Multiplexing a Thermocouple Input

Does anyone have a suggestion on what multiplexer I should use for 4 thermocouples into one MAX6675?

Thanks John

This guy uses a CD4052 Multiplexer to multiplex RTD sensors.

http://openenergymonitor.org/emon/node/75

I haven't done it myself, but plan to soon.

Relays are the safe bet.

Why would you use relays? Are you worried about the chip changing the reading of the thermocouple?

Read the MAX6675 data sheet very carefully. As I recall using relays in the thermocouple wiring can be a major problem.

Systems I have seen use separate Thermocouple to DC value chips and THEN multiplex the outputs.

I saw one example that used an ADG608 multiplexer but it is on the other side of the Earth so shipping would suck. Also I found an example on a Analog Devices data sheet that shows using an ADG609 multiplexer I'm going to use the latter and switch both T+ and T- to the MAX6675. That is the plan ATM.

http://www.oceancontrols.com.au/KTA-259.html

John

AFAIK any thermal junctions in the path between the TC and the amplifier are a problem, and that would include relays, MUXs etc. So I don't think it's ideal to MUX before the amp, but I don't know how critical it is.


Rob

None, unless you put the cold junction (and compensator) on the input side of the MUX. TCs really want a straight run of wire with constant composition from the measured junction to the cold junction. Anything else requires careful thought and is usually a bad idea.

BTB, you might want to consider RTDs unless you have some really specific requirement that demands TCs. I've seen them for TC-like prices on eBay (~$5 for a sheathed probe), and they're much easier to MUX in a reasonable fashion. Personally, I find them much easier to use.

BTB, you might want to consider RTDs unless you have some really specific requirement that demands TCs. I’ve seen them for TC-like prices on eBay (~$5 for a sheathed probe), and they’re much easier to MUX in a reasonable fashion. Personally, I find them much easier to use.

As well as much more accurate and less expensive to process on the electronics side.

Lefty

All you need is a constant current supply (i.e. a regulated voltage, a resistor, and an NPN transistor) and a differential amplifier... and then you are done.

rocketgeek: None, unless you put the cold junction (and compensator) on the input side of the MUX. TCs really want a straight run of wire with constant composition from the measured junction to the cold junction. Anything else requires careful thought and is usually a bad idea.

BTB, you might want to consider RTDs unless you have some really specific requirement that demands TCs. I've seen them for TC-like prices on eBay (~$5 for a sheathed probe), and they're much easier to MUX in a reasonable fashion. Personally, I find them much easier to use.

I didn't see any RTD's on fleabay for less than $20...

rocketgeek: All you need is a constant current supply (i.e. a regulated voltage, a resistor, and an NPN transistor) and a differential amplifier... and then you are done.

Do you have a link with a diagram on how you might hook this up and some code to read it?

Personally, I've not been able to get a thermistor to work so I'm trying thermocouples and that is my compelling reason... I figure if I keep bashing my head against this wall something will break.

Any suggestions are welcome.

Thanks for the reply John

I didn't see any RTD's on fleabay for less than $20...

Sureelectronics is a good seller and has the following showing on E-bay:

http://cgi.ebay.com/0-2-Class-Platinum-Resistance-Thermometers-PT100-/250776061715?pt=LH_DefaultDomain_0&hash=item3a636b0713

RTD are a simple but precision resistor that changes value per temp, just like a thermistor. So you can wired it to a fixed resistor and form a variable voltage divider and read it with a arduino analog input pin, just like you would with a thermistor. However there are IC RTD interfacing chips avalible that will process a rtd as part of a Wheatstone bridge circuit that can compensate for the length/added resistance of long lead wires on the rtd.

Lefty

Thanks for the link, that looks rather small compared to the dime. The type I need to use are all armor cabled and sheathed and some are pointy so you can jab them in the food.

