Bimetal Thermometer

What is a Bimetal Thermometer

A bimetal thermometer is a mechanical temperature indicator that converts a change in temperature into the rotation of a pointer over a graduated dial, using nothing but the physical expansion of metal. It belongs to the broad family of dial thermometers alongside gas-actuated and liquid-in-glass instruments, and it is the most common direct-reading temperature gauge in the field because it is rugged, self-powered, and inexpensive. Where a thermocouple or RTD answers the question "what does the control system see," a bimetal thermometer answers "what can the operator read at the equipment right now."

The heart of the device is the bimetallic strip: two metal layers with markedly different coefficients of linear thermal expansion, metallurgically bonded face to face. The low-expansion layer is almost always Invar, a nickel-iron alloy of roughly 64 percent iron and 36 percent nickel whose expansion coefficient is near 1.2 micrometres per metre per kelvin, an order of magnitude lower than ordinary steel. The high-expansion layer is a brass, nickel, or stainless alloy selected for the working range. Because the two layers cannot slide relative to each other, any temperature change forces the bonded strip to curve toward the low-expansion side, and that curvature is the raw signal.

To turn curvature into a usable rotation, the strip is wound into a coil and fixed at one end. As the metal heats, the coil tightens or relaxes and the free end rotates a central shaft that carries the pointer. The amount of rotation per degree is set by the coil geometry, the alloy pair, and the strip thickness, all of which the manufacturer tunes to spread the chosen scale range evenly around the dial. The completed assembly is sealed inside a stainless steel stem so the element is protected from the process while the dial and pointer sit in a weatherproof case.

The physics dates to 1759, when clockmaker John Harrison used a bimetallic strip to compensate his marine chronometers for temperature. The bonded-strip principle later underpinned thermostats, circuit breakers, and the industrial dial thermometer that took shape in the early twentieth century. The modern process thermometer is codified in the United States by ASME B40.200, the consolidated standard that absorbed the older B40.3 for bimetallic actuated thermometers, and in Europe by EN 13190. These documents fix the accuracy grades, dial markings, over-range behaviour, and test methods that let buyers compare instruments from different makers on equal terms.

Four engineering attributes decide whether a bimetal thermometer is the right tool: the temperature range to be covered, the accuracy grade required, the resistance to vibration and over-range, and the compatibility of the wetted stem with the process medium. Get these four right and the instrument runs for a decade with no power and almost no maintenance. Get them wrong and the symptoms are familiar: a pointer that flutters off a vibrating pump, a zero that has crept after an over-temperature excursion, or a corroded stem that has perforated into the line.

Element Types and Mountings

Bimetal thermometers are classified two ways that matter at purchase time: by the geometry of the sensing coil, and by how the dial is oriented relative to the stem. Both choices are driven by where the instrument is installed and from what direction the operator must read it. The table below compares the two coil geometries on the attributes that decide which one a maker uses for a given product line.

Helical elements wind the bonded strip into a long cylinder coaxial with the stem, much like a slender spring. This distributes the active sensing length down the immersion stem, which suits the long stems used to reach the centre of a pipe or the depth of a tank. The helix gives a comparatively linear and wide-range response, which is why it dominates process-grade thermometers from makers such as WIKA, Ashcroft, and Trerice. A helical instrument from the WIKA TG54 family, for instance, offers insertion lengths from 63 to 1,000 mm (2.5 to 39 inch) on a single product platform.

Spiral elements wind the strip flat, in a single plane, like a watch mainspring. The result is compact and shallow, which suits short-stem gauges, surface-clamp thermometers, and the low-cost domestic and HVAC dials where deep immersion is neither possible nor needed. Spiral construction is cheaper to manufacture but covers a narrower range and is more sensitive to mounting stress, so it is uncommon in heavy process duty.

The second classification is the connection and reading orientation, which the standards and configurators describe with a short vocabulary. The choice is dictated by whether the operator stands in front of the pipe or above it, and whether the stem enters from the back or the bottom of the case.

