If you’re in any process plant that runs distillation—think refineries, chemical factories, even craft breweries—chances are you’ve got a lot of thermometers around. And if you’ve ever stopped to think about which ones work where, you might’ve wondered: can a bimetallic thermometer be used in a distillation column? As someone who’s supplied bimetallic thermometers for 10+ years, I get this question all the time, and it’s not a straightforward yes or no. Let’s break it down like we’re chatting over a coffee after a long shift, no stuffy engineering jargon to trip us up. A Bimetallic Thermometer
First, let’s refresh what a bimetallic thermometer actually is, because if you’re in distillation, you might’ve used one a hundred times without noticing. It’s the little round dial thermometers you often see mounted on pipes or vessels, right? Inside, there’s two different metals bonded together—like steel and copper, or steel and brass. Different metals expand at different rates when heated, so when the temperature changes, the strip bends, and that turns a needle to show the temp. Super simple, no electricity needed, just mechanical. That’s the big sell for a lot of people: no power, no fancy wiring, just reliable, low-maintenance temp readings.
Now, distillation columns. These are those tall, giant towers where you separate liquids based on their boiling points—say, turning crude oil into gasoline, or making ethanol from water. The column has different sections: the top is where the light, low-boiling stuff rises, the bottom is heavy, high-boiling stuff sinks. Each section has a specific temp range, and that temp is make-or-break. Mess up the temp even a little, and your product’s off-spec—too much water in the ethanol, or leftover heavy compounds in the gasoline.
So, can a bimetallic thermometer handle that? The short answer is: most of the time, yes, if you spec it right, but there are caveats. Let’s start with the things it does well in distillation columns. First, temp range. Most bimetallic thermometers we supply go from around -50°F to 1000°F, which covers 90% of what you’ll see in a standard distillation column. The top of a distillation column, where the lighter hydrocarbons boil, might be around 150°F to 300°F—total cake for a bimetallic. Even the bottom, where the heavy stuff is at its boiling point, rarely hits over 800°F, and our high-temperature bimetallic strips handle that no problem.
Then there’s the environment. Distillation columns can be harsh: there’s corrosive vapors (like hydrochloric acid in some refining processes), maybe condensation, sometimes vibration from pumps or the column itself. Bimetallic thermometers are way more rugged than, say, a mercury thermometer (which is banned in most plants now anyway) or even a basic RTD. You don’t have to worry about a broken bulb if someone bumps it, because it’s a solid metal strip inside. We build ours with 316 SS stems and cases, which stand up to most common distillation corrosives—no extra coatings needed for standard use.
Another big win: cost and ease of installation. A bimetallic thermometer is way cheaper than a digital temp transmitter, especially for lower-precision applications. And since it’s mechanical, you don’t need to run power to the top of a column that’s 100 feet in the air. Just stick the stem through the wall of the column, tighten the fitting, and you’re done. No calibration software, no batteries to replace every year—just check it once a year, maybe, and it’s good to go. We’ve had clients use these on the side columns of their distillation units for years, and they rarely have issues.
But wait—there are times when a bimetallic thermometer won’t cut it, and that’s where people get confused. Let’s talk precision. If you’re running a high-purity distillation—like making pharmaceutical ethanol, or separating specialty chemicals where a 1°F error means your product is worthless—bimetallic might not be accurate enough. A standard bimetallic has an accuracy of around ±1% of the full scale. So if you’re using a 0-500°F thermometer, that’s ±5°F. For some processes, that’s too much. RTDs or thermocouples can get you ±0.5°F, which is what you need for those tight specs.
Then there’s the issue of temperature lag. The bimetallic strip is inside the stem of the thermometer, right? If the column has fast-changing temp—like if you’re adjusting the reboiler heat and the temp shifts quickly—the bimetallic strip takes a second to catch up. It’s not instantaneous. If you need real-time temp data to adjust your process in seconds, that lag can throw things off. RTDs and thermocouples respond way faster, so they’re better for control loops where the temp reading is sent to a PLC. Bimetallic is more for local monitoring, like a dial on the column so an operator can walk by and check it quickly.
