Thread Number: 79963  /  Tag: Modern Dryers
What is the difference between Automatic Dry and Sensor Dry?
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Post# 1038862   7/20/2019 at 17:31 (1,685 days old) by norgechef (Saint George New Brunswick )        

Do they both use a sensor? and Is one more accurate than the other? I would assume its sensor dry being more accurate with how common it is among dryers currently on the market. I know dryers with sensor dry have a moisture sensing strip in the drum, but what do dryers with ''Automatic dry'' use?

Post# 1038863 , Reply# 1   7/20/2019 at 17:44 (1,685 days old) by wayupnorth (On a lake between Bangor and Bar Harbor, Maine)        

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Those dryers use a thermostat to sense the exhaust temp. As the temp goes up as the load is dry, it shuts off.

Post# 1038868 , Reply# 2   7/20/2019 at 18:08 (1,685 days old) by DADoES (TX,U.S. of A.)        

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Either way refers to "auto"matic drying without specific cycle timing.

Thermostatic autodry has the air temperature thermostat control the timer. The timer does not run while the heat source (electric element or gas burner) is operating. Evaporating moisture at the beginning of the cycle keeps the air temperature down so the heat runs longer and for shorter "off" periods to reach and maintain the target temperature, and the timer runs less. Heat runs shorter and less often as the load progresses toward dryness so the timer runs more.

Sensor autodry involves a moisture sensor that controls the timer independently of the air temperature. Typically there are two metal bars in the drum connected to a low-voltage electric circuit. Wet/damp clothes brushing across the bars completes the circuit and stalls the (mechanical) timer. Same premise ... that the timer is stalled less, runs more as the load progresses toward dryness. This method also works with fully electronic controls. The control software can count the moisture hits and be programmed with different algorithms for various types of fabrics.

Thermostatic autodry can be affected by running the dryer in a cold environment .... very cold incoming air takes longer to heat to the target temperature, although the load IS giving up moisture during the interim. Moisture sensing isn't affected.

Post# 1038943 , Reply# 3   7/21/2019 at 13:21 (1,684 days old) by Supersurgilator (Indiana)        

If I had a choice I would go with the sensor dry. I've used too many dryers with typical auto dry and there is too much variability in them. I ran 2 identical loads of towels in a Whirlpool with autodry, once it shut off and the towels were still very damp. The next time the timer hadn't moved from where I set it, but when I stopped the dryer, the towels were bone dry. The autodry on the GE I have no is useless, it will seriously run for 2 hours before it thinks the load is dry. Never again, if I had the choice I would get one with the moisture sensors in the drum.

Post# 1038952 , Reply# 4   7/21/2019 at 14:38 (1,684 days old) by ea56 (Cotati, Calif.)        

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Ive had both types of automatic dryers, and Ive had better luck with the thermostatically controlled automatic dryers getting each load consistently dry, every time. And the dryers that were the most expensive, with sensor auto dry systems were the most disappointing.


Post# 1038971 , Reply# 5   7/21/2019 at 15:41 (1,684 days old) by vacerator (Macomb, Michigan)        
Oh, I am

understanding that higher end models use both a thermostat to sense temp. in the dryer, and a sensor on the drum to sense dryness in the clothing.

Post# 1038978 , Reply# 6   7/21/2019 at 16:56 (1,684 days old) by DADoES (TX,U.S. of A.)        

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ALL dryers have a thermostat (or thermistor if electronic) to control the air temperature. The heat source would otherwise never shut off for the duration of the cycle, leading to overheating of the clothes and potential fire.

Post# 1038980 , Reply# 7   7/21/2019 at 17:22 (1,684 days old) by chetlaham (United States)        

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@DADoES: if your dryer had a lower wattage heating element you would not need a cycling thermostat ;)

Auto Dry: Typically the cycling thermostat is wired such that the timer does not advance when it is closed. As the clothes loose moisture, the exhaust temperature rises. This causes the cycling stat to open and the timer to advance. As the clothes become dry the length of the open time increases the the length of the closed time (calling for heat) decreases. This helps advance the timer faster into cool down.

Auto dry is a solution to the down falls of timed dry. With timed dry its guess work and people often set the dryer above the required time because no one wants to come back and reset the timer for another 10 or 15 minutes.

Sensor dry works by having a metal strip which passes a tiny amount of electricity through the fabric. As the clothes loose moisture, the resistance of the fabric increases and less electricity can pass. At some point the resistance is high enough that the electronic control trigger the timer and it starts advancing.

In theory sensor dry is more accurate because you are actually "feeling" the clothes for moisture instead of relying on the exhaust temperatures.

Post# 1038985 , Reply# 8   7/21/2019 at 17:53 (1,684 days old) by henene4 (Heidenheim a.d. Brenz (Germany))        

Sensor drying has 2 huge downfalls though: Big items/loads and in my experience synthetic items.

Many less advanced sensing systems often only sense a very small area.
Thus, thick bulky items or very large loads sometimes under dry.

Same appears to happen with synthetic items.

That's why for many years sensor dryers over here used a circuit from either a drum paddle or a strip to the drum itself.

That was highly accurate, but more expensive to make and caused a lot of drag on the drum as there was a brush contact to the drum needed.

Post# 1038986 , Reply# 9   7/21/2019 at 17:54 (1,684 days old) by DADoES (TX,U.S. of A.)        

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It has two heating elements.

1,400 watts operates during reverse tumble.

3,600 watts during forward tumble for low (127F) and medium (140F) temp.

Both during forward tumble for high temp (149F).

5F differential from the target temp at the thermistor in the airflow outlet housing for heat to cycle back on.

(edited to remove erroneous URL entry)

This post was last edited 07/21/2019 at 21:16
Post# 1039038 , Reply# 10   7/22/2019 at 00:15 (1,684 days old) by LowEfficiency (Iowa)        

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Neither system is infallible, and both have to use some guesswork on top of sensor data to dry a load appropriately.

If you want some insight, reading patents is a great resource. For example, read some of the 1950s auto-dry patents - that's when the basic systems that were common in the 60s-70s were really born, and the descriptions are the most succinct. Everything sounds so rosy and magical - wrap it up folks, the problem is solved.

Then skip ahead to the 1980s-1990s, and read the patents on the equivalent modern systems. They will describe the new systems and their merits, going into great detail about what was so wrong with the 1950s-1970s designs, and the complex systems and tables of data it took to improve upon those results.

Remember too that "dryness" will always be subjective. Maytag struggled with this a lot in the 1970s - erring on the side of under-drying to avoid going over, but receiving frequent customer complaints for incomplete drying as a result. Both their dryness thresholds as well as machine settings and labeling were adapted over the years, in response to customer feedback, and to ultimately move more of the "automatic" system's control and configurability back into the hands of the operator.

Post# 1039045 , Reply# 11   7/22/2019 at 04:19 (1,683 days old) by Iej (.... )        

Miele has this *very* right.

The machine uses the surface of the stainless steel drum as the sensor. The connection is made via a sliding contact on the outside of the drum that allows the machine to maintain an electrical connection with the drum surface. They've been taking this approach since the 1970s, so it's very well proven electromechanical mechanically. There are no sensor pads in the drum at all. It measures anywhere the clothes touch the entire surface.

Then you can programme the sensor both with preset on the control panel and fine tune it in porgramming me menus, to your water hardness and personal preferences.

Post# 1039128 , Reply# 12   7/23/2019 at 02:09 (1,682 days old) by askolover (South of Nash Vegas, TN)        

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My 20 year old GE gas dryer has always worked great on sensor dry.  I always use low heat and everything except my big comforter and quilt dry completely and it shuts off. 

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