The adjustable angle or "every angle" variant is the most flexible: a jointed connection lets the dial pivot up to 90 degrees and rotate 360 degrees relative to the stem, so the face can always be aimed at the person reading it regardless of how the stem enters the line. This costs more and adds a sealed joint, but it removes the classic field problem of a thermometer installed where the dial points at a wall. A further reading aid common on process instruments is the dished, anti-parallax dial, recessed so the pointer sits close to the scale and the reading does not shift with viewing angle.

One more practical feature distinguishes process-grade from budget instruments: an external recalibration adjuster. Better thermometers, including the WIKA TG53 and TG54, provide an external zero-point screw on the back of the case so a technician can reset the reference against a known temperature without opening the instrument. Budget gauges lack this, so once they drift they are simply replaced.

Accuracy Grades and Standards

Accuracy is where bimetal thermometers are most often misunderstood at purchase. Two standards govern the market: ASME B40.200 in North America, which consolidated the earlier B40.3 for bimetallic actuated thermometers, and EN 13190 in Europe. Both express accuracy as a percentage of the full scale span, not as a fixed number of degrees, so the same percentage translates into a wider error band on a wider scale range. The table below lists the grades a buyer will actually see on datasheets and the tolerance each carries.

The vast majority of industrial bimetal thermometers ship as ASME B40.200 Grade A, plus-or-minus 1 percent of span, or the equivalent EN 13190 Class 1. On a 0 to 100 degrees Celsius scale, 1 percent of span is plus-or-minus 1 degree Celsius; on a 0 to 600 degrees Celsius scale, the same 1 percent is plus-or-minus 6 degrees Celsius. This is the single most common cause of disappointment: a buyer reads "1 percent" as "1 degree" and then finds the error grows with the range. The lesson is to size the scale range as tightly as the process allows, because a narrower span makes the percentage tolerance shrink into a smaller absolute error.

A subtle but decisive point appears on every EN 13190 dial: the stated accuracy is guaranteed only inside the marked measuring range, not across the full printed scale. WIKA prints two triangular marks on the TG54 dial to show the start and end of the guaranteed measuring range; readings between those marks meet the class tolerance, while readings near the very ends of the scale do not. This is why an experienced engineer keeps the normal operating temperature roughly in the middle third of the dial, where both the accuracy and the angular resolution of the pointer are best.

A premium grade exists but carries trade-offs. Grade AA, the tightest letter grade in ASME B40.3, holds a plus-or-minus 1 percent mid-scale tolerance while also tightening the permissible error near the dial ends, where ordinary Grade A is allowed to drift wider. It is offered on larger dial sizes, typically 5 inch (127 mm), because the finer accuracy demands a longer pointer sweep and finer graduations that a small dial cannot resolve. Even at Grade AA, a bimetal thermometer remains a local indicator: it cannot approach the 0.1 to 0.5 degree Celsius accuracy of a calibrated platinum RTD, and it cannot transmit a signal. When the application genuinely needs both tight accuracy and a remote reading, the correct answer is an RTD or thermocouple, not a higher thermometer grade.

Two further specifications round out the accuracy picture. Repeatability and hysteresis in a well-made bimetal element are small but real, caused by friction in the pointer bearing and mechanical play in the coil; this is the practical reason a thermometer should be gently tapped before reading. Response time is slow compared with electronic sensors, on the order of tens of seconds to a minute for the pointer to settle within the immersion medium, and longer still when the stem sits inside a thermowell. A bimetal thermometer is therefore unsuitable for fast transients and is best on steady-state or slowly changing processes.

Materials, Thermowells, and Media

Because the bimetal element is sealed inside the stem, only two material families touch the world outside the instrument: the case and window that face the environment, and the wetted stem and process connection that contact the medium. Process-grade thermometers are built almost entirely from stainless steel for both. The WIKA TG53 and TG54 use stainless steel 304 as standard for case, ring, stem, and process connection, with stainless steel 316 (or 316L on the TG54) offered as an option for more aggressive media. The window is instrument glass, laminated safety glass, or shatterproof polycarbonate depending on the safety requirement.

