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Can you see how is the temperature & the change inside a chamfer with your temperature controller?

The heat chamber, a closed environment with heat element inside, it is used to convert the electrical power into heat, a means to elevates the temperature within the chamber. More or less, these heat chambers may be used for MOCVD or similar process in semiconductor or LCD manufacturing or some others, one of your kitchen oven.

To control the temperature as precise as possible for the heat chamber, it aims to yield a more reasonable temperature consistency, a temperature controller could be equipped with control algorithm as PI, PD or PID in more advance model. Except your controller brand name, how well do you know about your chamber and the temperature inside? and how do you see if any overshoot of temperature from the whole system ?

A well known technique, the thermometer could be used to probe the chamber for temperature, and notepad & technician could be helpful to record the reading from time to time. The log chart can then be plotted later on to visual the temperature change against the time domain, or a mechanical log chart recorder is always provided, this is what a traditional experimental approach. Now a day, the mid of 90's, computer is more powerful for daily computational task, and some advance model of heat chamber has been built-in electronics temperature logger for daily operation. This is a given of another interesting question, what to do if you had your stone age heat chamber ? throw it away then buy a modern new one, or something can be done for those old dogs ? 

This project represent low cost approach, more or less, an experimental task to equip PC based temperature logger for your stone age oven in your kitchen, a bread baker, or the industry oven in the manufacturing plant. A simple block diagram is completely draft the idea of design.

Note :

1) Thermal sensing element, K-type thermal couple or P-N junction of an ordinary Transistor or Diode, available widely;

2) Conditioning circuitry, special design of Integrated Circuit (IC) or discrete component circuit, convert thermal current to temperature data;

3) Data Link, industry standard for data communication, RS485 or RS422 or RF Transcevier, send / receive temperature data to the host;

4) Host, computer (PC) used for recording and display;

A diode used as thermal sensing element, see the reference for further discussion, according to it says, a Diode for temperature measurement can be used up to 150*C or 200*C momentary, but we did not have any experimental data to confirm this,

Transistor or Diode to be used as temperature probe
http://schematics.blogspot.com/2006/10/diode-thermometer.html

http://hbd.org/mtippin/thermometer.html

 

description of what is a current mirror by bi-polar transistor,
http://www.allaboutcircuits.com/vol_3/chpt_4/12.html

 

Simple lab experiment of a Diode is used for temperature sensing,
http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p033.shtml

 

CPU temperature monitoring with Diode and DMM
http://www.overclockers.com/tips12/

 

PC com port (RS232), control charing time with NTC to measure temperature
http://www.electronics-lab.com/projects/pc/013/

 

why is it possible to measure temperature with a Diode
http://www.hobby-elec.org/e_ckt26_3.htm

 

simple and cheap thermometer by uses of transistor or diode
http://www.micro-examples.com/public/microex-navig/doc/098-temperature-sensor

 

sensor design with thermocouple or diode
http://extenv.jpl.nasa.gov/presentations/Sensor_Design_and_Operation.pdf

 

 

K-type thermocouple used as thermal sensing element,

K-type thermocouples (chomel-alumel) are used mostly, because they are useful up to over 1200 degrees Celsius. It is also much more linear in its response than T-type with respect to temperature. This is a well known element for industry purpose of any temperature measurement. They are facts where thermocouples are possible the best choice as temperature sensors, in terms of cost-effective. The output is standardized, determined only by the bi-metal junction that form the thermocouple, no effect of the size or shape of the junction, no change in calibration, the metals can be formed into a variety of sizes or shapes suited to different applications.

In biology, thermocouples are used to construct thin & tiny probes with improved response time by lower thermal mass or thermal inertia. It is also true any other measurement event. Low thermal inertia is necessary to gain shorter response time. Rapid response is always expected as in system dynamic to the temperature, pressure or force. T-type thermocouples are often used for biological studies, because the ease of construction, and resistance to oxidation by aqueous environments at temperatures typical in biology.  of interest in biological work. Range from -200°C to +350°C for measurement. Out of all the thermocouple types, T-type has the lowest "limit of error", which means that the level of purity attainable in the metals allows them to adhere to the standard type T curve most closely. J-type (iron constantan) thermocouple is also used for its low cost, but the iron is problematic in oxidizing situations.

Consider other unique properties of thermocouples, such as high temperature performance, it is only importance in industrial applications, as well as the ordinary appliances such as the kitchen oven. There are many more types of thermocouples useful for special purposes and up to much higher temperatures. The OMEGA Engineering site has a great technical reference if you are eager to see profound technical detail of thermocouples, http://www.omega.com/temperature/z/pdf/z021-032.pdf

Anyway, the difficulty in using thermocouples is another consideration,

a) the thermal voltage signal produced by theromocouple is so small and has to be amplified, 

b) the thermal voltage signal is produced to a relative of a reference temperature or cold-junction temperature which must be known or measured independently

 

MAX6674 or MAX6675, thermocouple conditioner

Maxim (http://www.maxim-ic.com) has manufactured couple IC chips that provide a bridge for digital interface & K-type thermocouple. The chip is available as an 8 pin SOIC, and cost about $7.8 each in small quantities. According to Maxim,

MAX6674, it covers the range from 0 to 128 degrees Celsius, 10 bit resolution (0.125 degree)

MAX6675, it covers a range from 0 to 1024 degrees Celsius, 12 bit resolution (0.25 degree)

These chips make the interface to a thermocouple never than easy before. Simply attach the thermocouple, read the digital output by SPI interface. From the pass, signal produced by thermocouples require sophisticated analog circuitry and adjustment. A high gain amplifier boots microvolt up to a certain level, and cold-junction compensation is a must, With this IC chips, it measures its own temperature and subtracts the appropriate offset internally, included the necessary voltage references, and auto-zeroing, to offsets the temperature effect for amplifier itself. The last stuff, there is a 10 or 12 bit analog to digital converter and SPI serial interface.

