Free-filament wire strain gauges pre-bonded to a flame-sprayed alumina (Al2O3) carrier measure dynamic strain on hot-section components up to +1038°C (+1900°F), as published in the Micro-Measurements ZWH-, ZWN-, and ZWP-Series product note (2017-05) [S6][S8].
The construction is the same across the two main U.S. suppliers: a sensing wire is wound as a free filament, then flame-sprayed with a dielectric ceramic coating that doubles as the strain-isolation carrier and as the lead-wire insulator. Micro-Measurements high-temperature weldable gages use flame-sprayed alumina on a shim-stock substrate [S9], and HPI's free filament HFN, HFP, HFK, HFH, and HFM series wire strain gages are installed with the same flame-spray approach for high-temperature service [S5].
Construction: Why a Free Filament Inside a Ceramic Shell
Free-filament design minimises thermal-strain pickup by isolating the active wire from the substrate except at the two end loops, a geometry that Micro-Measurements highlights as the core differentiator for fluctuating-temperature work [S4]. The carrier doubles as both the mechanical strain-transfer medium and the dielectric between the gauge grid and the test part, which is why flame-sprayed alumina, with a melting point around 2072°C and established use as a high-temperature dielectric, is the standard coating material [S9][S7].
Because the gauge is pre-bonded to a metallic shim, installation on a host component is a weldable or flame-spray operation rather than a room-temperature adhesive cure. NASA test work on ceramic thin-film strain gauges has run active gauges to 1495°C in laboratory settings, demonstrating the upper temperature boundary of ceramic-based resistive strain elements [S1]. The Micro-Measurements published +1038°C (+1900°F) limit for the ZWH/ZWN/ZWP wire gages is the practical field ceiling, well below the 1495°C laboratory record but proven on production hardware [S6][S8].
Operating Envelope and Performance Numbers
Micro-Measurements high-temperature strain instruments are specified to operate accurately up to 1150°C (2100°F) at the system level, with the ZWH/ZWN/ZWP wire gages themselves rated for dynamic strain measurement to +1038°C (+1900°F) [S4][S6]. The +1150°C figure represents the broader instrument and accessory envelope, while the +1038°C figure is the published gauge-only rating.
For comparison, printed additively manufactured strain gauges tested at Idaho National Laboratory under the DOE Advanced Sensors and Instrumentation program were characterised at 20-300°C using ASTM E8 standardised tensile testing on a separate-effects basis, with reliability and robustness methods developed for the AM gauge/substrate interface [S2]. A separately reported AgPd thick-film printed gauge showed a temperature coefficient of resistance of 191.6 ppm/°C measured from 50°C, which is the TCR benchmark for that chemistry [S3]. A free-filament wire gauge is chosen instead when the TCR contribution from a thick-film print is too high or when the printed pattern cannot survive the thermal cycling on a real hot-section part.
Selection Criteria vs Printed and Thin-Film Alternatives
Three options cover most high-temperature strain work: free-filament wire gauges with flame-sprayed ceramic, thin-film ceramic gauges, and printed (additive) thick-film gauges. The free-filament wire route wins on proven field life at +1038°C and on supplier maturity, since both Micro-Measurements and HPI ship stock parts. Thin-film ceramic gauges extend the ceiling to 1495°C in NASA work but require custom deposition [S1], and printed AgPd thick-film gauges stay in a 20-300°C envelope with TCR values around 191.6 ppm/°C that need compensation in the data path [S2][S3].
A worked comparison against four decision criteria:
1. Temperature ceiling: free-filament wire +1038°C, thin-film ceramic 1495°C, printed thick-film 300°C. 2. Mounting method: free-filament uses flame-sprayed ceramic or weldable shim, thin-film is sputtered or plasma-deposited, printed thick-film is direct-write or screen-printed. 3. Thermal-strain error: free-filament geometry minimises it by design, thin-film requires matched substrate compensation, printed thick-film needs active TCR compensation at ~191.6 ppm/°C. 4. Commercial availability: free-filament is a stock product at two U.S. suppliers, thin-film is mostly a NASA/lab build, printed thick-film is at TRL 4-5 in the DOE INL programme.