This is the thermistor I tried to get working...

http://cgi.ebay.com/Maverick-ET73-ET72-Thermometer-3ft-1m-Replacement-Probe-/290526307877?pt=LH_DefaultDomain_0&hash=item43a4b7a625

Thanks John

[quote author=J T link=topic=52085.msg381048#msg381048 date=1298506432] Thanks for the link, that looks rather small compared to the dime. The type I need to use are all armor cabled and sheathed and some are pointy so you can jab them in the food.

This is the thermistor I tried to get working...

http://cgi.ebay.com/Maverick-ET73-ET72-Thermometer-3ft-1m-Replacement-Probe-/290526307877?pt=LH_DefaultDomain_0&hash=item43a4b7a625

Thanks John [/quote]

Hey John,

Your project so far sounds very similar to the one I'm building. I'm pretty sure that is the same probe as the ones I'm using in my project to track meat temperatures in a smoker. I bought them here: http://www.thermoworks.com/products/low_cost/oven_temp_timer.html

Although I strongly suspect they're being OEM'd via Amwei: http://www.amwei.com/views.asp?hw_id=61 (search for 'meat probe')

I've tried contacting Thermoworks/Amwei for more info, but haven't heard back. :(

Anyways, I have 3 probes and am going to see if one of them can be setup as an ambient temp sensor for the smoker itself. Anyways, the probes + LM34 communicate via an Arduino via 315Mhz RF to another Arduino which has an LCD display, SD card for data logging and buttons for setting alarms, etc. Honestly, the Arduino on the transmitter/probe end is a bit overkill, but was easier for me to design.

I'm pretty new to this whole thing (this is my first real electronics/Arduino project), but I'm learning fast and have a deep programming background. I'd be happy to work with you if there's some overlap in our projects. Probably the next step in my project is calculating the Steinhart-Hart constants for the probes so I can calculate the temperature via the Thermistor4 library.

Regards, Aaron

FYI, 99% sure the OP is trying to interface to thermistors, not thermocouples.

Original posting said:

Does anyone have a suggestion on what multiplexer I should use for 4 thermocouples into one MAX6675?

I worked with thermocouples for decades in a refinery. They are a real pain to interface with correctly and are not all that accurate to begin with. They were cheap and could be run many hundreds of feet in length, so if you needed to have hundreds of temperature measurement points in a chemical plant they were the preferred method of times bygone. Now they use mostly RTD sensors for new construction.

Unless one needs to utilize their high temperature capabilities (above 500f) then there are cheaper more accurate and easier to use temperature sensors avalible. RTD are nice and several IC temp sensors are good also. Thermistors can be inexpensive and useful, but calibrating can be a pain the ass.

Lefty

Do you have any links to RDT's that might be useful in a smoker project? 3-4' long braided cover pointy tips for meat, not so pointy for cabinet. As I have found out thermistors are a PIA to try and get right... thermocouples (the kind I need) are easy to get and the MAX6675 gives you a SPI output.

As always I'm open to sugesstions.

John

KE7GKP: What temperature range are you needing to measure? That is one of the most important specifications for selecting a sensor.

And don't just tell us cold to HOT. :D

If he’s smoking meats like I am, then 50-200F would be interesting. 50-400F would be great for also measuring the smoker temp. I too would love to find a cheap, accurate & food safe alternative to thermistors if available.

KE7GKP: http://cgi.ebay.com/ET-73-Maverick-ORIGINAL-Food-3ft-1m-Replacement-Probe-/290359161273 US$ 11.85

If you want to use thermocouples, then you must live with the limitations (the finicky wiring, and ice-point interface in particular). You will have to make the trade-off decision whether using cheap thermocouple probes is worth the higher overhead for your project.

I think that's the thermistor that JT said he tried. Actually reading his comments again, looks like he tried the above thermistor and thermocouples (hence the title of this post). Honestly, sorta surprised that I haven't been able to find the Steinhart-Hart coefficents listed anywhere for that part since it seems everyone points at it as the solution for tracking food temperature (pretty sure it's the same used by Thermoworks)... actually come to think of it, I own one of the Mavericks- I should test their resistance values to make sure.

Anyways, looks like I won't be finishing building my race bike this weekend afterall, so looks like I'll have time to figure out the S/H coefficents myself.