Dimplex Opti-Myst fires

[theoldtimer]

Thanks for the interesting technical information (AlastairE) about the misting device and the frequencies used. I have a knowledge of electronics and radio due to my lifelong hobby, but I know little about these transducer devices as used in the Dimplex fires. I would like to understand more fully the principles of operation and about the frequencies you refer to. Is the amount of the mist production related to/proportional to the frequency of operation and/or voltage applied. In other words, what exactly does the user adjustable potentiometer knob actually control, perhaps it adjusts frequency in which case the circuit could be modified to reduce the minimum "smoke" generation. Are there any websites where I can download the service manual for a Danville fire? Any suggestions appreciated - thanks. I have no intention of "stealing" your customers or business honestly ! I have far too many other things on my plate than sorting other folks faulty fires. I just want to understand the operation and thus be able to fix it when it packs up, which I am convinced it will before long )

 

[SimonH2]

As for the physical process, I assume it's related to cavitation & ultrasonic cleaning. There is a Wikipedia page on Ultrasonic Fogging but it has no detail on how it's done.

I assume the process is that the ultrasonic waves reach the surface, but instead of creating cavitation bubbles that attack a hard surface, the bubbles collapsing eject tiny droplets into the air. BBC Four did an amazing program last year about bubbles and one of the things shown was how the shape of a glass affects the taste of Champagne ! It's to do with how long the bubbles has to grow as it rises through the glass, that affects it's size, and that affects what happens when the bubble hits the surface. As the bubble breaks the surface, in effect there is momentarily a "hole" in the liquid. Liquid rushes in to fill the void, but when it meets in the middle, kinetic energy results in a vertical jet being fired out of the surface carrying some of the liquid into the air as a tiny droplet. The size of the bubble affects the size of the droplet, and that (from what i recall of the program) affects the "potency" of the airborne mist that forms a large part of the "taste" of the drink (a big part of "taste" is actually down to smell, which is why food loses it's taste when you have a cold).

If the program gets repeated, it's highly worth watching.

 

[AlastairE]

Its an odd process. Real wacky science....

Ultrasonic Cleaning is done at say 50 to 100KHz and causes cavitation of the fluid/cleaner/water at the surface of whatever is to be cleaned. Apart from slight fluid disturbance, there's nothing much to see IME.

Mist-Making is done at a Very much higher frequency, in the Megahertz range, around 2MHz.

To observe one of these things working is most odd. It generates a tower of water approx 3-4 inches above the the fluid surface, above the transducer, the peak of which is where the mist is derived--or so they say....

The Transducer in Opti-myst is a Piezo Electric disc of a ceramic material coated both sides with a metallic coating, to which electrical connections are made. It has similar properties to Quartz in that it will Resonate at a certain frequency, (Depending on thickness and diameter) when stimulated appropriately by electrical pulses generated by electronics.

Like Quartz the frequency can be very stable and sometimes Ceramic resonators replace Quartz in time-keepers and other frequency dependent devices, in radios and all sorts, as its much cheaper, and can be made with other properties in addition to resonance/piezo-electric effect, not possible with Quartz

--So endeth the science-lesson!

 

[SimonH2]

To observe one of these things working is most odd. It generates a tower of water approx 3-4 inches above the the fluid surface, above the transducer, the peak of which is where the mist is derived--or so they say....

Which would match with the idea of creating bubbles which eject small droplets vertically as they break at the surface.

 

[theoldtimer]

Thanks for the information SimonH2, that's all very interesting and now I can do some more reading on the subject. I assume the frequency of operation of the transducer is in the KHz region not MHz i.e. above audible frequencies. The previous reference to MHz is perhaps for the inverter in order to arrive at 24 volts? Rather than use a small mains transformer Power supply, they are using SM PSU? Its all a bit too complex for my liking and for what is just a room heater and needs to be inherently reliable over the long term.

 

[bernardgreen]

A transducer gives the most mechanical ( sound or vibration) output when it is driven at its resonant frequency. The resonant frequency will vary with temperature and other factors. Being immersed in water will alter the resonant freguency

There are two ways to drive a ceramic transducer.

One is a fixed frequency oscillator driving a two terminal transducer where the design requires the oscillator to be the same as the transducers resonant frequency. Cheapest but not the most efficient. Some drivers can adapt the oscillator frequency depending on the amount of power the transducer is taking

The other is to use a three terminal transducer which provides feedback from transducer to the oscillator which enables the oscillator to adapt it's frequency to exactly match the transducer. It also allows the power fed to the transducer to be controlled to prevent it being overdriven which can shatter the transducer.

I have no idea which method is used in the OptiMist fires.

 

[AlastairE]

A chip based boost-regulator with switching MOSFET having very low RDS-On is used for raising the 12V supply to 24 volts for the RF power-stage that drives the transducer.

--WHY its done this way is to ensure a stabilised, dedicated 24V rail with over-current protection for the RF stage and to minimise RFI.
--Small variations in the RF stage Supply-Voltage will have a large effect on RF stage output, thus it needs good supply regulation.

The transducer is a simple two-pin affair and must be (reasonably) well matched to the RF stage in frequency.

The switching-boost-regulator runs at some hundreds of KHz--and isnt important for the operation of the transducer excepting it provides sufficient current and a stable voltage, for the RF stage.

The RF stage runs at 2.2MHz, matched to the transducer.

--IF you have a suitable radio-set, You could tune in to 2.2MHz and receive the carrier produced by these fires, as the screening of RF isn't perfect--especially on the single-disc sumps that have no tinwork around the electronics!