Mounting Methods and Field Installation
Micro-Measurements high-temperature weldable strain gages are pre-bonded to a metallic shim using flame-sprayed alumina and then welded to the host part with a capacitor-discharge or tungsten-inert-gas spot, leaving a ceramic-isolated filament ready for lead-wire attachment [S9]. HPI's HFN, HFP, HFK, HFH, and HFM series follow the same recipe, with the ceramic layer acting as both strain-transfer medium and electrical insulation, and the HFK/HFH/HFM variants extending upper limits for more aggressive thermal exposure [S5].
For hot-section rotating hardware such as turbine blades and combustor casings, flame-sprayed ceramic coatings remain the primary means of attaching strain and temperature gauges, as documented in a NASA Tech Briefs durability study of thermally sprayed dielectric ceramic coatings (2007-05) [S7]. Practical constraints are real: gauge lead-wire transitions need a second ceramic bead or a high-temperature lead-wire sealant, and the minimum bend radius of the free filament limits how tightly a gauge can be wrapped around a leading edge.
Standards, Test Methods, and Application Boundaries
Mechanical and thermal characterisation of strain gauges in the 20-300°C band at Idaho National Laboratory followed ASTM E8 standardised tensile testing procedures for the printed-gauge programme [S2]. For the Micro-Measurements product line, published specifications cite +1900°F (+1038°C) for dynamic strain measurement on the ZWH/ZWN/ZWP-Series free-filament wire gages with ceramic cements [S6][S8]. The choice between a free-filament wire gauge and a thin-film printed gauge is, in practice, driven by three numbers: the peak service temperature, the allowable thermal-strain error, and the lead time the user can absorb. A strain gauge that exceeds the process temperature is scrap.
Engineers sourcing a free-filament wire gauge for service above 800°C should confirm the supplier's published upper limit in writing, since the +1038°C figure applies to the ZWH/ZWN/ZWP-Series specifically, while the wider Micro-Measurements instrument line is advertised to 1150°C (2100°F) [S4][S6]. For reference, a temperature monitor on the same test article, paired with a temperature controller at the heat source, lets the operator verify the gauge actually saw its rated temperature rather than assume it from the controller setpoint.
Where Free-Filament Wire Gauges Are Used and Where They Are Not
The free-filament wire gauge with flame-sprayed ceramic is the workhorse for strain on jet-engine turbine blades, combustor liners, rocket-nozzle test sections, and exhaust-manifold dynamometer rigs, anywhere strain on a metal part at steady metal temperatures of 600-1000°C is the measurement goal. It is also a common pick for hot-steam piping in power-plant tests where conventional adhesive-backed foil gauges would delaminate. [S4]
It is the wrong tool for in-pile nuclear instrumentation at the same temperatures, where the DOE Advanced Sensors and Instrumentation programme is actively developing additively manufactured printed strain gauges for in-pile sensing under high-temperature and radiation fields simultaneously [S2]. It is also the wrong tool for non-metallic substrates such as ceramic-matrix composites, where flame-sprayed alumina does not bond and where a thin-film sputtered gauge or a printed thick-film gauge is the more compatible approach. Procurement notes: the Micro-Measurements high-temperature catalogue and the HPI HFN-HFM datasheets are the two U.S. sources with off-the-shelf free-filament wire products, and installation consumables (ceramic cements, lead-wire sealants, weldable shims) ship as accessories from the same vendors [S4][S5].
Trackable signals: Micro-Measurements publishes a High-Temperature data book with weldable-gauge installation procedures [S9], and the INL printed-gauge line is approaching higher temperature qualification rounds beyond the 300°C band already documented in INL/RPT-23-75257 [S2]. For selection context on a competing foil-gauge option, see the Foil vs Semiconductor Strain Gauge Sensitivity decision spec for 2026, which lines the foil and semiconductor families against the high-temperature wire family covered here.