The Fresh Loaf

News & Information for Amateur Bakers and Artisan Bread Enthusiasts

Thermocouple question.

headdown's picture
headdown

Thermocouple question.

Hi everybody,

Next week I will pour the hearth slab of my Alan Scott oven, and I am trying to decide whether or not to install a thermocouple in a hearth brick. What I am having trouble with is envisioning how this is done with a flexible wire and a long run through the concrete and bricks. It would seem to me that it would be like trying to push a chain up a hill. Even if I insert a metal tube before I pour the hearth slab so I don’t have to drill a hole, how do I get a flexible wire to thread through about 8 inches of tubing through the concrete and brick?  And what heppens if/when in the future the thermocouple needs to be replaced?  Any insight appreciated. 

 

Dean

jbovenbread's picture
jbovenbread

Dean .... when I constructed my oven more than 10 years ago (a scaled down version of the Allan Scott design) I installed four thermocouples.  I simply placed the sensor tips in the locations where I wanted them  and essentially added the various layers of cladding until I got to the final outside layer.

Three of the thermocouples were set about 1/2 inch from the hot surface of the firebrick while one was set about 2 inches out in order to measure the temperature within the oven cladding.

At this point three of the four are still in regular use with the remaining one now requiring a new connector.  I'll get around to replacing it sometime 'real soon now' !!!.  Although the oven is currently out of the elements inside a combination greenhouse / oven shelter built a number of years after the fact the thermocouples survived several years of the spring, summer and fall weather, and being enclosed in a tarp to protect them from the winter here about 70 km south of Ottawa, ON  Canada.

I sourced the thermocouples and a relatively low cost reader from omega.ca based in Montreal QC ... although I believe they have a US address as well.

II attempted to upload a couple of photo's but no luck so far.

Something you may wish to consider ..... install a dial type thermometer (also available from omega.ca) during the build.  I did not and really wish I had done so.  Again, one of these days I'll bore a hole 'just about' through to the inside of the firebrick and place one where I can see the relationship between the thermocouple readings and the dial thermometer.  At the moment I use data I get from both the thermocouples and an inexpensive point and shoot IR thermometer that of course works best when there is not active fire in the oven.

I hope this helps.

 

 

deblacksmith's picture
deblacksmith

Assuming you leave a passage way to your thermal couple location you can always pull in a new one.  I would use a metal conduit as my passage way.  Now when you first put it in you can leave a string in it to pull your wire in - but this is not what is common in industry.  We "suck" the string in.  We tie a small piece of foam or a small roll of paper to the string and then suck it in with a vacuum clear.  Once you have the string in place you can pull in your wire.

Another way is to use a "fish"  this is a spool of flat spring wire that can be pushed through the conduit.  For short runs even a wire from a hanger can be used.

Good luck with the oven.  My son-in-law made one and he makes the best pizza I have ever had - and I had a lot of pizza in my life time. 

gavinc's picture
gavinc

When I build my brick oven back in 2005, I installed two K-type thermocouples, one in the dome and the other in the hearth. I drilled them in to within about 5mm of the surfaces.  I used them for several years until I purchased a non-contact infra-red laser gun that I use to measure the floor temperature and can find the hot and cooler areas of the oven.  I no longer use the thermocouples.  The size of the fame and the temperature of the hearth determines the cooking environment, but bread environment has no flame.

Brick Oven Environments for Cooking.

These notes are summarised from “The Art of Wood Fired Cooking” by Andrea Mugnaini.

No matter what cooking method you are planning to use, you must first bring the oven up to pizza temperature and then let it drop down to the appropriate environment. Once the oven is fully saturated with heat, each of the environments can be identified by the floor temperature and the size of the flame.

The only tool to use for measuring temperatures is the infrared thermometer.  The only place to check is the floor.

Pizza Oven Environment

Floor temperature 345-400°C (650-750°F) with a flame rolling to the middle of the dome.

Roasting Oven Environment

Floor temperature 290°C (550°F) with a vertical flame extending up the wall only.

When roasting meats or poultry, start at 315°C (600°F) and then let the temperature drop slightly, maintaining 290°C (550°F).

Bake Oven Environment

Floor temperature of 180-235°C (350-450°F) with hot coals but no live flames.

Grilling Environment

Live coals only. The only time the oven doesn’t need to be preheated.  Pull some live coals towards the archway.

Bread Oven Environment

Floor temperature of 235-290°C (450-550°F) empty oven.

 

Regulate the oven by closing the door. The oven will regulate to an even temperature throughout.   If you need to oven to cool down faster, open the door for 10 minutes at a time. Check the floor temperature only after the door has been closed a minimum of 10 minutes, as the heat will migrate down.

