Simple kiln controls

Kilnforming can be done in kilns with a wide range of controls; from the most rudimentary right through to those with fully programmable controllers. 

This section discusses the various controls which have been used over time and which may be found on pre-owned kilns, as well as on new budget units.

For those interested only in programmable controllers, see Programmable Controllers.

ON-OFF switch

The most basic kiln control is an ON-OFF switch. Full power will be applied to the element only when the switch is on. Unlike the gas hotplate in the kitchen, power to the electric element cannot be part-on or part-off. When the power is full-on the kiln will heat up at a slowly decreasing rate, depending on the power of the elements and the mass of the load. It will be fast at first, then gradually slow down; exactly the opposite of what suits the glass.

Energy regulator

Also known as an infinity switch, and sometimes as a simmerstat.

Fig 53.01

A simple device to interrupt power going to the elements for varying proportions of time, depending on the setting of the control knob. At their heart is a bi-metallic strip; two thin pieces of dissimilar metal having different COE which have been fused together. The strip bends when heated and straightens again as it cools. Current going to the elements heats the strip and as it bends it opens switch points and cuts off power to the element. The control knob is used to vary pressure on the strip, thus varying the proportion of time the points are open and closed.

They usually have a scale marked downwards from 100% but the settings can be quite arbitrary as it depends on the installed position of the device and the rate at which heat can be lost from inside and around it.

They are sometimes fitted to tall 3 phase kilns, one to each phase, where they can be used to minimise vertical temperature differences. In this situation they allow the power to be applied for progressively shorter time from bottom to top.They are still offered on pottery kilns with basic controls but their use is limited to element currents below about 13Amps. Because they take time to respond, their performance can be unpredictable when used with digital devices which may switch power too rapidly for them to work properly. In those situations, set them to 100% (full on). 

Kiln Sitter

Fig 53.02

Used on pottery kilns, this is a switching device which turns off the power to the elements when sufficient heat-work has been done on the ware. It senses this by using a ‘pyrometric cone’ which softens and bends at some precise heat-work level.  It allows potters to do what is called ‘cone firing’. The kiln sitter is currently offered in conjunction with an energy regulator to provide basic pottery firing control.

It acts simply as an Off switch, gives no indication of temperature. It is not suitable for use as a control when kilnforming; other than as an overheat cut-off switch.

Temperature switch

Fig 53-03

Panel mounted control fitted to pottery kilns which could be set to switch off the kiln at any pre-set temperature. Consists of a rotating scale which can be set to the required soak temperature. Internal contacts change over when the set value is reached and cut off power to the elements. They give no indication of temperature during the firing.

Temperature switch with analogue temperature indication

Fig 53-04

Similar to the temperature switch above, but with the addition of an analogue temperature indicator. A leading maker was ‘Shimaden’, who fitted an edge mounted scale across which a pointer moved to indicate the temperature. The device operated only as an On-Off switch to turn off the power when the set temperature was reached.

Analogue pyrometers

Fig 53-05

Consists of a thermocouple driving a moving coil voltmeter. The thermocouple produced a voltage proportional to temperature and drove a pointer across a scale .They could be prone to considerable calibration and operator error.

As much pottery firing is done using LPG or Natural gas, the self powered analogue pyrometer was all that was needed; the potter exercising manual control of the gas controls. Analogue pyrometers have been superceded by digital devices which are more robust.

Parallex error

When viewing instruments with a pointer above a scale, moving one’s head sideways can give different readings. This is called parallex error. The correct way to read such an instrument is to position one’s head so that the reflection of one’s eye appears directly over the pointer.

Methods of power control.

Development of digital displays saw the introduction of instruments which could be read from afar, and where readings were definite and without risk of parallex error. They could not only show the temperature but could also store data and control power switching devices such as contactors or Solid State Relays.

This latter is done by a 'Control output' signal which can be either;

• On-Off, or
• Proportional. (commonly referred to as P.I.D.)
  

On -Off control

With this type of control the power is turned on whenever the kiln temperature (process value or PV) is below the planned or Set Value (SV) and switched off when the SV is reached. There is always a lag between when the power is turned off and the temperature ceases rising, so during a soak the PV will overshoot the SV and there will be a finite time period during which there will be no power going to the element;until the temperature drops to the SV.

