Best way to read multiple temperatures?

[jmgelba]

Hi folks, I need to read at least 4 temperatures that will range from -30 deg F to + 300 deg F and one from -30 deg F to + 1800 deg F. I will use a thermocouple and amplifier for the higher temp sensor but which method is accurate and fast for the lower temperatures? Can I go digital here and have all sensors share an spi/I2C/etc port or should I go with 4 adc channels and NTC's? I think I prefer digital as I need to read to .1 deg F accuracy over the entire temperature range.
Also to note is that these sensors will be in a hostile environment with lots of vibration, dust, chemicals and obviously heat.

Thanks.

[falingtrea]

The only digital sensors I have seen are restricted to semiconductor temperature ranges (-40C to +125C) For the temp ranges you are working with you will have to use analog sensors with conditioning. The conditioning circuit can either have a digital or analog output to your application. I know that some semiconductor manufacturers have chips that take a sensor's direct input and can generate digital output data. They even have cold junction compensation built in.

[BH_epuk]

Hi, I've used the MAX6675 in the past with two connected to an spi bus, this has now been replaced with the MAX31855. Should have no problems getting the accuracy you require.

[Acetronics2]

1800 °F @ +/- .1° F ???
even 300°F @ +/-.1°F ....

Please ... try to be serious !!!

This never will be in the Hobbyist or semipro range ... so you'd better buy REAL thermometers and deal with their ASCII outputted result !!!

do not forget you'll have to spare somewhat before getting them ...

A "homemade" thing will give you @ best 1 or 2 degrees precision ( NOT resolution ...)

Alain ( Thermal Engineer ... )

[languer]

Some quick math:
For -30 deg F to + 300 deg F and 0.1 deg F you're talking 12-bit ADC (either external for analog sensor or internal to digital sensor).

For -30 deg F to + 1800 deg F and 0.1 deg F you're talking 16-bit ADC.

I'm with Alain on this; really? I'm not saying this is not doable; just pointing out the obvious. Hopefully we are already stating the obvious. If that's the case, then for the -30 to +1800: yeah thermocouples, amplifiers, probably of windows/ranges to get better resolution. For the -30 to +300: something similar to LM135 (probably break it out in ranges as well).

[retepsnikrep]

You could consider the Melexis MLX 90614 infra red sensor for the lower of your two ranges. -70 to +380c

I agree with the others who say you will really struggle with that accuracy over such a huge range.

Why such accuracy?

[jmgelba]

This is all automotive based for testing a piston in a prototype lean burn engine. All numbers are customer specs. Interestingly, the only thing on a engine that gets to 1800 is exhaust gases. I have no idea why they need - 30 to 1800 though.


24bit ADC as SPI slave and set to 8 bit. Output 4 words for each result?