Stainless steel 304 is the default wetted material and handles water, steam, air, oils, and the majority of mild process media. Stainless steel 316 and 316L add molybdenum for better resistance to chlorides and moderately corrosive chemicals, and 316L's low carbon content suits welded sanitary fabrications. For genuinely aggressive media, the limiting factor is the thin-walled stem, which is why most corrosive applications protect the thermometer inside a thermowell of an upgraded alloy rather than exposing a bare stem. The table below summarises the common stem and case material choices.

The thermowell is the single most important accessory for process duty. It is a closed-end protective tube, threaded or flanged into the line, into which the thermometer stem slides. Its first job is to let the instrument be removed or recalibrated without draining the line or breaking containment, which is essential because the bare WIKA TG54 stem is rated to only about 25 bar static. Its second job is to shield the stem from pressure, corrosion, abrasion, and flow-induced vibration. Thermowells must be sized by a wake-frequency calculation per ASME PTC 19.3 TW so that vortex shedding from the flow does not resonate with the well and fatigue it; this is a mandatory check on high-velocity or high-density flows.

The cost of a thermowell is heat-transfer lag. The well adds an air gap between the medium and the stem, slowing the already modest response of the bimetal element. Two remedies are common: fill the well bore with a thermally conductive paste, or specify a spring-loaded stem tip that presses against the well bottom for direct metal-to-metal contact. For sanitary and food service, the wetted parts are upgraded again: 3-A authorized models use electropolished 316L stems and Tri-Clamp connections so the assembly can be cleaned in place, and gaskets are FDA-compliant elastomers rather than the standard process seals.

For vibration, the answer is not a material but a fill. A liquid-filled case, charged with silicone or glycerine, damps the pointer so it does not flutter on pumps, compressors, and engine skids, and it slows wear on the pointer bearing. The WIKA TG54 offers a silicone case filling usable up to about 250 degrees Celsius at the probe; some makers instead damp the coil itself with a non-silicone lubricant rated to roughly 200 degrees Celsius. Liquid filling does narrow the allowable case ambient, typically to about -40 to +70 degrees Celsius, so the filled and unfilled versions carry different environmental ratings.

Key Specification Parameters

A bimetal thermometer datasheet lists a dozen or more fields, but only eight truly drive a selection decision: dial size, scale range, accuracy grade, stem diameter and length, process connection, case and stem material, ingress protection, and over-range and pressure rating. Each is decoded below, with representative values drawn from the WIKA TG53 and TG54 process catalogues.

Dial size (nominal size) sets both readability and the achievable accuracy grade. Standard sizes are 63, 80, 100, and 160 mm, marketed in inches as 2.5, 3, 4, and 6 inch; North American lines add a 2 inch and a 5 inch (127 mm) dial. A larger dial gives a longer pointer sweep, finer graduations, and the room needed for tighter grades, so a 5 inch dial is the usual home for premium Grade AA. A smaller dial is chosen when panel space is tight and a 1 percent indication is good enough.

Scale range is the printed span of the dial, chosen from a catalogue of overlapping ranges. The TG54 family alone offers ranges from -70 to +70 degrees Celsius up to 0 to +600 degrees Celsius, with matched Fahrenheit ranges from -100 to +150 up to 200 to 1,000 degrees Fahrenheit. The selection rule is to keep the normal operating temperature in the middle third of the scale: this maximises both the percentage-based accuracy and the angular resolution of the pointer, and it leaves headroom below the over-range limit for excursions.

Stem diameter and length set the mechanical fit and the immersion. Process stems are commonly 6 or 8 mm (0.24 or 0.31 inch) in metric lines, and a quarter inch (0.250 inch, 6.35 mm) in North American lines, with insertion lengths from 63 to 1,000 mm (2.5 to 39 inch). The stem must immerse far enough that the active coil sits fully in the medium, usually a minimum of the coil length plus a margin; too short an immersion produces a stem-conduction error where heat leaks out along the stem and the reading falls low.

