It looks like the function you ar calling is taking a temperature, converting it and then passing it back i the same argument. This would indicate that the first argument is actually a pointer to a double instead of a double value. Try to modify your DLLImport statement to be as follows (note the added ref before the double):
[DllImport("OP930M.DLL")]
private static extern int OP930_GetTemperature(ref double tempC, int units);