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Frequently Asked 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 32°F. 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 34°F to 45°F. 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 50°F.
    • 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. One of my ovens shows an absurdly high temperature (over 5000°F) and has stopped heating. What's going on?
    Answer: The temperature is reading off scale because the thermocouple circuit is open. If this channel has a sending unit, it should be buzzing, too. If it is not, perhaps it has a bad buzzer. Since the temperature is clearly above any temperature you could have programmed, the unit will be off. Replacing your thermocouple should solve the problem. For more details, see our troubleshooting web page.
  7. 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 50°F, it will work normally. Do this for each channel that won't come on.
  8. Why do my temperature readings jump around?
    Answer: Likely possibilities:
    1. bad thermocouple, 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.
  9. 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 electricity, touching a thermocouple to heating elements, lightning strikes in the neighborhood, etc.
  10. 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.
  11. 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.
  12. How is a thermocouple different from a thermocoupler?
    Answer: A thermocoupler is simply a mispronounced thermocouple.
  13. 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.
  14. 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 1300°F.
  15. 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.

  16. 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.
  17. 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.
  18. Why shouldn't I always use Type S thermocouples? 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.
  19. 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.
  20. My temperature readings seem to be a little bit off. Can you calibrate my thermocouple for me?
    Answer: A thermocouple cannot be calibrated. Its temperature curve is solely a function of the alloys from which it is made, so you cannot do anything to the thermocouple itself to adjust it. Over time, the chemical composition of the alloys can become contaminated, e.g., via oxidation or adsorption. This can change the temperature response of the thermocouple. Because they are enclosed by an impervious cladding, Digitry's stainless steel sheathed thermocouples give better long-term temperature stability than either ceramic sheathed or the very common, exposed tip thermocouples.

    Also, you must use the correct extension wire with your thermocouple. Be sure to connect the extension wires with proper polarity — remember, in the world of thermocouples, red means negative. Digitry's stainless thermocouple has integral extension wires, and the sending unit has colored markings, so in this case, it is simply a matter of matching the color of the wire with the marking on the sending unit.

  21. I have on-off control. When holding at 900°F, I notice that the temperature overshoots by about 10°F and then undershoots by about 10°F. What causes these temperature swings?
    Answer:If the observed temperature is below the target temperature, the standard GB calls for heat by turning the contactor on; if above, it turns the contactor off. Once the contactor disconnects power from the heating elements, they begin to cool, but they are still hotter than the interior of the kiln, so the temperature continues to rise for a while. Conversely, when the contactor applies power to the heating elements, it takes a little while for them to heat up. Until they do, the temperature of the kiln will continue to fall. How much the temperature swings depends on a lot of things, like how big the kiln is, how powerful the elements are, what material the kiln is made of, how well insulated the kiln is, how full the kiln is, and so on.
  22. As a first quick test, to be sure everything was hooked up correctly, we did an initial run up to 500°F degrees in 5 minutes. Why did it overshoot to 576°F?
    Answer: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 500°F 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 500°F 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.


  1. What is the difference between a mechanical relay, a mercury relay, and a solid state relay?
    Answer: All relays are basically switches that open and close. When closed, they complete a circuit that allows electricity to flow.

    A mechanical relay uses an older technology, involving an electromagnetic coil to close the contacts that complete the connection. This action causes a clicking noise. Some people find it annoying; some find it reassuring. Because the relay is mechanical, it can fail for all the usual mechanical reasons e.g., the parts rub against each other and eventually wear out, dust hinders the motion, etc. Also, each time the contacts close, there is a slight arcing between the contacts. Eventually the contacts become pitted and rough and can even weld together, making it impossible for the relay to open. The predicted lifetime for mechanical relays is generally in the hundreds of thousands of actuations. The ones in our plug boxes are typically rated for a million actuations.

    A mercury relay is a special type of mechanical relay. Instead of mechanical contacts, it uses liquid mercury to make the connection. This mercury is in a sealed, oxygen-free tube, so there are no contacts to pit, and no oxide contaminants form. To turn power off, a mercury relay relies on gravity instead of a spring to open its connection. Because of this, it is essential that mercury relays be mounted upright. The specified maximum deviation from vertical is 15°. Mercury relays are very quiet and have an expected lifetime of about five million operations. Note that some states may restrict the use of mercury relays.

    A solid state relay is an all-electronic device, so it is silent. It uses solid state electronics (like transistors) to complete the connection and it is not subject to mechanical failures, but of course it does have some other disadvantages. These relays generate heat during normal operation, so they require a heat-sink and a clear air-flow path. Our solid state relay plug boxes have the heat-sink built in and have air holes (that you must not cover). Solid state electronics are susceptible to electrical surges, e.g., those caused by lightning. Inherent in the design of solid state relays is that even when in the "off" state, a small amount of electricity (called "leakage") flows through the circuit. Consequently, you must be sure to disconnect your kiln from its power source — e.g., throw the circuit breaker — before you do anything that may involve touching its elements. Finally, if you choose a solid state relay, you can order a GB that uses proportioning output [no extra charge for this option]. This provides tighter temperature control (less over- and under-shoot), which is particularly important for kilns that are not made with fire brick.