These MAX6675 is designed to work with the popular K-type thermocouple. It is made of a nickel-chromium / nickel-aluminum junction, called Chromel / Alumel, the other end, + in yellow shield & - in red shield, produce 12.2 millivolts at 300°C, sensitivity is about  41uV/°C. The AMX6675 is possible be useful with the T-type (copper-constantan), because the two types have very similar sensitivities around room temperature, where the cold junction compensation is concerned. The basic accuracy for type K is no greater than 3 degrees Celsius. The IC has no any linearity correction, but software interpolation can be done additional corrections in your own software. Namely, the thermocouple itself generates a microvolt that is very subject to noise pickup by a variety of error sources, the human body induced AC hum, or inductive loading emitted strong electric field. One must aware to the apply engineering practices for use of thermocouples, including the use of shielding, thermo wells, and appropriate grounding or insulation of the thermocouple tip. The MAXIM data sheet gave recommendations, as any standard references on thermocouples application.

You can also make K-type thermocouple yourself, buy thermocouple wire, NOT thermocouple extension wire. They will know what you are asking for if Chromel wire or Alumel wire. Argon inert gas and the arc welding (Gas tungsten arc welding, http://en.wikipedia.org/wiki/Gas_tungsten_arc_welding or http://tw.myblog.yahoo.com/md8168/article?mid=38&prev=776&next=9&l=f&fid=5) will bring very good result to make the thermocouple junction for you to form tiny dot, see the picture below for this junction. If you want longer thermocouple, uses the K-type thermocouple extension wires and connector & plug, do not use copper wire, because the copper wire will create another juncton to form another theromocouple, it affect the measurement.

The photo below shows the MAX6675 soldered with DIY PCB (just a thicker card paper) other than any FR-4 etched PCB for electronics, and some copper wires striped off from salvaged LAN cable, a quick & cheap solution with a immediately available platform for evaluation.

The circuit is simple enough, a direct connection to the printer port (LPT), it should be available for those PC or notebook shipped before 2005. This is no any battery or external power supply for the circuit, only draw a little power from the printer port itself. 

No use any DB-25 Male connector or ribbon cable when connection is made by some panel pins, can be found in shops for stationery. The design is completely base on the schematic of Max6675 evaluation kit, but ommited any unnecessary, retain the Max6675 itself. Of course, the location for those panel pin, it must be the match to an exact location for pin assignment of DB-25. The left behind, plug this paper made PCB with circuit in assembly to LPT, yes, plug those panel pin into the DB-25 connector right with your PC, it will be just fine as charm.

We have no attemp to design software for PC side at this moment, but a course to use the software provided by Maxim. In our DIY design, Pin-3 LPT port is used to supply the power to this DIY logger, so it must tick this setup for Pins, this will enable the Pin-3 output 5V to the logger. For further application need, design the software with calibration feature is possible, also own preference for user interface is not a big deal.

with no thermocouple connected to the logger, it detected as Thermocouple OPEN.

In case you are not comfortable with this DIY stuff, MAXIM is offering a commercial evaluation kid, see the following ULR if you want more detail,

The data sheet of MAX6675 is avalialbe here,

http://datasheets.maxim-ic.com/en/ds/MAX6674.pdf

http://datasheets.maxim-ic.com/en/ds/MAX6675.pdf

Read the recommendations in the MAXIM data sheet, try to maximize the isothermal for IC & connection to the thermocouple, in order to minimizes the effect of cold-junction error & improve the accuracy. In our LPT powered design, the whole assembly has no any other active component such as voltage regulator and power transistor, the IC should be no way to close any sources of heat or thermal gradients of any kind, except the PC itself. A more better way to do this, use a extension cable for LTP port and put this circuit inside an enclosure. Be a simple test to proof if you want to see what is cold-junction error, put your finger over the IC, you will observe changes in the temperature reading, because body temperature offsets the cold junction temperature measured by the IC itself.

Uses thermocouple with epoxy coated for an isolated tip only to cut the noise sources. In contrast, if you hold a bare thermocouple junction with your fingers, you may induce a huge 50Hz hum signal into the circuit but it can not be compensated, the temperature reading may be fluctuations as long as your finger there. The fluctuations will be much less if use an insulated thermocouple tip. More other means, uses thermocouple with grounded & shielded type, a much more robustness design, BUT DO NOT CONNECT THIS GROUND TO YOUR PC GROUND IN INDUSTRY ENVIRONMENT, BECAUSE INDUSTRY POWER GROUND MAY BE A FLOATING GROUND OF WHICH HAS UNSPECIFIED POTENTIAL VOLTAGE, IT CAN BURN YOUR LPT PORT OR YOUR PC / NOTEBOOK.

For use with T-type thermocouples, the connection is (T+) blue copper, and (T-) red constantan. Remember though that the T-type thermocouple is not nearly as linear as the K-type, cold-junction compensation is not exactly the same, within certain range about room temperature, you can use software technique to lookup the cross referrence table, and the thermocouples other than K-type are possible to be useful with this IC.

To Do List :

1) 4-channel logger for a standard LPT port, 

2) MCU based channel relay for distributed sensing network, i.e. one host computer for logging via RS485 or RF.

3) MCU + SD memory for hand held logger with huge memory storage.