)){
   htcnt = 0;
  }
  if (levelstreamon == 1) {
   Serial.println(celsius);
   delay(500);
  }

 if(Serial.available())
 {
    char c = Serial.read();
    if (c== '\n')
    {
      currpos = parseCommand(command, currpos);
      command = "";
    }
    else 
    {
      command +=c;
    }
 }
 delay(30);
}

float parseCommand(String com, int currpos)
{

  if(com.equalsIgnoreCase("washon")){
    analogWrite(washpin, washval);
    delay(100);
  }  
  else if(com.equalsIgnoreCase("washoff")){
    analogWrite(washpin, 0);
    delay(100);
  }  
  else if(com.equalsIgnoreCase("dryon")){
    analogWrite(drypin, dryval);
    delay(100);
  }  
  else if(com.equalsIgnoreCase("dryoff")){
    analogWrite(drypin, 0);
    delay(100);
  } 
  else if(com.equalsIgnoreCase("info")){
    Serial.println("wash_dry_pcv_electrocaloric_kill_stepper_valve");
  }
  else if(com.equalsIgnoreCase("turnon5v")){
    analogWrite(turnon5vpin, 255);
  } 
  else if(com.equalsIgnoreCase("turnoff5v")){
    analogWrite(turnon5vpin, 0);
  }

  else if(com.equalsIgnoreCase("manpcv")){
    fillflag = 1;
    analogWrite(pcvpin, 0);
  }
  else if(com.equalsIgnoreCase("feedbackpcv")){
    fillflag = 0;
  }  
  else if(com.equalsIgnoreCase("pcvon")){
    if (fillflag == 1){
     analogWrite(pcvpin, pcvval);
    }
  }  
  else if(com.equalsIgnoreCase("pcvoff")){
    if (fillflag == 1){
     analogWrite(pcvpin, 0);
    }
  }

  else if(com.equalsIgnoreCase("levelstreamon")){
   levelstreamon = 1;
  } 
  else if(com.equalsIgnoreCase("levelstreamoff")){
   levelstreamon = 0;
  } 
  else if(com.equalsIgnoreCase("heatoff")){
   analogWrite(heatpin, 0);
  } 
  else if(com.equalsIgnoreCase("heaton")){
   analogWrite(heatpin, heatval);
  } 
  else if(com.equalsIgnoreCase("readlevel")){
     int rawvalue = analogRead(THERMISTOR_PIN);
     int celsius = read_temp();
     Serial.println(celsius);
  }
  
   else if (com.substring(0,10) == "setwashval") {
    washval = com.substring(11).toInt();
  }
   else if (com.substring(0,9) == "setdryval") {
    dryval = com.substring(10).toInt();
  }
   else if (com.substring(0,9) == "setpcvval") {
    pcvval = com.substring(10).toInt();
  }
   else if (com.substring(0,11) == "setlevelval") {
    levelval = com.substring(11).toInt();
  }
   else if (com.substring(0,7) == "heatval") {
    heatval = com.substring(7).toInt();
  }
  
  else if (com.substring(0,10) == "valveservo") {
    myservo.write(com.substring(com.indexOf("valveservo")+11).toInt());
  }
   return currpos;
}

int read_temp()
{
   int rawtemp = analogRead(THERMISTOR_PIN);
   int current_celsius = 0;

   byte i;
   for (i=1; i<NUMTEMPS; i++)
   {
      if (temptable[i][0] > rawtemp)
      {
         int realtemp  = temptable[i-1][1] + (rawtemp - temptable[i-1][0]) * (temptable[i][1] - temptable[i-1][1]) / (temptable[i][0] - temptable[i-1][0]);

         if (realtemp > 255)
            realtemp = 255; 

         current_celsius = realtemp;

         break;
      }
   }

   // Overflow: We just clamp to 0 degrees celsius
   if (i == NUMTEMPS)
   current_celsius = 0;

   return current_celsius;
}