Vibration is another thing. Most bimetallics hold up, but if the column is super vibratory—like in a refinery with lots of large pumps nearby—the internal gear that turns the needle can wear out faster. We make heavy-duty versions with a reinforced movement for high-vibration areas, but even that has limits. If vibration is constant and severe, a digital transmitter is more reliable.
Wait, also: deep insertion. The inside of a distillation column, especially the trays or packing, has temp differences from the wall to the center. If you’re only inserting the stem 2 inches into the column, you might get a reading from the wall, not the actual process fluid. Bimetallics need enough insertion depth to get a good sample. The rule of thumb is 10 times the stem diameter—so a ½-inch stem needs 5 inches inserted. If you skimp on insertion depth, your reading will be wrong, no matter what type of thermometer you use, but it’s a common mistake with bimetallics.
Let me throw in a real example to make this concrete. Last year, a craft distiller reached out to us. They were making their own whiskey, and their distillation column’s top section was running at 173°F—right at ethanol’s boiling point (172°F, for those who don’t know). They were using a basic dial thermometer that was off by 8°F, so they were losing a lot of ethanol to the tails because they couldn’t tell when the temp hit the right point. We sent them a bimetallic with ±1% accuracy, a 0-250°F scale, and a 6-inch stem inserted 5 inches into the column. Now their readings are spot on, no more lost product, and it cost them 1/3 of what a digital RTD would’ve. That’s exactly where bimetallics shine—simple, local, cost-effective monitoring for distillation columns.
Another client: a small chemical plant that separates benzene from toluene. Their column has three sections: top (150°F), middle (210°F), bottom (310°F). They used bimetallics on all three, because they just need to check the temp when they do their daily rounds. The plant has a control system for the reboiler, but the operators want a visual check right there on the column. The bimetallics have been working for 5 years, no calibration, no breakdowns. They say if they had used digital ones, they’d have to fix wiring issues every few months, which is a pain on a 50-foot column.
So what’s the bottom line? If you’re using a distillation column for general, non-high-purity separation—food-grade ethanol, basic chemicals, fuel products—a bimetallic thermometer is totally fine, even better in some cases, because it’s low-maintenance, no power needed, cheap. But if you need super precise temp control for high-purity products, or you’re using the temp reading as part of your automated control system, you’ll want a different type. Even then, we still supply bimetallics as a backup—operators love having that dial right there.
Now, as someone who’s in this business, I’ll be honest: there’s no one-size-fits-all for thermometers in distillation. The key is knowing your process specs. If you’re not sure whether a bimetallic will work for your column, don’t guess. We’ve helped hundreds of distillation operators pick the right thermometer over the years, whether it’s a basic bimetallic or a more high-precision option. We don’t push expensive stuff you don’t need, because we’ve learned that the best part of our job is solving a real problem for you, not making a quick sale.
If you’re looking to replace old thermometers on your distillation column, or you’re building a new unit and aren’t sure which temp sensors to spec, reach out. We can walk through what you need, no pressure, no confusing sales pitches. Just tell us your column’s temp ranges, the environment (corrosive? vibrating?), and whether you need local monitoring or automated control, and we’ll point you to the right bimetallic thermometer for the job. It’s way better than having a temp reading that’s off and messing up your distillation runs.
Resistance Temperature Detector References:
- Perry, R. H., & Green, D. W. (2008). Perry’s Chemical Engineers’ Handbook (8th ed.). McGraw-Hill.
- Bimetallic Thermometer Basics. (2022). Instrumentation and Process Control Association.
- Distillation Column Temperature Control Guidelines. (2021). American Institute of Chemical Engineers.
Jiangsu Zhaolong Electric Co., Ltd.
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