Is this a K Type sensor?

[jessey]

Hello everyone,

A friend of mine just bought a used Crucible Furnace for melting metals. http://www.johnsongas.com/industrial/frn-crucible.asp The package deal came with a temperature probe connected to an analog meter to display the temperature of the metal being melted. The meter was left outside for some time and exposed to the elements and needless to say it doesn't work. I disconnected the wires from the probe (picture of the probe & meter are attached below) and connected them to a volt meter, when the sensor is cold the meter reads 0 volts and when heated up with a propane torch it produces a voltage in the milli volt range that continues to rise as the heat is applied to the sensor.

I was thinking that the sensor is a "K" type sensor (but not certain) and was wondering if anyone here could confirm that this is a "K" type sensor and possibly offer any advice as to how to approach writing code and interfacing the temperature sensor for this project. I'm assuming that I'd be using an ADC input to measure the voltage and do some math to print the temperature to my LCD. Any help in pointing me in the right direction would be greatly appreciated.

Thanks
jessey

[Dave]

jessey, If the wire colors are yellow and red for the sensor then it is a K type thermocouple. If the wire colors are white and red then the type is J. If the colors are blue and red then the type is T. K is good for over a thousand degrees F. and is made of Cromel/Alumel. Type J has a greater output voltage but a lower range and is made of iron/constantan. Type T is copper/constantan and is good for lower ranges and has a greater output voltage. Lookup OMEGA Temperature for more information...

Dave Purola,
N8NTA

[Acetronics2]

Hi, Jessey

I'm assuming that I'd be using an ADC input to measure the voltage and do some math to print the temperature to my LCD. Any help in pointing me in the right direction would be greatly appreciated.

I think you should first have a look to MAX 6675 or AD 595 chips ...

the Pic ADC's are way too weak to measure those sensors outputs ...

Alain

[Byte_Butcher]

Ditto on what Dave said about the wire colors. They will tell you what type of probe you have. Red/Yellow is type K.

The output from a type K probe is very low and you won't be able to use a PIC ADC to measure it directly.

I'm working on a project now that uses the MAX6675 sensor with a type K probe and it works very well and is super easy to read the MAX6675 and display to an LCD.


Steve

[Byte_Butcher]

I've never used the AD595, but from a quick look at the data sheet it appears that the AD595 is good for measuring up to 1250C (2285F), but the MAX675 is only good up to 1024C (1875F), so depending on what your temperature requirements are, the MAX6675 *might* not read high enough for you...

[jessey]

Thanks Everyone,

I just checked and the wires coming off the sensor are red and yellow confirming that it is indeed a "K" type sensor. The maximum temperature that we need to check for is approximately 1832 degrees Fahrenheit which is the melting point of Bronze. Thanks for the information and I'll be looking to purchase a MAX675 voltage reference chip as soon as possible. My friend was thinking to have me build the circuit with an LCD in a small box and mount it just ahead of the handle on the sensor and have it operate on batteries which sounds like a good idea to me. I'm assuming that the red and yellow wires coming off the sensor are Cromel/Alumel as Dave said and that it's not a good idea to put any other type of wires in-between the sensor and the micro, I seem to remember someone telling me that years ago. Sounds like an interesting project. So when I get the MAX675 chip I'll read the data sheet and connect it as per instructions to an ADC input and see what numbers I get displaying on my LCD. When using a K type sensor is there a set number of milli amps rise that would equal a one degree rise in temperature for either Celsius or Fahrenheit? I can't wait to fire up the furnace and start melting some aluminum. Is anyone else here into melting metals in a furnace and casting their own parts?

Thanks Again
jessey

[b1arrk5]

Jessey,

Take a look at STEVECHASTAIN.COM I have his book on building a small foundry, and highly recommend it. You might also take a look at Lindsay Books, they reproduce a lot of older technical manuals, and they carry the Dave and Vince Gingery books. I have the Gingery book on building a charcoal foundry, and they are easy to read, and inexpensive. Both books will give you ideas as to where to find materials and supplies for melting metal.

Jerry.

[jessey]

Jessey,

Take a look at STEVECHASTAIN.COM I have his book on building a small foundry, and highly recommend it. Jerry.

Hi Jerry,

Yes my friend did order some plans for a furnace then he found an advertisment on craigslist for a furnace and got a smoking deal on the one he bought. It's a manufactured unit made by Johnson Gas Appliance Company that runs on natural gas. Neither one of us has ever done any castings before so it's going to be very interesting to say the least. Have you been using your furnace much?

Thanks for the clarification on the MAX6675 Steve and the code to get it up and running, that's cool. I phoned Digi-Key and they have a 5 week waiting period for a back order for the MAX6675. I will still get some of the MAX6675's but I think I'll phone Digi-Key tomorrow and see if I can get a couple of the AD595's especially if I can get some without the long wait, as I can't wait to get this project up and running. I'll let you know how I make out.

