Sensing air pressure

[parts-man73]

I had an idea for a project, just looking for an appropriate sensor to do the job.

My idea is as follows.

Clothes dryer vents can plug up with lint. This can be bad for 2 reasons. First of all, it's a safety hazard, the lint could combust and cause a house fire. Secondly, the restriction in air flow will make it take longer for the clothes to dry, which will give you higher elecrtic bills.

My idea is to plumb a sensor into the side of the dryer vent, close to the output of the dryer that senses the air pressure inside the vent. When the backpressure builds up, that would indicate a clogged vent and an indicator light would let the homeowner know it's time to fix the problem.

I would need a sensor that could tolerate heat and humidity and measure small differences in air pressure.

Thank you,
Brian

 

[hippy]

How about just a temperature sensor in the duct near the dryer end to detect the increase in temperature which would be indicative of a blockage ?

 

[parts-man73]

There are 3 different temperature setting on this dryer(and I presume all dryers are similar), and the dryer regulates the temp thoughout the drying cycle (controlled by a thermostat, so when the proper temperature is reached, the heating coils or gas burner is shut off) so I don't think measuring the temperature is the way to do it.

 

[manuka]

As you just want to detect when air is not being vented, maybe a flow sensor would do it? Hence even something like a hanging "flag" that normally displaces out of the vertical with air flow?

But,given the nature of the task, my first thought related to humidity measuring- an easy job for Picaxes. See Peter Anderson's pages etc =&gt; www.phanderson.com/picaxe/sht71.html <A href='http://www.phanderson.com/picaxe/sht71.html' Target=_Blank>External Web Link</a> If humidity values were not falling over time then the vent would probably be blocked.

Of course you could -ah-always dry your clothes by the sun &amp; gain that &quot;healthy outdoor aroma&quot;! It's a telling cultural/climatic reflection, but here in NZ/Oz. lack of an outdoor clothes line is viewed as poor housekeeping in even millionaires row. <A href='http://www.orbit.com.au/images/870%20FoldnPlay%20Clothesline%20Out.jpg' Target=_Blank>External Web Link</a> Glass/plastic &quot;greenhouse&quot; enclosed versions make great use of low grade sun, ease an possible eyesore, keep rain at bay &amp; - ahem- save money. We've not used an indoor drier for years here- 4 kids &amp; my wife is a washing fanatic!

Edited by - stan. swan on 15/03/2006 19:49:16

 

[desulfator]

Hi Brian,

Hope I don't spoil your PICAXE fun with a low tech solution.

I've had a lot of fun using pieces of rubber roofing (rubber sheeting approx .03 inches or 0.75mm thick) to make up a bladder. You can score tons of the stuff at any roofing construction where they trim the material to fit places.

Another source for a bladder might even be something like rubber gloves cut to suit the intended lifting task.

I found a home made bladder useful for lifting light objects or it could be used as an indicator of pressure and used to trip a lever operated microswitch.

To calibrate your bladder you can offset the lifting force with weights applied with silicone adhesive to the top of the bladder.

The bladder was constructed using square pieces of rubber that were sewn together with common thread. Prior to sewing the pieces together a 1/4 inch band of liquid silicone rubber was used to seal the inside seam, thread holes and the rubber tubing leading into the bladder.

Some experimentation would be needed to see how big of a bladder you would need (lifting force opposing spring tension of the levered microswitch). They are cheap to make so you could make up a few of various sizes.

A long copper tubing from the tap at the vent outlet of the dryer to the bladder should dissapate the heat and the rubber roofing is capable of getting quite warm without ill effects.



I used a home made bladder to interface a digital programmable thermostat to a mechanical fuel oil regulator of a fuel oil heater.


The fuel oil heater had a top mount rotory control that went from LOW to HIGH. It had a shaft connected to a fuel flow regulator at the bottom of the heater.

Naturally using this mechanical control it was always either too hot or too cold in the house.

I noticed that when rotating the control from Low to High there was a corresponding movement of a pin jutting out of the regulator. When on Low it was flush. When on High it was fully extended.

When I placed the rotary control High and the pin extended I noted that the fire would reduce if I manually pushed the pin down with my finger. So I installed a heavy hinge on the regulator so that the weight of the hinge half would keep the pin down (low fire) if the rotary control was set to High.

The bladder was used to lift the hinge half up and allow the pin to rise (high fire). Air pressure was provided by an aquarium pump. There was a bleed valve that prevented excess pressure from being developed and allowed the bladder to deflate when the pump was turned off. The 120VAC pump was controlled by a relay which was connected to a digital thermostat.

When the thermostat called for heat you would hear the click of a relay and the buzz of the aquarium pump. Worked well during a cold winter and the programmable thermostat was used to automatically lower the temperature during sleeping hours.

Not to mention the savings of tightly regulating the expensive fuel oil burned.


Edited by - desulfator on 16/03/2006 01:04:31

 

[Screech]

How about using a automotive MAP sensor.
MAP = Manifold appsolute pressure.

It's just a pot. wich moves to vacum/pressure.

 

[parts-man73]

I also know of another low tech solution... A simple ping pong ball in a suitable diameter clear plexiglass tube with some holes drilled into the side of it. The air pressure would make the ball &quot;float&quot; and the higher the air pressure, the higher the ball floats in the tube. I guess I was looking for the elegant solution.