Process connection is the thread or fitting that seals the stem into the line. The TG54 offers plain unthreaded stems plus G 1/2 B, 1/2 NPT, M20 x 1.5, and M24 x 1.5 in male and female forms, with union-nut and compression-fitting variants for adjustable insertion. Sanitary lines substitute Tri-Clamp connections. The connection must match both the mating fitting on the line or thermowell and the pressure rating of the service.

Ingress protection (IP code) per IEC 60529 describes the case sealing against dust and water. Process thermometers span IP65, IP66, IP67, and IP68; the TG54 reaches IP68 rated for continuous immersion to 5 metres (16.4 feet). Outdoor, washdown, and submerged installations need IP67 or higher, while a sheltered indoor panel can accept IP65.

Over-range and pressure rating protect the instrument from excursions. Bimetal elements tolerate limited over-temperature: on the WIKA TG54, scale ranges ending at or below 120 degrees Celsius typically allow 100 percent over-range, ranges from 120 to 200 degrees Celsius allow only about 50 percent, ranges ending between 280 and 400 degrees Celsius are capped at about 430 degrees Celsius, and the highest ranges are limited to the full scale value. Exceeding the over-range plastically shifts the coil and permanently offsets the zero. On the pressure side, the bare stem of a TG54 is rated to about 25 bar static maximum, which is the practical reason higher-pressure service is routed through a rated thermowell.

Selection Decision Factors

To turn the preceding five chapters into a specific model and order code, work through the decision sequence below. Most selection errors come not from a single wrong field but from deciding a downstream field before an upstream one is settled, so follow the order. These eight steps double as a reusable RFQ template.

1. Confirm a dial thermometer is the right tool: a bimetal thermometer is correct when you need a self-powered local reading and a person is present to read it. If the value must reach a PLC, DCS, or logger, or if accuracy better than about 1 degree Celsius is required, specify an RTD or thermocouple instead, or pair a thermometer with one in the same well.
2. Scale range: choose the printed range from the catalogue so the normal operating temperature falls in the middle third of the dial. Verify the medium peak stays below the over-range limit, and prefer the narrowest range that covers the process so the percentage accuracy yields the smallest absolute error.
3. Accuracy grade and dial size: Grade A (plus-or-minus 1 percent) on a 3 or 4 inch dial covers most process duty; step up to premium Grade AA on a 5 inch dial only when a tighter local read is genuinely needed. Confirm the grade is the marked-range tolerance, not a full-scale claim.
4. Stem diameter, length, and immersion: select 6 or 8 mm metric or quarter-inch stem and an insertion length that fully immerses the active coil with margin, accounting for any thermowell. Too-short immersion causes a stem-conduction low reading.
5. Mounting and reading orientation: back (axial), lower (radial), or adjustable every-angle, chosen so the dial faces the operator from where they stand. Add a dished anti-parallax dial for installations read off-axis.
6. Process connection and thermowell: match the thread (G 1/2 B, 1/2 NPT, M20 x 1.5) or sanitary clamp to the line, and specify a thermowell sized per ASME PTC 19.3 TW for any pressurised, corrosive, abrasive, or high-velocity service. Add conductive paste or a spring-loaded tip to recover response.
7. Materials and ingress protection: stainless steel 304 for mild media, 316 or 316L for chlorides and sanitary, an upgraded thermowell alloy for aggressive service. Pick the IP rating for the environment, IP67 or higher outdoors or in washdown.
8. Vibration, certification, and serviceability: specify a liquid-filled case for vibrating equipment; confirm 3-A authorization for food, ATEX or IECEx where required for hazardous areas, and an external recalibration adjuster for field maintainability.

One dimension is routinely overlooked at the purchasing stage and then determines the next decade of operation: serviceability. A thermometer with an external zero adjuster and a standard thermowell can be recalibrated and swapped in minutes without breaking the line, while a sealed budget gauge in a welded pocket must be cut out and replaced. Established makers including WIKA, Ashcroft, Trerice, REOTEMP, Tel-Tru, and Dwyer maintain calibration documentation, spare windows, and consistent order codes that keep a fleet of instruments maintainable for years, which is the quiet reason large projects standardise on them.