Bad Temperature Sender or Gauge?

My question is: Is the Temp Sender bad? Reads 490 Ohms at 93 degrees.

I have a teleflex temperature gauge that has started to bounce around. Thanks to Pete's earlier suggestion, I tested the Ohms at the temperature sender located near the water circulating pump.

Teleflex says to remove the tan wire from the top of the sender and connect an ohmmeter - one lead to the terminal of the sender and the other to the block. Approximate values are: 75 (room temperature) = 600 to 800 ohms; 212 degrees = 55 ohms.

Is this a linear relationship or is there a chart somewhere? Air was 93 degrees in the shade, engine was also 93 and my Ohm reading was 490.   A linear calculation shows a range of 530 to 705 ohms. Is 490 an indication of a bad sender?

Gauge read 160 with all power off. This makes me think the gauge is bad. I removed the tan wire from sender and cleaned its ring terminal. With key on and wire removed the gauge went to Zero showing infinite resistance. Next, grounded the ring terminal and watched the gauge spike to 240+.

I'll check voltage at the gauge next. It has 6 pin Deutsche connectors.. Guess I will separate the connector and check the lead in voltage for 10-16 VDC.

http://www.jamestowndistributors.com/userportal/document.do?docId=842&title=Troubleshooting+Teleflex+Water+Temperature+Gauges