Mercury Displacement Relays

    For all very large Paragon pottery kilns and nearly all of the Paragon professional glass kilns the Mercury Displacement Relay is standard equipment.  Mercury Displacement Relays (abbreviated and commonly referred to as MDR by Dogwood Ceramic Supply) are required in the large pottery kilns, such as the Paragon Dragon, because of the amperage drawn by these kilns exceed the amperage ratings of the standard mechanical relays found in smaller Paragon kilns.  Mercury Relays are used in the glass kilns, not because of the amperage drawn (even though that is an issue on the biggest boys), but rather the typical usage of these glass kilns with long hold times means the MDR is a critical component.

What is a Relay?

    The purpose of the relay is to distribute (send) power to the element when commanded to do so by the Digital Controller.  On and Off, On and Off, the mechanical relays are constantly cycling.  In the typical small pottery kiln that on-off cycling is usually a few seconds on followed by several seconds off as the temperature is slowly built up in the kiln. This slow cycling allows the relay to cool off between periods of being on and prevents premature relay failure.  Most lower amperage pottery kilns can operate quite well with the standard mechanical relay (shown to the right in the photo above).  The basic glass fusing kiln will follow a similar pattern of on-off and the mechanical relays can work for these kilns as well.

Why is the MDR needed?

    But for the serious glass artists long hold times are common, especially in a kiln such as the GL 24 ADTSD.  These long hold times often cause the relay to frequently cycle with extremely short on-off cycles to maintain the kiln temperature without increasing or decreasing by even the slightest amount.  This on-off cycling is so frequent and of such a short duration that the noise generated from the relay is best described as chattering.  In a manner of speaking the relay is doing it's job at a normal pace of performance.  But in a kiln environment, with all the heat of the kiln chamber very close by, it is difficult for a chattering relay to dissipate the heat generated from the frequent on-off cycling.  Heat is not very friendly to a mechanical relay and premature relay failure is common.  Enter our friend the Mercury Relay (shown to the left in the previous photo).  Mercury Relays are ultra heavy duty, capable of handling large amounts of electrical current.  Mercury bathing the relay components also is wonderful to dissipate the heat from the frequent on-off cycles.

Why not make the MDR standard in every Kiln?

    The issue is not the Mercury, but simply because the Mercury Displacement Relays can be very expensive and not every customer will require the heavy duty relays.  Depending upon kiln amperage the Mercury Relays can range in cost from $100 to as much as $250 per relay and some larger kilns can require 2 to 5 Mercury Relays depending upon the electrical specifications of the kiln.

Should you add Mercury Relays to your kiln?

    For large pottery kilns and all of the professional glass kilns the questions is moot as the kilns are equipped with Mercury Relays as standard equipment.  For everybody else, the best answer to that question is how hard will your kiln work, especially holding temperatures for extended periods of time plus the location of the kiln.  Is your kiln going to be fired with hold times of 2 hours at one temperature while located in an outdoor work shed near Death Valley?  Then add the Mercury Relays as the high ambient temperature alone will most likely cause premature relay failure long before the long hold time causes any damage.  Is the kiln going to be located in an air conditioned room with the kiln temperature never held for more than a few seconds?  Then skipping the Mercury Relays is a possibility as the replacement cost of the regular mechanical relays ($28 to $48) is low enough that it might be difficult to recover the added cost of the Mercury Relays. Kilns situated between these two extremes are difficult to judge in absolute certain terms.  Replacing standard relays are not very difficult for those who are handy with basic tools such as a screwdriver.  But there are those issues of premature relay failure that always seem to occur during a critical firing.

    Historically better than 80% of all glass customers will opt for the Mercury Relays for their small glass kilns.  The cost of the relay for the small glass kilns is not that significant compared to the alternative of losing the load of work or worse.... loosing the kiln.  Only a small percentage of pottery customers will add the Mercury Relays because 1) the length of hold times are seldom excessive (which means less wear on the relay), 2) The wares are often tolerant of the maximum temperature rating of the kiln which means even a stuck relay may not be a catastrophic event and 3) the upgrade costs for 3 to 5 high amperage Mercury Relays in a large pottery kiln is just too expensive versus the benefit.  Dogwood Ceramic Supply conveniently prices the MDR option on all glass kilns when the MDR is not standard equipment.

Are there environmental issues with Mercury Relays?

    In theory yes, but in application the mercury is sealed in a metal chamber to act as a coolant and if the internal relay mechanism were to ever fail the mercury cooling chamber can not leak into the environment.  In the event the mercury relay does fail, even out of warranty for the rest of the kiln, Paragon has already arranged with their supplier to handle the proper disposal of the unit to keep the mercury out of landfills.  Speaking of failure, MDR failures are usually due to a faulty connection following element replacement or other repairs.  With a little caution when making repairs, the MDR is one of the most reliable components that could ever be placed in the kiln and should never fail under practical and not-so practical applications.

Can I upgrade my old kiln to use Mercury Relays? 

    Check the photo near the top of this section.  There is a significant mounting difference between the two types of relays and user upgrading is not easily accomplished, even IF there is sufficient room in the control box.  Alternatively, you can send the control box for the kiln (if there is enough room for the upgrade) back to the Paragon factory and request the factory staff to install the MDR.  Unfortunately many small kilns lack the required space in the control box to handle the physically large Mercury Relay.

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Mercury Relays are used in the glass kilns, not because of the amperage drawn but rather the typical usage of these glass kilns with long hold times means the MDR is a critical component.