At this point the power will be again turned on to the element. However, there will be a lag before this becomes effective, so the PV will continue to drop below the SV for a short period until it again rises toward the SV. This see-was action will continue, above and below the SV, often by considerable amounts.

Fig. 53-06

Fig 6 shows the power and temperature curves during a soak using this type of control.

On-Off control is widely used on current low priced American kilns imported to Australia, in the form of the Bartlett 3K or three button controller. It or a similar device has also been fitted to a small Woodrow bead annealing kiln. This is a pity as the much more effective proportional control mentioned below could be fitted for a small additional cost. With the exception of the Woodrow kiln mentioned, I am not aware of On-Off control being used on Australian made kilns for many years.

 Proportional or PID control.

 This is where the power is applied in short bursts for whatever proportion of the total cycle time is necessary to maintain the programmed temperature profile or pattern. To illustrate; when using Solid State Relays (SSR) for power switching a cycle time of three seconds is commonly used. The controller works out for what proportion of that time the power should be turned on to best maintain the SV. It then monitors the result and amends the proportion for the next time period. The result is packages of power varying from off to full on every three seconds being applied to the elements; and much more precise tracking of the firing pattern than can be achieved with On-Off control. 

Most instruments using proportional control can maintain the SV within one degree C.

Control output voltage

SSR's and contactors work best with different control signals; 240Volt or 24Volt a.c. for contactors and 12Colts d.c. for Solid State Relays. This is discussed at length in 55.Power switching devices. Most devices discussed below can be fitted with one or the other but not both, so it is always necessary to specify the output voltage when ordering.

Digital control devices 

These versatile devices can be configured (select from options within the instrument software) to perform various functions. The most common kiln application is as a ramp and soak controller.

Ramp and soak controller

Fig 53-07

Subject to limitations, the ramp-and-soak controller allows the kiln to be heated up at a maximum rate of rise to a set temperature and held there for a selected time; when the power will be turned off. The limitation is that the entered rate of rise must be achievable by the kiln when it reaches soak temperature.

Fig 53-08

As has been discussed elsewhere, the heating curve of an uncontrolled kiln will resemble something like curve A in Fig 8.

If the pattern entered in the controller calls for a rate of heating slower than that of which the kiln is capable, as curve B; then the controller will switch power to follow that pattern.

Fig 53-9

If, on the other hand, the pattern entered calls for heating rates of which the kiln is not capable, then the controller will apply power to the kiln continuously but will arrive at the soak temperature before the kiln does so; and will immediately start timing out the soak time. In some firings the soak will be cut short, whilst in others the kiln may not reach soak temperature before the power is turned off.  

This type of controller does not WAIT for the kiln, but goes blindly on its way. Only  programmable controllers have 'wait' capability, where the controller halts the running of the pattern at the end of a step until the kiln catches up. Different makers use various terms, but wait says it most succinctly.

 When the Riley GS hobby kilns were introduced, controllers with wait capability cost as much as the rest of the kiln, so a Brainchild 9090 digital controller configured in ramp-and-soak mode was fitted. Users overcame the premature cut off of power by entering a long soak period and monitoring the kiln during the firing.

They are still an effective controller for simple operations such as firing paint or bead annealing but have been superceded for more complicated firings.

With the introduction of the Shinko PCD300 series controllers the use of single-ramp-and-soak controllers was discontinued and the Shinko PCD was fitted to all Riley Glass kilns, of whatever size.

Plug-in controllers

Fig 53-10

A convenient way of exercising programmer control of single phase pottery style kilns fitted with a Kiln Sitter or other temperature switch is as shown in Fig 10

This comprises a housing containing the programmable controller which is plugged into the power outlet and into which the kiln is plugged. Also in the housing is a Solid State Relay (SSR) which switches power to the outlet socket in response to instructions from the controller.

Why a SSR?  Because SSR's allow shorter bursts of power at more frequent intervals, smoother temperature curves can be achieved with them than with contactors.

A thermocouple on a wander lead from the housing is inserted into the kiln at some convenient point to monitor kiln temperature.