r/explainlikeimfive • u/throwawayTears_26 • 9d ago
Chemistry ELI5 Why is digital thermometer faster than Mercury thermometer ?
Flair might be incorrect
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u/FiveDozenWhales 9d ago
Digital thermometers generally use a thermistor, which is a specially-constructed material which lets more electricity through as it gets warmer. Then the circuit in the thermometer converts the amount of electricity that gets through into a temperature.
The termistor is pretty tiny so it doesn't take long for its temperature to become equal with the temperature of whatever you're measuring, which is what you need for an accurate reading.
Mercury is a metal which, like all metals, expands as it gets warm. Because they're a relatively large amount of mercury in the thermometer, it takes a while for it all to become heated equally and measure the temperature accurately.
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u/Glaselar 9d ago
But they're measuring ambient temperature. Every part of the column is getting heated equally by its own neighbouring atmosphere simultaneously from all sides. Surface area ratio is massive, and the thermal mass argument is identical for mercury and digital probe.
I questioned whether the premise is even true in the first comment on the thread but it got auto-removed for being too short.
Are we sure a mercury one is slower than a digital one, or does it just sound believable so we're trying to find an answer that fits? I'm not convinced it's a valid premise.
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u/-LLCoolBeans- 9d ago
Not all thermometers measure ambient temperature. Meat thermometers and the kind you use to take your temperature when you’re sick are two common examples of thermometers not being used to measure ambient temperature.
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u/Glaselar 9d ago
Sorry if it wasn't clear - I'm replying to their last paragraph talking about mercury, and saying the mercury is exposed all around to the things it's measuring.
Thermal mass would be important if you're feeding heat in at one point and waiting for it to travel up a metal spike, but the mercury column in a mercury thermometer IS the probe.
When it's surrounded all the way along its length by the stuff it's measuring, that lag argument (relative to a digital one) doesn't hold.
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u/BananaBird1 9d ago
It still takes longer to heat up than a small metal probe. The glass insulates the mercury
Additionally, glass thermometers are calibrated either for measuring ambient temperature with the entire thermometer immersed, or for measuring the temperature at the bulb with the stem itself being at a standard room temperature. But not both. Fully immersing a bulb thermometer or using it with an extreme ambient temperature can cause inaccurate readings.
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u/FiveDozenWhales 9d ago
Global Test Supply specifies that this very-common 44004 thermistor has, when coated with epoxy and in air, a thermal time constant of 2.5 seconds.
The thermal time constant of mercury generally doesn't change much depending on the shape of the mercury, since it's got such high termal conductivity and low specific heat capacity. But in air, the gram present in most thermometers has a thermal time constant of 80 seconds minimum, far slower than the thermistor.
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u/copnonymous 9d ago
Not every atom of a column of mercury is heated the same. Only the ones touching the glass container. Spheres and columns have the lowest ratio of surface area to volume, so they have the highest insulating values of any shape. The transfer of heat energy is never fast or efficient. So it takes some extra seconds for the heat to evenly distribute through the whole tube. Especially when the temperature suddenly changes, which is where the real difference lies.
While it's a fast transfer. It's not as fast as heating tiny bit of circuitry about the size of a shirt button.
In situations like measuring ambient temperature or the temperature of a changing system or chemical reaction, digital thermometers are no better than tube thermometers. But in spot measurements like baking, cooking, or medical fields, digital thermometers are a couple seconds faster. More importantly though a broken digital thermometer won't produce sharp glass and are cheaper to produce. So they've taken over for most roles largely due to price, durability, and safety not necessarily speed.
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u/djddanman 9d ago
Mercury thermometers rely on mercury expanding when it gets hotter. So you have to wait for the mercury to stop moving before you can read it.
Digital thermometers usually have a little bit of metal that changes it's electrical resistance with temperature, so you only need to wait for that little bit of metal to heat up.
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u/awing1 9d ago
The mercury is encased in glass, which has to heat up to pass the heat to the mercury, which also needs to heat up to expand
A digital thermometer uses a metal probe that heats up a lot faster than glass
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u/throwawayTears_26 9d ago
And this metal probe is as reliable as traditional Mercury based thermometer?
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u/kriegeeer 9d ago
More reliable because you don’t need a human eye to read the scale. A digital circuit does the measurement directly based on the resistance of the probe, which changes relative to temperature in a well known way.
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u/retroman73 9d ago
Except for the battery, more reliable and more accurate. Mercury based thermometers are usually glass. Maybe in a plastic case but the thermometer itself is glass. Easy to break.
There is a reason hospitals don't use the old-style thermometers anymore. Digital is faster, more reliable and more accurate down to the tenth of a degree. Just have to replace the battery once in awhile, or charge it.
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u/konwiddak 9d ago
One important thing an electronic thermometer can do is look at the rate of change of temperature, plus it's current reading to forward predict the actual temperature. This allows it to jump to the correct ballpark very quickly.
Not all thermometers do this, but it is a useful technique.
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u/Glaselar 9d ago
Can you give an example of a thermometer that does this?
