Questions using flexwatt on a rack

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Originally Posted by kitedemon

Dimmers only work if the room temps are absolutely stable. If they are not they don't work well. I agree the flexwatt is variable and racks are more an 'average' system. I have never had dead on temps top to bottom. I would say that a failsafe on a second spot is a needed bit of safety gear.

Generally a room temp should be 79-81 I would say 80ºF personally.

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Quote:

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Originally Posted by suzuki4life

what?
A dimmer regulates the tape to only receive a certain amount of power PERIOD. POWER = temp. If you set the dimmer properly, they work right. If you think you will use the dimmer as a thermostat, you are asking for troubles. The dimmer is used to keep the temp from running at max power. The thermostat should be used to keep the tape itself from continuously running. In a proper setup, the tape should be turning on an off many times an hour and not in a continuous "on" position.

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It sounds like suzuki4life is saying to use the dimmer like a backup thermostat. If the primary stat fails, the dimmer would limit how how that tape can get without the stat. Kitedemon is also right in saying that a dimmer without a thermostat would be highly unstable in a room with varying temps.

no...

dimmer goes first. It controls the temp to be under 95 degrees.

This allows a buffer in case of a power spike so the tape does not burn.
You plug the dimmer INTO the thermostat.

The thermostat is there to keep the tub from getting above desired temp.

Thermostats use external probes that are designed to measure ambient temps not surface temps. Pyrometers are designed to measure surface temps.

No is no such thing as a way to guarantee flex watt or heat cable does not catch fire. By reducing power to it, reducing its actually "on" time intervals, allowing air flow and replacing aged tape, you can try to make it as safe as it possibly can be.

Let's apply this to an engine.

The heat tape is the engine.
the dimmer is the throttle
the thermostat is the starter and kill switch.

Without the throttle control(dimmer), and engine(heat tape) when started would go to wide open throttle and run at max RPM(speed/power)

The thermostat would shut off the engine at select intervals but would not control the RPM it runs at.

So, go out and try this with your car. Fire it up and hold it to the floor and shut it off and then start it, do this about 100 times.

Or start your engine and let it idle at a comfortable level. Then shut it off and start it 100 times.

The engine MIGHT last through both tests for 100 tries. However which do you think will fail first?

I did miss what you intended. I get what you are saying but you have made two assumptions. Your scenario is great if you are using a on off t-stat and if your room temp is reasonably stable.

A proportional t-stat feathers the power and usually does not deliver 100% power or at least only rarely. The second issue is if the ambient air temps drop the rheostat will limit the max heat and if that temp is not enough to over come the rooms ambient temp the hot spot gets too cold. Racks don't do well in cool rooms so that isn't a big issue with a rack but when applied to an enclosure it can be.

Your car analogy is great but if using a proportional t-stat it is more like an accelerator (t-stat) and a governor (rheostat) restricting the flow of gas to the engine. That is great until you need just a bit more speed than normal and the rheostat will not allow it.

The new generation of t-stats are very 'smart' they are able to shut down power if they detect over temps but also under heating too. So if the probe gets pulled away from the heat source. (very real possibility with a rack) they add an extra bit of safety margin. I still recommend a dual t-stat set up especially in cool environments it allows the primary to do its job but if it fails or something happens the secondary will shut down the power to the primary.

The numbers are my own numbers and variable but this is the basic idea. Either way we are saying the same thing a T-stat should have a secondary safety system. A t-stat should have something to else in the advent of catastrophic failure.

http://images9.fotki.com/v114/photos...drawing-vi.jpgHosted on Fotki

so what happens when you have a 6 tub rack and you place your probes in 2 bins where the tape does not burn? The tape catches fire. I've seen it with 500r's, herpstats, helix and ve300's.

I would never use anything but a proportional t-stat with a rack.

suzuki4life,

are you saying that there are spots on the flexwatt that do not heat at all?

Quote:

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Originally Posted by Tzeentch

I would never use anything but a proportional t-stat with a rack.

suzuki4life,

are you saying that there are spots on the flexwatt that do not heat at all?

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use a pyromemter on a piece of flexwatt and you will see the variation yourself.