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HubblePie t1_iy9ncvt wrote

That reminds me of one of those silent switches they used to sell.

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Prindagelf t1_iy9nocp wrote

it's not entirely accurate, the assembly housing the Mercury and contacts sit on a metal spring, of a material I can't remember, the spring flexes as the temperature changes causing the Mercury to slide from one end of the glass to the other connecting the contacts

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lorddoa t1_iy9r071 wrote

Replying to Prindagelf (#776,068)

Yep. Us po' rednecks used mercury in straws as the trigger to make our parachutes deploy in our homemade rockets. Like you said, it was about completing the circuit as opposed to thermal expansion

Just be sure to wait until the fucker is on the launchpad before hooking the wires up ;)

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RadBadTad t1_iy9rf7a wrote

This is how most/all thermostats worked before the recent digital ones. It's always neat to me, looking back and seeing purely mechanical solutions to problems that we solve digitally now.

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bhops24 t1_iy9t031 wrote

All thermostats prior to going digital use the Mercury switch.

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gilareefer t1_iy9ujkz wrote

The mercury does not expand/contract to activate these. It only acts as a switch

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Imbalancedone t1_iy9vdsp wrote

Its a bulb with mercury attached to a bimetal strip. The strip bends with temperature change and moves the bulb to make the switch action.

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jayg76 t1_iy9wf78 wrote

they all used to.

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Gambit3le t1_iy9wwat wrote

Mine is the same. It works well with my oil fired steam boiler.

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5_under t1_iy9yod8 wrote

So a thermostat?

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damaltor1 t1_iy9zufa wrote

Replying to Prindagelf (#776,068)

exactly. setting the temperature higher tips the vial to one side, and actually rising temperature tips it back to the other side. it will find its equilibrium point when set temp and measured temp are equal.

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KruxAF t1_iya3359 wrote

As someone else said, “The mercury does not expand/contract to activate these. It only acts as a switch”

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flannelmaster9 t1_iya4362 wrote

Think it's called a Mercury switch. Spring expands and contracts and the Mercury shifts completing the circuit.

Haven't seen a mercury stat in the wild in a long time. And I've replaced hundreds of stats.

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Skoodge42 t1_iya5btz wrote

Anyone else get MacGyver nostalgia when seeing a mercury switch?

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HTorv t1_iya5mmx wrote

Yep … old one for sure!!

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RadBadTad t1_iya6k1b wrote

Replying to BoringCrow3742 (#780,113)

Digital thermostats are WAY more accurate, and react a lot faster than analog thermostats. Old analog thermostats also need to be calibrated semi-regularly, as even bumping against it wrong can throw off the metal pieces inside that are used to react to the temperature in the room.

I don't know what you're referring to when you say that they "fuck up" all the time, but there isn't really any widespread issue with the accuracy or reliability of a digital thermostat, and certainly nothing related to the mechanics of actually reading the temperature of your home. Hell, they can even account for being in direct sunlight by subtracting the warmth they gain from the light. Analog thermostats can not.

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your_Assholiness t1_iya7a7y wrote

The spring is a BI- Metal strip that expands at different rates to tip the mercury switch and make contact.

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RadBadTad t1_iya8s9i wrote

Replying to Kurotan (#780,622)

Modern thermometers use a variety of different methods and sensors to determine the temperature, but some of them include, thermocouples, Resistance Temperature Detectors (RTDs), thermistors, and Solid State sensors. These all use the ideas of voltage, resistance, and current to determine what the ambient temperature is.

The technology is a lot more complex than something like a bimetallic spring and some mercury, but if you're interested there is good reading about it!

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nayhem_jr t1_iya9bjk wrote

Replying to Kurotan (#780,622)

Thermocouples measure voltage across a material that is sensitive to temperature, same thing that digital thermometers use. This allows for much more precision and faster response than that old bimetallic switch.

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Fartingonyoursocks t1_iyap6dh wrote

Replying to RadBadTad (#780,231)

What do you mean when you say analog? Mine doesn’t look like this but looks like an analog clock. It’s connected to a gas furnace in the floor. Is this the same thing? I’m just curious.

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CyberNinja23 t1_iyaub2f wrote

BiMetallic strip. The thermal expansion properties of the different metals at different room temperatures allows the metal holding the mercury to flex back a forth, ideally allowing the mercury in the tube to break the circuit or comeback to complete the circuit.

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dscottj t1_iyax999 wrote

True story: my brother and I were rural latchkey kids, left alone for hours in an (for then) advanced house. I got bored one day, I think I was twelve, my brother was ten. We had this kind of switch for our thermostat, but it was in the cover. I'd pried that thing free months before my bright idea.

I thought the giant jiggling lump of mercury was fun to watch as it went back and forth. I didn't know what a bimetallic spring was, but I did know if I touched one side of the spring it made the whole thing twist in an interesting way and the mercury ball slid back and forth.

Here's where it gets fun: we were one of the first families on the block with a microwave oven. We were alone. My brother was bored and driving me bonkers.

"I know!" I said. "Let's put this spring thing with the mercury switch in the microwave!"

Oh yes I did. I didn't count on all the sparks. A half second after I turned it off I yanked it out and that spring was wound TIGHT. Physics probably saved me from mercury poisoning that day.

There's a reason why women live longer.

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iwillneverreadthiscr t1_iyaxnw5 wrote

Nope... It has nothing to do with the temperature of the mercury; the actual sensor is a coiled piece of metal onto which the capsule containing the mercury is attached; when that coil expands, the capsule is tipped over enough to have the mercury close the circuit and power up the cooling system; as the room cools, the coil contracts, tipping the capsule in the opposite direction and breaking the circuit.