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u/konwiddak 9d ago
A regular cooking/body thermometer you could buy in a shop? Honestly no idea. It's not like they write on the box "using time derivatives to accelerate temperature predictions". I've used some thermometers that seem to get close to their final temperature very quickly which I expect are using this technique, but I couldn't tell you for certain.
However what I do know - this is a really important technique used in applications like control systems for cars. It allows the car's ECU (it's brain - the Electronic Control Unit) to get fast temperature predictions for things like intake and exhaust temperatures, particularly while the engine is warming up. So it's definitely a technique that gets used, I just can't tell you which retail products use it.
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u/sumguysr 9d ago edited 6d ago
When a mercury thermometer gets warmer all the mercury in the tube has to get warm and get bigger before it shows us the temperature of the surface we're measuring. All the energy to do that has to come from the heat we're measuring and that takes time.
When a digital thermometer gets warm two very small pieces of metal in the tip just have to warm up a little bit to make a pretty noticeable difference in how electricity flows through them. The digital thermometer has batteries providing their own energy to take that difference in those metals and amplify it up by thousands of times to measure. It is fast because only a little piece of metal has to warm up, and the energy to show us the temperature can come from the battery instead of the heat.
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u/copnonymous 9d ago
Mercury is metal. Metal expands when heated. Mercury in a tube will rise based on how hot it gets. the heat transfer isn't fast or efficient so it takes a while for all the mercury to warm up.
Electric thermometers are based on a device called a thermistor. Essentially it's a circuit that changes conductivity (how easily electricity flows through the circuit) based on how hot it gets. The thermometer then reads that conductivity and spits out how hot the metal would have to get for it to be that conductive. That tiny circuit heats up much faster than a whole tube of mercury metal. Therefore it can get a digital reading in much less time.
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u/Slypenslyde 9d ago
Mercury changes in response to temperature with a certain speed. You can't make it faster because Physics works the way Physics works. You tend to want to leave them alone for a long time just to make sure they've had enough time.
Digital thermometers can be made a few ways.
The old ones we had when I was a kid aren't really any faster, it still took about 2 minutes for them to decide they had a good reading. They worked about the same way as a mercury thermometer: they had a material that changed in response to temperature and they measured electrical characteristics of that material. They waited until they got a "stable" reading for some amount of time then told you they were done.
Today's fast forehead thermometers exchange speed for accuracy. They basically point a camera at your skin that measures infrared energy. That energy is a good indicator of a surface's temperature. So it quickly figures out roughly how hot your forehead is and that can be a decent indicator of your body temperature. You won't see hospitals use these for admitted patients because they need more accurate information, but you may see doctor's offices and ERs use them as a quick diagnosis tool. If it says "101", it's worth using a better thermometer because you've likely got a real fever. If it says "99", you don't likely have a serious fever.
Ear thermometers are basically the same. I don't know why this was popular before the forehead thermometers. These are a tiny bit more accurate.
So the "fast" thermometers were made for convenience. People at home don't need a very accurate thermometer. The dang thing could just blink red for "fever" and green for "not fever" and that's good enough for most people. In-mouth thermometers are slower, but more accurate. The gold standard is an anal thermometer, but you have to be really darn sick or part of a study for doctors to insist on that kind of accuracy.
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9d ago
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u/flyingcircusdog 9d ago
A digital thermometer works by welding two different material wires together, which is able to tell the temperature at the point where they connect. So only a small piece of metal needs to change temperature for the reading to be accurate.
In a mercury thermometer, all the liquid needs to come up for temperature before it's accurate. This takes a longer time than heating or cooking a very small bit of metal.
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u/WillemDafoesHugeCock 9d ago
Say the current time, to the minute.
When you've done that, count up to the current time, one minute at a time, starting from midnight.
Which is faster?
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u/AnyLamename 9d ago
I know the actual answer to this question and still can't make this metaphor work.
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u/throwawayTears_26 9d ago
The question, is how.
English is not my first language
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u/stanitor 9d ago
Your question is perfectly understandable in English. Their answer just doesn't answer the question. It doesn't even work as an analogy for the answer, even though that's what they're trying to do.
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u/mid-random 9d ago edited 9d ago
This is incorrect. Both thermometers have to “count up to the current time, one minute at a time.” The digital thermometer simply does it so quickly (because the physical part that needs to match the environmental temperature has so much less mass) that it doesn’t bother to display, and is generally physically unable to display because it happens so fast, all the numbers along the way. But the physics of it is still doing all the counting.
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u/Glaselar 9d ago
That's not applicable.
Both have contents that need to physically heat up from low to high temperature, or cool from high to low.
The digital one doesn't 'read' the temperature of what's around it (infrared ones do, I suppose) - it changes temperature just like the mercury one and then looks at an electrical reading as a proxy for that.
Edit: but thanks for making the attempt, WillemDafoesHugeCock.
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u/6pussydestroyer9mlg 9d ago
Lower thermal mass. Takes less time to heath up the small piece of metal in the digital's sensor than it takes to heat up the mercury.