Or to be more exact...I wanted the Picaxe to be the solution, and I was trying to think of a problem that it could be the solution to. I wanted to do something different with a picaxe, other than making some sort of robot, or interfacing with an LCD screen to say &quot;hello world!&quot; (which by the way has got to be the most annoying thing, why does everyone feel the need to make the first message their LCD displays &quot;hello world!&quot;????)

 

[parts-man73]

Now the Map Sensor is a good idea. I of all people should have thought about it. I work in an Automotive Salvage yard (thus the name parts-man) And I have access to all sorts of automotive sensors. I will have to grab a few and start experimenting. I'll inform y'all of my progress.

 

[bobrayner]

Sounds like you are trying to detect a pressure differential across the filter. Such things exist with varying sensitivity (some will trip if you breath lightly on them) and are commonly used as an interlock across critical filters (such as air intake filters in high power transmitters) to shut the equipment down or prevent start up in the event of a partial filter blockage or the opening of a critical door or panel etc.
Such devices are usually VERY expensive and I am sure if such a device were cheap enough the dryer manufacturers would include it even as a selling gimmick.
cheers BobR

 

[premelec]

Whistle! They used to make plastic whistle buttons that you punched through a furnace air filter - when the filter was backed up it whistled - lots of confused homeowners

Anyhow a small whistle tapped into the duct will start letting you know when the pressure goes up - not very temperature dependent...

 

[hippy]

<i>why does everyone feel the need to make the first message their LCD displays &quot;hello world!&quot;???? </i>

It's probably part of an ISO or ANSI standard somewhere

I suspect it is because it's the closest anyone will ever come to a micro giving any illusion of having sentinent intelligence. The nearest we get to 'playing god'.

&quot;It's alive !&quot; - &quot;Feel the power ! Bow down before me&quot;

 

[Jeremy Leach]

I actually think there's still some mileage in Hippy's suggestion of looking at the temperature.

Ok, the temperature is regulated inside the dryer - but if there's a blockage in the vent then the temperature just OUTSIDE the vent won't be raised as much as it normally is. So you could monitor a temperature differential.

Temperature measurement is such an easy thing to do accurately with a PICAXE this would be my first choice - even with a couple of temperature sensors. I'd experiment to see if there is a temperature solution first.

 

[desulfator]

premelec,

I love the idea of using a whistle as an alarm. Nothing like an irritating noise to alert you that a dangerous condition exists.

-

parts-man73,

If you wanted to do something different and highly rewarding with a picaxe, why not get involved in the emerging field of vegetable oil fueled diesel engine controls.

Working in a salvage yard you are in a prime position to be able to get a vehicle with a diesel engine at a better price than most.

If you are successful, you can reap the benefit of being able to virtually drive your car for free. Not a small accomplishment in these days of sluggish economys, low competitive wages and high priced fuels.

My son bought a well known conversion kit for his 95 Suburban. Just one of the restaurants in his area would be able to freely give him 140 gallons of vegetable oil per week. That is like getting over $300 of free fuel per week.

As the old saying goes, A penny saved is a penny earned.

Nothing like getting a $300/week pay raise. The added benefit is that the increased earnings are NOT taxed.

The kit came with a DPDT center off switch which controls two direct acting, three way solenoid valves which determine the three operating modes.

Run on Diesel (center off)
Run on Veg oil
Purge Veg oil

It is up to the operator to monitor engine temps and throw the switch to Veg oil when the engine comes up to operating temperature. The operator must remember to throw the switch to Purge a short time before reaching the destination and finally return the switch to Diesel when finished.

Forgetting to switch to veg oil after warmup wastes diesel fuel.

Forgetting to purge can result in the fuel system clogging up with congealing veg oil in cold weather.

Forgetting to return the switch to Diesel can result in diesel fuel being transferred to the veg oil tank.

All of the above is likely to occur if his wife drives the car or he gets preoccupied with another matter while driving.

-

I had thought it would be an ideal task for a micro to manage the system. PICAXE could monitor engine temps via thermistors and put the system into veg oil after warm up.

It could sense when the operator turned the ignition key off and begin the purge routine and finally killing the fuel shut off valve after the system was purged.

Although the engine would continue running for a short time after shutoff, the vehicle would not be going anywhere with a locked steering column. Nor would it get far with the limited time remaining on purge before automatic shutoff.

The advantages of having a micro handling the process would be optimum use of veg oil, minimum use of diesel fuel and always having purged fuel lines for the next start up.

You could have one override switch on the dashboard for forcing the system into diesel.

A DPDT switch could kill the power to PICAXE and reconnect the fuel shut off solenoid to the vehicular wiring system.

The two issues I have not discussed is that of the heat content of veg oil and the effects of lowering the viscosity by having to pre-heat the veg oil to use in a veg oil run diesel. Also, the effects of running hot oil in a fuel system designed for ambient air temp oil.

If you are interested, I can start another post to explore the benefits of having PICAXE also control the amount of heat gain of the veg oil.



Edited by - desulfator on 16/03/2006 16:17:04

 

[parts-man73]

I thought of engine control as a picaxe project... but I was leaning toward small engines. Like perhaps adding fuel injection and electronic spark control to a lawn mower engine. I bet Fuel injection would increase the reliability and economy of a small gasoline engine. Maybe it would be marketable to landscaping companies that mow lawns all day every day as a fuel saver.

I was going to try to trigger an ignition coil and a fuel injector with a picaxe for starts.