TEMPERATURE RELATED QUESTIONS

1. Are the GB4 and GB5 Sending Units different?
   Answer: No; they're the same. And so is the optional GB1 Sending Unit. However, Sending Units differ by thermocouple type. All Sending Units for Type K Thermocouples are the same, but they are different from the Sending Units for Platinum Thermocouples (Type R or Type S).
    
2. Why do Sending Units have shipping jumpers?
   Answer: To protect against possible damage from static electricity during shipping.
    
3. All of a sudden one of my ovens reads 32ºF. Why?
   Answer: Assuming the other channels are working correctly, the most likely cause is that one or more of the wires between the Sending Unit and the GB4 or GB5 is either broken or disconnected. A less likely possibility is that the Sending Unit itself is defective.
    
4. All of my ovens read very low, about 34ºF to 45ºF. Why?
   Answer: A GB4 or GB5 has an internal power supply for its Sending Units. This power supply protects itself from external short circuits by shutting down almost completely, causing a low reading on all channels, even though only one may be shorted out. Since this reading is below any reasonable setpoint, a running unit would remain on continuously, trying to attain its setpoint. Left unchecked, this could cause a serious over temperature situation: A GB4 prevents this potentially dangerous condition by refusing to allow a unit to run if the temperature is below about 50ºF. A GB5 handles this in a more sophisticated manner: it reports a "BAD1" error and shuts off the corresponding unit.
5. What could cause the short circuit you mentioned above?
   Answer: The most likely cause is a defective Sending Unit. Another possibility is that the insulation of the wires is damaged, allowing bare metal to touch something it should not.
    
6. My GB4 controller was working fine all summer and fall. Last night it got quite cold in the barn where my annealers are. In the morning none of my annealers would come on. Why?
   Answer: The GB4 interprets the cold temperature in your barn as the potentially dangerous problem mentioned above. Raise the temperature reading slightly to "jump start" you unit by holding the thermocouple in your hand or heating it briefly with a flame. Once it's above 50ºF, it will work normally. Do this for each channel that won't come on.
    
7. Why do my temperature readings jump around?
   Answer: Likely possibilities:
   1. bad theromcouple, or
   2. a loose connection between the thermocouple and the Sending Unit, or
   3. the "multiplexor" inside the GB4 or GB5 needs replacement.
   If the problem occurs on more than one channel, it is almost certainly the multiplexor.
    
8. What's a multiplexor?
   Answer: This is an integrated circuit that connects each sending unit in turn to the computer circuitry. Since it is electrically close to the input, it is more sensitive electrical damage than most of the other parts of the controller. These include static electricty, touching a thermocouple to heating elements, lightning strikes in the neighborhood, etc.
    
9. What is a pyrometer?
   Answer: It's an instrument used for measuring high temperatures. Sometimes "pyrometer" is used colloquially to refer to just a thermocouple probe instead of an entire instrument.
    
10. What is a thermocouple?
    Answer: A temperature probe based on the contact of two dissimilar metals. Any time two dissimilar metals are in contact, they generate an extremely small voltage that depends on the temperature of their junction. It also depends on what the metals are. To be useful, the metals must have certain characteristics and the correspondence between the temperature and the voltage it generates must be known.
     
11. How is a thermocouple different from a thermocoupler?
    Answer: A thermocoupler is simply a mis-pronounced thermocouple.
     
12. What does "Type K" mean?
    Answer: Type K thermocouples are one of the common, standard thermocouples. They use Chromel (a special alloy of Chromium and Nickel) and Alumel (a special alloy of Aluminium and Nickel) as the two dissimilar metals to generate the voltage. This pair of metals has been studied extensively and the temperature-voltage correspondence is well known.
     
13. Are there any other types of standard thermocouples?
    Answer: Yes, there are several, each based on a different pair of metals, and designated by a letter of the alphabet. For example, Type J uses iron and an alloy of copper and nickel; Types R and S use slightly different alloys of platinum and rhodium as one of the metals and platinum as the other. Different metals impart different properties to the various types of thermocouples, including the maximum temperature they can withstand, the strength of the voltage output, etc. For example, Type J does not hold up well at temperatures above 1300ºF.
     
14. Why do you use Type K thermocouples? What other ones could I use?
    Answer: For working with glass, Type K thermocouples are the de facto standard. They are relatively inexpensive, physically rugged, and easily handle most of the temperatures involved with fusing, slumping, kiln-casting, annealing, etc.

    You could also use Type R or Type S thermocouples, which withstand even higher temperatures than Type K, but being made of platinum, they are much more expensive and are generally reserved for high temperature applications such as furnaces. Also, to keep their cost down, platinum thermocouples are made of very thin wire and thus are very fragile. At any given temperature, they produce a smaller voltage, requiring more costly electronics.
     

15. Can I use any type of thermocouple with a Digitry controller?
    Answer: Digitry controllers are calibrated for only Type K, Type R, or Type S thermocouples. You must specify the type of thermocouple to be used for each channel when you order the controller.
     
16. What is the difference between type R and Type S thermocouples?
    Answer: There is very little difference between the two. We don't know why both exist. They both use the same kind of Sending Unit, but they do require slightly different calibration, so when ordering a controller, you must be clear which one you will be using. Type S seems to be more common than Type R.
     
17. Why shouldn't I always use Type S themocouples? It seems they might last longer?
    Answer: As noted above, Type S thermocouples are much more expensive and fragile than Type K thermocouples. You can buy many Type K thermocouples for the cost of a Type S thermocouple.
     
18. Do I need special wire for connecting my thermocouple?
    Answer: You should use special thermocouple extension wire to connect a thermocouple to a Sending Unit or to a GB1. You do not need special extension wire to connect the Sending Unit to the GB4 or GB5. A popular Type K thermocouple sold by Digitry includes an integral 7' lead wire, so no extra extension wire is required.
     
19. What is the difference is between your power surge supressor and one from [name your favorite store]? Is there a drastic difference in quality?
    Answer: YES! Our equipment is based on silicon avalanche suppressor diodes (SASDs) rather than the older technology of metal oxide varistors (MOVs) that most units use. SASDs have far better performance for modern electronic devices and a much longer expected lifetime, something like 10+ years. An MOV degrades over time and can be used up in a matter of days or years, depending on the number and intensity of transient voltages ("spikes") it experiences.
     
20. As a first quick test, to be sure everything was hooked up correctly, we did an initial run up to 500ºF degrees in 5 minutes. Why did it overshot to 576ºF?
    Answer: The GB calculates the required temperature for each minute, as determined by the profile you enter. If the observed temperature is below that, the standard GB calls for heat by turning the contactor on; if above, it turns the contactor off. When ramping up, when the kiln reaches the desired temperature and the contactor turns off, the residual heat in the heating elements will cause it to continue to heat. How much it heats depends on a lot of things, like how big the kiln is, how powerful the elements are, how well insulated the kiln is, what's in it, and so on.

    Whenever the kiln is going full blast for some while, the elements get to their hottest temperature, maximizing the overshoot. When the kiln is near the set point and the elements come on to merely correct for heat loss or to ramp up to a nearby value, they usually do not reach their hottest potential before the required temperature is attained. This reduces the overshoot. The situation you described will probably have the kiln on constantly until it gets to 500ºF degrees. Hence you have a lot of residual heat and more overshoot. If your program has a second step, one that holds the kiln at 500ºF degrees, the subsequent temperature swings will be much less. If there were a large piece in the kiln (thus increasing the "thermal mass"), it would be even less. We do have proportioning models that reduce the power to the elements when the temperature gets close to the target, thus reducing the overshoot.