Walk through any refinery, boiler house or utility skid and you will see round dial gauges on pipes reading temperature with no power or wiring at all. Their secret is two metals bonded together that bend by a predictable amount as they warm up.
Two metals with different expansion rates, welded into one strip, curl when heated. Wind that strip into a helix inside a stem and the curl becomes shaft rotation that moves a pointer across a dial.
What Is a Bimetallic Thermometer?
A bimetallic thermometer is a mechanical dial gauge that measures temperature using a strip of two metals with different coefficients of thermal expansion. It is one of the simplest local indicators among the types of temperature sensors and needs no power supply.
One metal, often a nickel iron alloy like Invar, barely expands. The other, such as brass or a high expansion steel, grows much more, so the bonded strip curls toward the low expansion side as it warms.
How the Bimetal Strip Bends
In a straight strip, bending is small, so every industrial bimetallic thermometer winds a long strip into a helical coil. The bottom of the coil is fixed in the stem tip, and the top end turns a thin shaft connected directly to the pointer.
Because the helix sits low in a bimetallic thermometer stem, the sensitive length must be fully immersed. Heat reaches it by conduction through the stem wall, so thermal conductivity of the stem and any fill affects speed.
5 Essential Parts of a Dial Gauge
Coiled strip that twists with temperature.
Stainless tube, commonly 6 or 8 mm in diameter.
Transfers rotation to the dial.
Sealed stainless case with printed scale.
Threaded, union or flange fitting.
WIKA data for the TG54 lists nominal sizes of 63, 80, 100 and 160 mm with insertion lengths from 63 to 1000 mm. The case is hermetically sealed stainless steel rated IP65.
An every angle bimetallic thermometer lets the case swivel and tilt so the dial faces the operator in any mounting. Back, bottom and adjustable connections cover most pipe layouts.
Bimetallic Thermometer Standards and Accuracy Classes
WIKA states that its bimetal gauges are made to EN 13190 in accuracy classes 1 and 2, with scale ranges between minus 70 and 600 °C. Class 1 gives a bimetallic thermometer the tighter permissible error.
In North America, ASME B40.200 covers dial thermometers and expresses accuracy as a percentage of span. Always read the actual tolerance on the datasheet rather than assuming a class.
| Feature | Typical Choice | Why It Matters |
|---|---|---|
| Dial size | 100 or 160 mm | Readable from a distance |
| Range | Normal value near mid scale | Best accuracy and life |
| Stem | 6 or 8 mm stainless | Fits standard thermowells |
| Filling | Silicone oil below 250 °C | Damps vibration |
| Mounting | Every angle | Faces the operator |
Deflection of a Straight Bimetal Strip
Tip deflection δ ≈ L² ÷ (2 × R)
Valid for equal layer thickness and equal modulus, t = total thickness
Example, brass on Invar:
α1 = 19 ppm/K, α2 = 1.2 ppm/K, ΔT = 100 K
t = 1 mm, L = 50 mm
1/R = 3 × 0.0000178 × 100 ÷ 2 = 0.00267 per mm, R = 374.5 mm
δ = 2500 ÷ (2 × 374.5) = 3.34 mm
This is the classic Timoshenko result for a strip with equal thickness and stiffness in both layers. A 100 K rise moves a 50 mm strip only a few millimetres, which is exactly why real gauges coil the strip into a long helix.
Bimetal Deflection Calculator
Thermowells, Stem Length and Response
Never insert a bare bimetallic thermometer stem into a pressurised line. Mount the gauge in a thermowell sized as described in the thermowell guide and matched to one of the types of thermowells.
The stem must reach deep enough for the helix to sit in the flowing fluid, yet the well must survive vortex shedding, as covered in thermowell U length and wake frequency.
A thermowell adds lag, often tens of seconds, as explained in temperature sensor response time. Heat transfer paste or a close fitting bore helps.
Silicone Filling and Vibration
Pumps and compressors make an unfilled bimetallic thermometer pointer shake until it cannot be read. WIKA explains that silicone oil filling damps vibration but limits the case temperature to 250 °C.
- No power or wiring needed.
- Rugged and low cost.
- Easy local reading.
- Wide range from minus 70 to 600 °C.
- No remote signal.
- Slower than electronic sensors.
- Needs full stem immersion.
- Accuracy lower than an RTD.
For remote reading, compare the bimetallic thermometer with the gas filled thermometer. When a signal must reach the DCS, see temperature switch vs temperature transmitter.
Common reading errors from poor immersion are discussed in temperature measurement errors. Check each gauge yearly in a dry block, one of the types of temperature calibrators.
WIKA TG54 Datasheet PDF
Dial Thermometer Working Video
Bimetallic Thermometer FAQ
Two bonded metals expand by different amounts when heated, so the composite strip bends toward the low expansion side. The bend is almost linear with temperature over the working range of the gauge.
A helical coil turns that bending into rotation of a thin central shaft. The shaft then moves the pointer directly across a calibrated dial without any gears.
A low expansion alloy such as Invar usually forms one layer of the strip. The other layer is a high expansion metal such as brass or a special alloy steel.
The larger the difference in expansion coefficients, the larger the movement for each degree. Manufacturers choose pairs that stay stable over years and resist corrosion in service.
A straight strip moves only a few millimetres even for a large temperature change. Coiling a long strip multiplies the total twist into a useful angle of rotation.
The helix also fits neatly inside a narrow stainless stem that enters a thermowell. It drives the pointer directly, so there are no gears to wear or stick.
EN 13190 is the European standard that covers dial thermometers for industrial use. Class 1 allows a smaller measuring error than class 2 across the scale.
WIKA builds its bimetal gauges to classes 1 and 2 under this standard. Check the datasheet for the exact permitted tolerance in degrees for your chosen range.
Silicone oil damps pointer flutter caused by pump, compressor and pipe vibration. It also reduces wear on the shaft, bearings and other moving parts.
WIKA notes that silicone filling limits the maximum case temperature to 250 °C. Specify a filled case wherever pointer flutter would otherwise make reading difficult for operators.
Yes, a thermowell is needed in any pressurised or flowing process line. The well protects the stem and allows the gauge to be removed without draining the system.
Choose the stem length so the helix is fully immersed in the flowing fluid. Also confirm that the well passes a wake frequency check at maximum flow.
Many plants check dial gauges once a year or at each planned shutdown. Critical services and gauges exposed to heavy vibration may need more frequent checks.
Compare the reading in a dry block or liquid bath at two or three points. Adjust the zero screw if the error stays constant across the whole range.
Related Articles
- Types of Temperature Sensors
- Thermowell Guide
- How Does a Gas Filled Thermometer Work
- Temperature Sensor Response Time
- Types of Temperature Calibrators
External References
- Bimetal Thermometer Model TG54 Datasheet, WIKA
- How Does a Bimetal Thermometer Work, WIKA
- Bimetallic Strip, Wikipedia
What We Learn Today
- A bimetallic thermometer uses two bonded metals with different expansion rates, coiled into a helix that rotates a pointer as temperature changes.
- EN 13190 defines accuracy classes 1 and 2, while stem length, thermowell design and full immersion decide how true the reading really is.
- Silicone oil filling damps vibration up to 250 °C, and a yearly dry block check keeps local dial gauges trustworthy for operators.