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Temperature Sensors Forum
Temperature Sensors for monitoring ambient temperatures for humans
I want hook up a sensor with some "dummy lights"...green, yellow, orange, red. We have some clients/workers who work in changing temperature environments and it would be nice to have a small sensor they could clamp on to their gear to help remind them when it's getting too hot. We would want to monitor heat between 100F up to 750F i think. Accuracy is not important. e.g. We would want to associate a green light with 100F to 200F, Yellow light with 200F+ to 300F, Orange light with 300F+ to 500F, Red light with 500F+. We can encase/ build the device for our purposes with materials we already have. What recommendations do you have? Please let me know.
Sounds like an interesting project. Texas instruments does not make any temperature sensing integrated circuits that will operate over the 100 - 750F range that you are looking for. I would recommend choosing either an RTD or a thermocouple for the temperature sensing device. Then use one of TI's sensor signal conditioning ADCs to condition and digitize the signal so it can be processed to determine the correct temperature. Since it sounds like your device will be portable and battery-powered, I would recommend using one of TI's MSP430 ultra-low power processors to maximize battery life.
Please note that the temperature range for most semiconductors is -40C - 125C, so the processor, ADC, and battery pack would somehow have to be placed away from the extremely hot temperature so they do not fail due to the heat.
For an RTD circuit I would recommend using the ADS1246, ADS1247, or ADS1248 depending on the number of channels you need.
For a thermocouple I would recommend either using the ADS1246/7/8 or using the ADS1118.
Links to some useful reading are below:http://focus.ti.com/lit/an/sbaa180/sbaa180.pdfhttp://focus.ti.com/lit/an/sbaa134/sbaa134.pdf
Regards,Collin WellsPrecision Analog
Regards,Collin WellsPrecision Linear Applications
You might try something as simple as a glass-encapsulated thermistor sensor and a bank of window comparators. Glass-encapsulated thermistors are rated to +300C (572F) and will probably survive short-term exposures to somewhat higher than that. See http://www.ussensor.com/prod_DO-41_std.htm and http://www.ussensor.com/prod_DO-35_std.htm Since thermistor resistance goes down as temperature increases and you want to go to a high temperature, you probably will need to choose a high resistance, say 1M @ 25C. This material has a resistance-temperature curve "R" http://www.ussensor.com/rt%20charts/SM105R1K.htm As you can see, the resistance varies over a large range, from 1M @ 25C (77F) to 2.894k @ 220C (428F) and even lower as you go to 300C, which is off their chart.
Since you won't need speed but you do need a low input bias current (to prevent loading on the thermistor at low temperature) and low input offset voltage (to keep errors low at high temperature), I'd suggest using an op amp as a comparator; simply put a reference voltage on one of the inputs and sense the voltage developed across the thermistor by its bias voltage on the other input. Without any feedback, the op amp output wil switch from rail-to-rail just like a comparator-- but not nearly as fast. A couple of OPA2333s should do the job.
If you need to have a higher temperature range than 300C, you will need to go to a thermocouple or RTD, An RTD is easier to work with since you don't need any cold-junction compensation. http://www.ussensor.com/prod_rtds_thin_film.htm Thin-film platinum RTDs are OK to 500C (932F) but have a smaller resistance change than a thermistor. http://www.ussensor.com/rt%20charts/ppg102b2.htm Unlike a thermistor, an RTD temperature coefficient is positive, going from 1k @ 0C (32F) to 2.75463k @ 500C (932F). OPA2333s are also appropriate for RTD sensors.
If you strap the electronics to the workers body or arm, etc, you will create a stable, safe temperature for the batteries and amplifiers while only the sensor is exposed to a very high ambient temperature.
Regards, Neil P. Albaugh ex-Burr-Brown
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