  2. How should I dispose of a mercury relay?
    Answer: As we indicate in the literature that accompanies our mercury relays, they should never be put into ordinary trash or land fill. Generally, the original manufacturer of the relay will accept returns for recycling at no cost. The EPA lists safe mercury disposal options on line at http://www.epa.gov/epaoswer/hazwaste/id/univwast/mercury.htm#recycle.
  3. Does the plug box of a Complete Digitry GB1 system replace the relay or do I have to get a relay in addition?
    Answer: The plug box contains a relay to control your kiln; you do not have to provide another one.
  4. I'm wiring up my annealer so it has a standard 240 Volt plug coming out of it. I'm assuming the plug box also has an outlet to plug the elements into. Is this correct?
    Answer: Yes. Assuming that you put the correct plug onto the kiln — one that is appropriate for the amount of current it draws, etc. — then the plug box will provide the appropriate, corresponding socket, e.g., NEMA 6-50 or NEMA 10-50. If you like, you can look at our web page http://www.digitry.com/plug.html, which has diagrams of the various socket configurations.
  5. I'd like to order a GB1 package with the Nema 10-50 plug. However my annealer will be pulling 56 amps when in full-on operation. Do you think it will be a problem using the 50 amp plug box?
    Answer: We cannot advise using either a NEMA 10-50 plug or our plug box for more than 50 amps. It is beyond the ratings of these items! Some people follow a rule of thumb that states you should not draw more than 80% of the rated current through a circuit, so we would suggest that you or your electrician directly wire the annealer using at least a 70 amp relay.
  6. If I get a 60 amp mercury relay from you, how does it interface with the GB1 without the plug box? Is it a simple plug-in thing?
    Answer: Although not so simple as using a plug box, direct wiring installation is really quite easy. You can read about it our manuals, which are on-line at http://www.digitry.com/manuals.html. However, most people do opt for an electrician because of the safety issues involving high currents and voltages.
  7. I read the website about the difference between a 24 Volt or 120 Volt coil for the relay. Which would you recommend if I'm only running one annealer at 240 Volts?
    Answer: Since you are using a 240 Volt annealer, you must use a relay with power connections rated for that voltage (or higher), but this rating is independent of the coil voltage. Mercury relays are generally rated for up to 600 VAC (which stands for "Volts AC"), but you should look on the information tag attached to the relay to be sure. For convenience, most people use relays with 120 Volt coils. If you have a relay with a 24 Volt coil, you need to provide a 24 Volt transformer to supply the voltage for the coil. The advantage of a 24 Volt system is that it is safer than a 120 Volt system and so does not generally require running wires in conduit to pass code. In any case, this is something your electrician can advise you on.

    Note that because of the increased safety and simpler installation guidelines, the mechanical relays in Digitry plug boxes all use 24 Volt coils.

  8. I have a pottery kiln that came with a kiln sitter. Do I have to remove it to use the GB1?
    Answer: No. On the contrary, it can be used as an emergency high-limit cut-off switch. Leave it in and use a cone for a temperature above any you expect to see in normal operation. Since cones degrade with repeated heating, be sure to change them as needed.


  1. While in Monitor Mode, I pushed ENTER, but the controller stays on IDLE and does not begin to heat.
    Answer: You probably forgot about the confirmation sequence. Press the profile or oven number immediately after pressing ENTER. See the manual for more information.
  2. I see all E's in the Temperature display and cannot advance to the next step or change back to Monitor Mode. What do I do now?
    Answer: Press Clear, and the Temperature will change back to 0. You may now enter the correct temperature. However, if you press Clear a second time, the Temperature display will become blank and the Time display will change back to 0, and when you press digits, they will be entered into the Time display.
  3. And how do I get rid of E's in the Time display?
    Answer: Just press Clear, and the Time display will change back to 0. If you now you press digits, they will be entered into the Time display.
  4. What causes those E's mentioned above to appear in the Temperature display?
    Answer: The E's indicate that there is an error in your input. You have tried to enter a temperature that is too high. This usually happens because you accidentally pressed too many digits: temperatures are limited to four digits. However, even if you pressed only four digits, there is a maximum reasonable working temperature for your thermocouple, and Digitry's controllers check for unreasonably high temperatures. So, for Type K thermocouples, the temperature must be under 2400°F, and for Type R and Type S thermocouples, the upper limit is 3200°F.
  5. And what causes E's in the Time display (as mentioned above)?
    Answer: For a GB1, this means that you have pressed too many digits. For a GB5, you tried to enter a time that exceeds the maximum time the GB5 can store, 546 hours. This usually happens because you accidentally press too many digits.

    For a GB4 or any Digitry that has a "classic" interface, this can happen for any of the reasons above, or it can happen if there the subsequent step in the profile has a time that is before the time you are trying to enter. Since the times in "classic" profiles are cumulative, the time in each step must be later than the time in the step before it. So, if step 5 has a time of 35:00 and you try to enter a time of 36:00 into step 4, this will be recognized as an error, and the GB will display E's. The easiest way to avoid encountering this problem is to clear out the entire program before you start to enter a different one. To do this, the GB4 must be in program mode, and then you press Cancel followed the number of the oven you are programming.


  1. Sometimes I have to push very hard on some keys to get them to register. Sometimes I press a key and it registers twice (or more).
    Answer: Either of these conditions indicates that your keypad is worn out and needs replacement.
  2. My keypad is worn out. Can I replace it myself?
    Answer: The GB1 keypad is an integral part of the faceplate, so you will need a whole new faceplate. The GB5 keypad is screwed to the faceplate and connected to the rest of the computer by a plug-in cable. All but the oldest GB4 keypads are the same as the GB5 keypads. Replacement of any of these keypads is a simple mechanical operation.
  3. What is the difference between your power surge suppressor and those commonly available in consumer electronics stores? 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.


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