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skittlebog t1_iyaykiz wrote

That was a standard design. They would last for decades, because the contacts did not wear down. My grandfather's house had one of this design that kept working for 60 years.

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UnReAl0 t1_iyb0kp2 wrote

Replying to Prindagelf (#776,068)

The really cool part is in the center dial with the numbers. Thats the heat anticipator that helps increase or decrease temperature overshoot in the room. Technology and stuff

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LivingGhost371 t1_iyb1v5i wrote

House has been in the family for 50 years. The first 30 we had a mercury thermostat. It was still working when my parents ripped it out to put a digital one in, which needs new batteries every year and the unit itself seems to need replacing every five years.

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seanmorris t1_iyba390 wrote

Na, its using a bimetallic strip to move the glass in the tube. The mercury slides back and forth, and makes contact with the conductors only one one side.

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ftminsc t1_iybcja0 wrote

Since it’s been covered, I’ll just add that based on the coefficient of thermal of expansion of mercury, a 10mm blob of mercury at 60F will become a 10.003mm blob of mercury at 70F. So while you could design a thermostat based on thermal expansion of a mercury blob, it would have to be seriously precise, and resistant to arcing.

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RJFerret t1_iybes2y wrote

All of our thermostats work with bimetalic strips prior to digital battery ones.

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gilareefer t1_iybffpy wrote

Replying to D_daKid (#791,039)

Because it works well for this type of switch. Mercury is used as the conductor material in the switch... As the spring it's attached to heats/cools, the glass tube tilts & rolls the mercury into "on" position

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dwarfgiant6143 t1_iybhyn7 wrote

I usually change those out when I can. They worked great for their time, but now people want them gone.

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crispy1989 t1_iybk11u wrote

Replying to D_daKid (#791,039)

/u/gilareefer explained how the mechanism works, but to answer your question more generally, this type of switch mechanism is used because it's very resistant to oxidation/corrosion, and because a traditional mechanism used with a bimetallic strip would be prone to arcing.

The temperature-sensitive part is a bimetallic strip shaped in such a way so as to "tilt" one way or the other based on temperature due to the difference in the metals' thermal expansion coefficients. A simple switch mechanism based on this might be to just allow the strip (or a contact attached to the strip) to touch another contact in a certain position; but this type of switch is prone to corrosion over time due to being exposed to the elements. The complicated liquid-metal-drop-inside-sealed-vial mechanism prevents corrosion because there's no oxygen inside the vial.

Additionally, a simple switch contact mechanism used here would likely cause arcing and damage to the contacts since the switch would open and close extremely slowly. Things like normal light switches prevent this by integrating a "snap" mechanism to very quickly open and close the circuit; but a bimetallic strip supplies very little force, not enough to operate such a snap mechanism. The liquid-metal-inside-sealed-vial mechanism solves this problem by a) surface tension keeps the drop of mercury together so once the vial tilts even a little the whole drop will quickly slide over and complete the connection; and b) any damage due to arcing would be minimized in a low-oxygen environment inside the vial.

For both of these reasons, mercury metal is used because it's the only metal that's liquid across the usual range of human environmental temperatures, so is the only metal suitable for use in this type of switch mechanism.

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leannerae t1_iyblwh0 wrote

Replying to BoringCrow3742 (#780,113)

I'm on your side, I've come home twice to a cold house because the batteries died. Another fancy programmable one just quit working and again, house was cold. I bought one of those old fashioned round analog ones and I love it.

Before anyone mentions it: I am capable of remembering when to turn it up or down to save energy and I don't want one that will email me when it's time for new batteries

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cipherd2 t1_iybod33 wrote

No, it doesn't. You're getting warmer, though. Pun intended.

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Funny-Record-5785 t1_iybolp6 wrote

Silent switches were sick too sucks they took them off the market

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Gunch_Bandit t1_iybrfzu wrote

Tell us you've never removed the cover of the thermostat without telling us.

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kneecapped33 t1_iybv4xp wrote

Mercury is viscus enough of a liquid to not bounce back and forth on the relay

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Professional-Eye8981 t1_iybvq1s wrote

The mercury doesn’t expand and contract. The mercury vial is mounted on a bi-metal coiled spring that causes the vial to tilt during a temperature change. When it does, the mercury blob rolls to the other end of the vial and opens (or closes) the circuit.

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Dylsnick t1_iybw9gf wrote

HVAC tech here. You're all wrong. It's magic. Don't ask questions, just pay us to wave our magic wand and fix shit.

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mr78rpm t1_iybzrke wrote

No, it for sure ridiculously does not.

The mysterious "bimetallic strip" consists of two pieces of metal bonded together. The metals do not expand or contract at the same rate as one another, so when the air in the room changes temperature, the bimetallic strip bends, shifting the position of the glass bulb and hence the mercury... when the mercury moves such that it connects together the two wires at the left side of the bulb, the heater comes on.

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Ok-Drink-1328 t1_iyc2f0w wrote

it's a mercury switch, it works by leaning on one side or the other (mechanically), when the mercury touches both contacts inside the glass vial the contact is formed and stuff turns on

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LogicallySound_ t1_iyc93fo wrote

There is something quintessentially “Reddit” about someone showcasing very old technology as if it were new and confidently describing it’s function incorrectly as if Google didn’t exist.

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InterestedDudeFr t1_iyccs4f wrote

bro went to far to science lessons, that's actually impressive but also not that much accurate in my opinion

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snuffy_tentpeg t1_iycd87y wrote

When I upgraded my thermostat I took the ampule of mercury to the waste management personnel at my factory for disposal.

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Buschwick66 t1_iyeehmw wrote

Um. That's called a mercury switch. And that's not how they work.

I took one years ago and tied it up at just the right angle in a buddy's engine bay. I had the switch completing the 12v circuit for the horn when the front end nose dived while braking.

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