Spec a bimetal thermometer in five steps, not one: pick the dial size, fix the stem length and diameter to the thermowell, choose an EN 13190 accuracy class, lock the wetted material to the process fluid, and verify the IP rating against washdown or outdoor exposure. A dial thermometer in 304 stainless steel with a 6.35 mm stem and 1% FS accuracy covers the bulk of HVAC, brewing, and light chemical service [S2][S4].
Common catalog offerings span case diameters from 25 mm to 160 mm, scale ranges from -70 to +650 °C, and stem lengths from 63.5 mm to 1,000 mm depending on the line [S4][S5]. Units for hazardous, high-vibration, or sterile duties step up to 316L wetted parts, IP65 sealing, and a separate thermowell; economy units for indoor panels use plastic cases, copper-alloy C1220T stems, and a basic IP rating [S3][S6].
How a Bimetal Thermometer Works and Where It Fits
A bimetal thermometer uses two metals with different expansion coefficients (commonly Invar and a high-expansion alloy, or steel and brass) welded into a helix; temperature twist is read on a calibrated dial, with no power and no signal wiring [S2]. Compared with a bimetal thermometer baseline, an infrared thermometer gives non-contact spot readings but needs a clean optical path and emissivity input, while bimetal units win for sustained, eye-level local indication on a pipe or vessel.
Operating envelope per multiple OEM datasheets runs from below -100 °C to above +500 °C, with WSS-series industrial units published at -80 to +500 °C and a class 1.5 indication accuracy [S7]. Scale-range choices in EN 13190 catalogs run from -40 to +60 °C up to 0 to +600 °C, with the measuring range typically narrower than the scale to leave calibration headroom [S5].
Sizing the Stem, Dial, and Connection to the Service
Match stem length to the thermowell or pipe ID, not the case: minimum immersion is the full active stem or 100 mm, whichever is greater, and a thermowell is the standard recommendation on any pressurized or corrosive line [S4]. Standard stem diameters are 6.35 mm (1/4 in), with 8, 9, 10, and 12 mm options, and 6 mm available on smaller 25 to 50 mm cases [S5][S8].
Dial size drives readability, not accuracy: 63 mm (2.5 in) suits local-panel mounting, 100 mm (4 in) is the workhorse for floor-readable process indication, and 160 mm (6 in) is used where the gauge sits 4 to 6 m above floor [S4][S6]. Process connections are typically 1/2 in NPT or BSP, with G 1/2 B, M20 x 1.5, M24 x 1.5, plain shank, union nut, and compression fitting options for special piping [S5].
Accuracy Classes and How to Read EN 13190
EN 13190 governs dial thermometer design and indication accuracy, and a 100 mm NS63 or larger case is typically class 1 (about ±1% of full scale), while 40 to 50 mm cases sit at class 2 and 25 to 33 mm cases at ±5% of scale range [S5]. Indication accuracy is independent of readability: a 63 mm class 1 gauge and a 100 mm class 1 gauge read the same process temperature, the larger dial only makes the reading easier to resolve at distance [S5][S6].
Working range versus scale range is a key distinction: continuous load should stay inside the measuring range, while the full scale range is the short-time (24 h maximum) envelope, and published ranges show measuring range bands of 20 to 50 °C narrower than the scale on every EN 13190 catalog row [S5]. For comparison, an HVAC-grade 75 to 100 mm indoor bimetal reads -30 to +50 °C at ±1 scale division in a plastic case with a C1220T stem [S3], while a process-grade 100 mm stainless bimetal typically carries EN 13190 class 1 and a -30 to +500 °C capability [S6].
Materials of Construction: 304 vs 316L vs Hastelloy
Default wetted material across most catalogs is 304 stainless steel for case, ring, stem, and process connection, with 316L as a step-up option for chloride-bearing or pharmaceutical services [S4][S5][S8]. High-corrosion or acid services step further to Hastelloy C-276 protection tubes, offered by HMK as part of their 304, 316, 316L, and C-276 protection-tube family on the WSS series [S7].