 

headdown's picture
headdown

Hi everyone,

I am still undecided as to whether or not to bother with installing any thermocouples.  If they can't be replaced fairly easily, they will not likely be worth the money and hassle to install.  I am 100 miles west of Ottawa myself, and if the thermocouples can't last for at least 10 years in our climate, then at the price they are asking it is not worth it to me.
Each passageway to the thermocouple will be a dead end..no using suction to pull the thermocouple in here.  A fish tape would get it in there alright, but how would you make it let go to withdraw the fishtape without unknowingly pulling it partly out of position, leading to innaccurate temperature readings?  

The main reason I would want a thermocouple would be to know how deep into the cladding that the oven was heat saturated.  This knowledge would let me, or a future bread baker better estimate the number of bread loads that could be done without refiring.  Other than that, I agree than an IR gun is likely sufficient. 

Dean

deblacksmith's picture
deblacksmith

While your thermocouple needs to be sealed off from the inside of the oven it doesn't have to be that way at installation.

You can seal that area with refractory cement after the couple is in place (which means you can use a pull string to get it there.)  Also if it goes bad on you can dig this area out and install a new couple.  We would install couples this way in the refractory brick in melting furnaces for aluminum.  Like you we installed these mostly to know the dry out history and establish a working understanding of a new design or refractory material.  Molten aluminum eats up melting furnaces refractory and so we spent a lot of money repairing and rebuilding furnaces.  Folk always wounder why we had a brick mason craft in an aluminum rolling mill - it was because they had a never ending job repairing and rebuilding furnaces.

As an aside - we had very complex heating schedules to dry out refractory on a new or rebuilt furnace.  Too fast and you would damage the refractory - too slow and you were wasting time and money.  They we learn how they did it in China.  No fancy program - they just built a small wood fire, follow by a larger wood fire, followed by a big wood fire - all done. Simple is sometimes way better.

deblacksmith's picture
deblacksmith

PS, in Canada it would be aluminium and not aluminum.  The rest of the world uses the correct spelling but we are stuck with aluminum because the word was misspelled on the first stationary printed for the Aluminum Company of America back in 1890 or so.

headdown's picture
headdown

Hi deblacksmith,

The depth of heating of the cladding is my main interest, since the IR gun gives a good indication of hearth temperature.  But it won't tell me anything about how deeply I have heated the walls and dome of the oven, where one or more thermocouples would.  I am pouring the hearth slab this weekend and will not bother making any provisions for a hearth thermocouple.  I will have a little while to decide on installing any in the cladding.  Once they go in though, digging them out to replace is not going to be something I would ever want to do. I would have to tear into the finished brick or stone work, the perlite concrete, and then the cladding. If I don't have a way of pulling the old thermocouple out and somehow pushing the new one in (through a stainless steel tube?) when replacement is needed, I won't bother with them.
I was surprised to read recently that Portland cement loses half of its strength at 750F, and it doesn't regain it when it is cool.  So I am using alumina cement for the suspended hearth slab and the cladding.  I would also like to have thermocouples just to see how long it takes to get the cladding to the point where if it were made of Portland cement that it would be losing strength. One day of firing?  Two? Three?  Inquiring minds want to know.

Dean

P.S. I fortunately have never heard a Canadian pronounce it aluminium.  I know I could not bring myself to do so. :) 

headdown's picture
headdown

Hi everyone,

Well I should be pouring the cladding on the oven in about a week, so I really have to make a decision about thermocouples.  I have decided to put two in the cladding, and none in the hearth.  The cladding where they will be installed will be about 4 inches thick.  One will be set one inch from the firebricks, and one three inches from the firebricks.  This should give me a good profile of heat saturation of the cladding.  I will use an IR gun for the hearth temperatures.
So my question now is - what exactly do I need?  I have decided to enclose the oven and use loose Perlite insulation over the cladding.  I will leave an access door that gives me access through the loose Perlite to the thermocouples if and when they need replaced.  My head is spinning trying to figure out which thermocouples, wires, and display I need. Some of the thermocouple wires I see online seem to have plastic plugs, which concerns me.  I have heard people talk about using a stainless steel tube and pouring the cladding around it, and then the thermocouple is inserted into the tube.  This would be ideal for me, but I have no idea how big a diameter of tube to get, or if this is bought with the thermocouples, or needs to be bought elsewhere.
I guess I need a shopping list.  I would like an analog dual dial display for the front of the oven, and I know I need K type thermocouples.  But other than that I am pretty confused.  Any advice appreciated.
I wonder if Omega is helpful with their customer support for something like this?

Dean

jbovenbread's picture
jbovenbread

Try contacting the Omega people.  I've found their customer support to be exceptional in the past.

I forget what sort of thermocouple I used in my oven but the cost of the thermocouple is minor (on the order of $15 C if I recall correctly).  You will have to put out about $100 for a device to read the temperatures however.

I suggest that you not get fancy ..... just lay the thermocouple tip where you want it and build the matrix of perlite / cement / cladding around the wires as you build it out.

You also may wish to get a dial thermometer from Omega for your oven and install it during the build.  I did not do this and wish I had.