【arduino】新人求教，用这个程序进行测电压值，误差一直比较大

// Code for dc voltage measurement by using a voltage divider circuit
// By deba168 dated 17/03/15
int temp=0;
float sum =0;
float VOLTS_SCALE =0;
float volt=0;
void setup()
{
Serial.begin(9600);
}
void loop()
{
for(int i = 0; i < 100; i++) // loop through reading raw adc values 100 number of times
{
temp=analogRead(A0); // read the input pin
sum += temp; // store sum for averaging
delay(2);
}
sum=sum/100; // divide sum by 100 to get average
// Calibration for Voltage
VOLTS_SCALE = 0.00488 ; // The voltage divider resistors are R1=100k and R2=20k // 5/1024 =0.00488
volt = VOLTS_SCALE * sum ;
Serial.print(volt);
Serial.println("V");
delay(500);
}