Thanks
jessey

[b1arrk5]

Jessey,
I haven't done nearly as much casting as I would like, but I've had fun with aluminum. A friend rebuilds diesel engines, and saves me the bad pistons. The aluminum used in the pistons contains alloys that make it machine nicely, and I have a small lathe that doesn't get used as much as it should either! Got to save something to do for retirement right?

Anyway, if you go to Maxim's website they will let you order samples of the Max6675. You might be limited in the packages that they offer in samples. You can order parts direct from them too, but the shipping would cost more than one or two parts.

Jerry.

[Byte_Butcher]

I think with the MAX6675 you're pretty much limited in package selection anyway... 8 pin SOIC is all that's shown in the data sheet.

I don't know why DigiKey said they have a 5 week back order on the MAX6675. According to their website they have several 1000 in stock...
http://search.digikey.com/scripts/Dk...75ISA%2BTCT-ND

[Byte_Butcher]

The MAX6675 doesn't need a ADC to interface with a PIC.
You just shiftin the serial data and it reads pretty much directly in degrees C on an LCD without too much else.

This is pretty much all the code involved...

'Alias pins - MAX6675 temp sensor
MXSCLK  var     PORTB.5   'Clock
MXCS     var     PORTB.4    'Chip Select
MXSO     Var     PORTB.3   'Serial out


 'Allocate MAX6875 Variables
MXTemp   var     word    'raw data from 6675/type K sensor
TempC   var     word    'converted to degrees C


'-----------Read and display temperature from MAX6675-----------
MXCS = 0  'Chip select low
shiftin MXSO, MXSCLK, 0, [MXTemp\16]   'read the data to MXTemp
MXCS = 1    'Chip select high
TempC = MXtemp >> 5   'right shift the data 5 places to get degrees C (Read the Data sheet)
LCDOUT $fe,2,"Temp = ", DEC TempC, "degrees C "  ' Spit it out on an LCD

Works for me...

Steve

[Byte_Butcher]

Thanks for the information and I'll be looking to purchase a MAX675 voltage reference chip as soon as possible.

I just reread this part....

What you want is a MAX6675, which is designed to read a type K probe, NOT a MAX675, which is a precision voltage reference.

Totally different animals!

[Sam]

Is anyone else here into melting metals in a furnace and casting their own parts?

I'm just getting started and have built a spincaster machine and an electric radiant furnace. The furnace can melt chunks of 6106-T6 aluminum in about 40 minutes ! Although my main purpose will be for Zamack-27 zinc aluminum alloy, I have the first ingot on the way.

I was thinking of ordering a pyrometer with a K sensor off ebay but since you started this thread, I'm thinking I'll build one too. Please keep us informed on your progress.

Here's a link to a thread on my spincaster build.

[Archangel]

Hi jessey,
sorry this is so late, I knew I had it but couldn't find it.
http://srdata.nist.gov/its90/download/type_k.tab

[jessey]

The MAX6675 doesn't need a ADC to interface with a PIC.
You just shiftin the serial data and it reads pretty much directly in degrees C on an LCD without too much else.

This is pretty much all the code involved...

'Alias pins - MAX6675 temp sensor
MXSCLK  var     PORTB.5   'Clock
MXCS     var     PORTB.4    'Chip Select
MXSO     Var     PORTB.3   'Serial out


 'Allocate MAX6875 Variables
MXTemp   var     word    'raw data from 6675/type K sensor
TempC   var     word    'converted to degrees C


'-----------Read and display temperature from MAX6675-----------
MXCS = 0  'Chip select low
shiftin MXSO, MXSCLK, 0, [MXTemp\16]   'read the data to MXTemp
MXCS = 1    'Chip select high
TempC = MXtemp >> 5   'right shift the data 5 places to get degrees C (Read the Data sheet)
LCDOUT $fe,2,"Temp = ", DEC TempC, "degrees C "  ' Spit it out on an LCD

Works for me...

Steve

Hi Steve,

I finally received 2 of each MAX6675's and AD595's and thought I'd use one of the 6675's first to try out your code. I found this article: http://emesystems.com/OL2therm.htm that shows a schematics for the 6675 and I have a couple of questions for you.

This article shows 200 ohm resistors on pins 5, 6 and 7 to the microprocessor for input protection which sounds like a good idea but are not shown in the data sheet. Do you think they could cause any problems in reading the higher temperatures?

Also in the same article the author dismisses grounding pin 2 as shown in the datasheet and I was wondering what you think of that?

Now that I have my pickit2 programmer working good on my laptop, I'll take a breadboard to my friends place next week sometime to try your code. Once again thanks for posting your code.

Thanks
jessey

[Byte_Butcher]

Jessey,
I didn't use any series resistors on my MAX6675, but it's probably a good idea to use them. It shouldn't cause any accuracy problems... it's all serial data, not analog signals.

I grounded pin 2 on mine. If it's grounded, then the MAX6675 can tell when there's an open probe. (type K probes almost always fail "open") The data sheet explains it.

Have fun!


Steve