The RF Power stage driving the transducer consumes around 20-30W at 24V.....

 

[AlastairE]

As a reference-point--The LATER TYPE Opti-Myst with the ORANGE Water-Tank uses the 'KERI' type transducer/mist-generator.

From Dimplex these are around £50.

Interestingly--They look a LOT like the ones Maplin sell for £20--

http://www.maplin.co.uk/p/mini-fogger-mist-generator-l38ak
--I have not confirmed they are identical/compatable but strongly suspect they are one and the same device....

The Earlier type with the BLUE water-tank is a complete sump assembly with the mist-maker fitted as part of the sump, This complete sump for the earlier type is currently (cheapest smallest sump) £185! and separate parts are not available for them They however can be repaired, contact me for more info.

 

[ban-all-sheds]

Prompted by this discussion.

But no answers.

 

[theoldtimer]

Thanks guys for responding with so much interesting information, this is a brilliant forum, I can see there is so much more I need to read on the subject and importantly understand. I certainly do have the equipment for monitoring the 2.2 MHz signal and the RFI EMC point is of particular interest to me and many others who have an interest in all EMC issues that are adversely affecting our radio hobby. Low energy, LED lamps, PLT devices etc. So these transducers are in fact driven at 2.2 MHz and are resonant loads on that frequency. Temperature will affect the resonant frequency and thus the mist generation which explains why it takes a while for the mist generation to settle down. The water gets fairly hot after the fire has been running for an hour or so. I wonder? With all your knowledge and experience with these transducers, is there a simple method to change the max/min setting on the misting control knob. The minimum setting still produces excessive mist? Sorry, I think Al has answered this before, I will go back and print previous postings and read again.

 

[AlastairE]

Prompted by this discussion.

But no answers.

From my experience,--Taken care of helps, but does not help the reliability all that much.

They are not what I would call reliable--in any way shape or form in their standard--from the factory--format. I upgrade the earlier type, but your'e stuck with what you buy with the later type, as they are a completely sealed block of plastic inside that shiny metallic 'puck'.

Disc life can be extended by ensuring the water tank/sump is cleaned out VERY regularly--at Least once a month, preferably every week, as slime/bacteria grow in it making the water somewhat acidic that attacks the metallic (Earlier) and the teflon (Later) surface of the discs.

I'm still on my original pair of discs, the fire was bought S/H --with a faulty sump, and we have had it 3 years, used every day in Winter--and even just for the effect Summer nights sometimes.

Wash out the sump, the diffuser lid, and the water-tank with a little kitchen-cleaner and warm water, wash out the soap well after.

It goes without saying--the water MUST be drained if not using the fire for some time.

Once the metallic surface has gone, thats it for that disc, and it also stresses the already taxed-to-the-limit driver electronics.

 

[AlastairE]

Oldtimer--

Not sure so much with your later version if the mist level is adjustable. I suspect that the amount the Keri 'puck' develops is constant, and the fan speed is altered to effect alteration of the amount of mist driven out of the sump box/diffuser.....

Easy way to tell--Check the connector that the mist-maker has where it attaches the sump/fire and see how many connections. I suspect there'll be only two,--Its just a power supply plug/socket arrangement with no control wiring....

Temperature will seriously affect the mist production quantity as its much easier to atomise water in this way when its warm than if its stone cold. The lamps impart some considerable heat (200W for a 4 lamper) to the bottom area of the fire, so the sump/water/mist-maker will warm up a fair bit....

 

[theoldtimer]

Great info, thanks a lot.
I am copying all these posts and printing for future reference and insert with the Users Manual so I can find it !
I am going to give the sump etc a really good clean now and at weekly intervals over the winter and empty out clean and dry during the summer. I only use filtered water (Brita jug etc) now, but at first I used to buy the cheap "value" bottled water which I think is basically tap water. When you cost the purchase of the jug and filters into the running costs, plus the KWHr's just to run the halogen lamps and electronics, it does work out to be a fairly expensive way of producing a flame effect. Never mind, its less work than a real fire and cleaner, so I save money on painting and decorating the lounge. In any case, I have just re roofed and taken down the chimney, so I am stuck with my high tech fire. Thanks again, everyone for your inputs, I am almost an "expert" myself now ! just kidding........

 

[ban-all-sheds]

I am going to give the sump etc a really good clean now and at weekly intervals over the winter and empty out clean and dry during the summer. I only use filtered water (Brita jug etc) now, but at first I used to buy the cheap "value" bottled water which I think is basically tap water.

IIRC Dimplex advise that sort of regime in the operating instructions.

 

[ban-all-sheds]

They are not what I would call reliable--in any way shape or form in their standard--from the factory--format. I upgrade the earlier type, but your'e stuck with what you buy with the later type, as they are a completely sealed block of plastic inside that shiny metallic 'puck'.

I thought from what you said it was the other way round?

As a reference-point--The LATER TYPE Opti-Myst with the ORANGE Water-Tank uses the 'KERI' type transducer/mist-generator.
.
.
The Earlier type with the BLUE water-tank is a complete sump assembly with the mist-maker fitted as part of the sump.

In any event (and I've never seen one of these appliances in the flesh, let alone delved inside one, so this might be a laughable question) - how big is the sump? No matter which sort you have, and no matter whether it is the fogger unit, the driver electronics or the membrane which has failed, could you not simply drop a self-contained unit, such as the one sold by Maplin, or your nearest garden centre, into the water?