I see you mention temperature there. The Arduino is not very accurate as far as I can determine using an external analogue sensor . So far the best I can get after much discussion is an accuracy of about 2 degrees C . Thats about 20mV variation in readings returned.
There is an internal temperature sensor if the whole device is in the fridge for instance but it is not great either -this from the datasheet
23.8 Temperature Measurement
The temperature measurement is based on an on-chip temperature sensor that is coupled to a
single ended ADC8 channel. Selecting the ADC8 channel by writing the MUX3..0 bits in ADMUX
register to "1000" enables the temperature sensor. The internal 1.1V voltage reference must
also be selected for the ADC voltage reference source in the temperature sensor measurement.
When the temperature sensor is enabled, the ADC converter can be used in single conversion
mode to measure the voltage over the temperature sensor.
The measured voltage has a linear relationship to the temperature as described in Table 23-2.
The voltage sensitivity is approximately 1 mV/°C and the accuracy of the temperature measure-
ment is +/- 10°C.The values described in Table 23-2 are typical values. However, due to the process variation the
temperature sensor output voltage varies from one chip to another. To be capable of achieving
more accurate results the temperature measurement can be calibrated in the application soft-
ware. The software calibration requires that a calibration value is measured and stored in a
register or EEPROM for each chip, as a part of the production test. The software calibration can
be done utilizing the formula:
T = { [(ADCH << 8) | ADCL] - TOS} / k
where ADCn are the ADC data registers, k is a fixed coefficient and TOS is the temperature sen-
sor offset value determined and stored into EEPROM as a part of the production test.
+/- 10C is not too useful is it?
The effect of the environment on any electrical device can be countered by firstly getting it right, attaching heavy contact points and then submerging everything else in epoxy, clear or dark . Epoxy rather than casting resin as the latter is polyester which is permeable.
If its a device you sell for say $1000 then dark resin is the go to slow your competitors down