Stem options also include a plastic-case C1220T (copper) build for indoor, non-corrosive HVAC and OEM panel use, where the sensing part is ⌀8 x 50 to 300 mm and a SUS-316 ⌀15 protective tube is available for higher-pressure or higher-velocity service above 2 m/s [S3]. Window material trades off impact resistance and chemical compatibility: instrument glass is standard, polycarbonate is used on smaller 33 mm cases, and safety glass or PC options appear on tougher industrial builds [S5][S8].
Ingress Protection, Case Pressure, and Environmental Ratings
Ingress protection varies widely across the catalog: IP43 is common on 50 to 160 mm cases in EN 13190 process lines, IP54 on 25 to 40 mm cases, and IP65 when a bayonet-ring stainless case is specified [S5][S6]. A 304 stainless case with a hermetically sealed crimped ring and IP65 rating suits outdoor, washdown, and food-grade panel builds [S8].
Static pressure on the stem is case-size dependent: NS25 to NS50 cases are typically rated to 6 bar static, while NS63 to NS160 cases extend to 25 bar static, which matters when the unit is direct-immersed without a thermowell [S5]. For aggressive, abrasive, or high-velocity flows above 2 m/s with sensitive lengths of 150 mm or more, an SUS welded or SUS solid-drawn protection tube is the published minimum, not an option [S3].
Who Should NOT Pick a Direct-Immersion Bimetal Unit
Skip a direct-immersed bimetal thermometer on any service that is high-pressure, high-velocity, corrosive, or requires in-place cleaning, and install a thermowell instead, because the wetted stem cannot be isolated, cleaned, or replaced without depressurizing the line [S3][S4]. Likewise, do not use a bimetal in fast-response control loops (typical time constant is seconds, not milliseconds), in flame or combustion gas streams (radiation error and case damage), or in strong AC or DC electromagnetic fields that disturb the operator reading [S2].
For temperature control loops, a temperature transmitter or RTD with a 4-20 mA or HART output is the correct fit, not a local dial gauge, even though bimetals remain the lowest-cost local indicator on a pipe. For explosion-risk zones, standard stainless bimetals are not Ex-rated; the appropriate path is an Ex-certified head transmitter or a separately certified indicator, not a standard bimetal case.
Selection Map: Indoor Panel vs Process Local vs Outdoor Washdown
Indoor panel duty (HVAC, OEM equipment, building services) takes a 75 to 100 mm plastic-case bimetal, C1220T stem, -30 to +50 °C or 0 to +100 °C range, and ±1 scale division accuracy with no thermowell [S3]. Process local duty (petrochemical, brewing, boilers) takes a 100 to 160 mm 304 stainless case, 316L stem, 6.35 or 8 mm diameter, EN 13190 class 1, IP65, with a stainless thermowell sized to the pipe [S4][S5][S6].
Outdoor, washdown, or hygienic duty takes a hermetically sealed 304 or 316L stainless case, crimped or bayonet ring, safety glass or PC window, IP65, and a 1/2 in NPT thermowell connection, with 1% FS accuracy and dual scale °C/°F [S4][S8]. For high-corrosion or acid service, step wetted material to 316L or Hastelloy C-276, keep the case at 304 stainless, and verify the protection tube material is rated for the specific process [S7].
Spec Checklist Before You Issue the PO
On the purchase order, lock seven items: case diameter (mm), stem length (mm), stem diameter (mm), process connection size and thread (NPT, BSP, M20), scale range (°C, °F, or dual), wetted material (304, 316L, Hastelloy), and whether a thermowell is included or by others [S5][S8]. Add working-temperature as a percentage of full scale (typical published value: 75% of full scale for continuous service) and required ingress protection (IP43, IP54, IP65) [S4].
For a deeper read on adjacent temperature instrumentation, see this thermal-imaging-camera spec map for non-contact comparisons, and this infrared thermometer supplier map when the application justifies a non-contact device. Two trackable signals to watch in late 2026 are wider EN 13190 catalog updates at the 25 to 33 mm size band (current accuracy ±5% of scale range) and more 316L-as-standard offerings replacing 304 in mid-tier process catalogs [S5].