TMP36 fine with USB, bad value with battery

Hello,

I'm fairly new to the Arduino world. I went through the starter kit and had success with all of the experiments, so I decided to combine two. I wanted to read the TMP36 temperature sensor with my UNO, and output temperatures in Fareneheit to the LCD display. It was pretty straightforward and I had success with it, showing around 74 degrees F.

Then I tried to check the temperature outside. When I powered my Arduino using a 9v battery to the Vin pin, 5v power supply to Vin pin, or 5v wall wart, the display showed 105-130. I read that the temperature sensor needs its own ground so I have a wire going to one of the Arduino grounds on the other side of the board from the LCD's ground.

Any ideas on why this would happen? I went over my code for about an hour, the whole time plugged in to USB and the temperature stayed between 72 and 74.

voltage = (sensorVal/1024.0)*5.0;
Serial.println(voltage);
float temperature = (voltage-0.5)*100; //new variable Temperature which is equivalent to the (voltage-0.5)100
float Fah = (temperature
9/5)+32;
lcd.setCursor(0, 1);
lcd.println(Fah, 2);
lcd.print("Deg F");

A 9volt battery can barely power an Arduino, and not for long.
Forget about powering an Arduino + LCD/backlight from a 9volt battery.
Leo..

I had some similar issues that ended up being intermittent breadboard connections. It doesn't like mechanical instability. I had about 3' of lead wire. I braided that and added a tiny random ceramic disc capacitor across the power leads, all of which seemed to improve stability. I dragged the laptop into the kitchen so I could dunk it it boiling water. As long as I kept the device leads out of the water it read properly 100 C and 212ish F.