A pressure sensor deployed in a 250°C industrial furnace fails within weeks — not because the sensor itself can’t handle the heat, it’s rated for 250°C, but because the potting encapsulating it was never built for that duty. Sensor potting fails in ways power-electronics potting never does.
Sensor potting has to preserve calibration accuracy and signal integrity, not just survive thermal exposure — a materially different bar than the one power-supply potting has to clear.
What Makes Sensors Different
Potting that expands significantly under thermal stress mechanically loads the sensor element itself, shifting its response curve. A pressure sensor potted with an incompatible compound can drift 1–5% per 50°C swing purely from mechanical stress, with no change in the sensor’s underlying physics. Moisture compounds the problem for humidity and pressure sensors specifically, and any potting with meaningful electrical conductivity or temperature-variable conductivity can add noise to high-impedance outputs like thermistors, strain gauges, or capacitive elements. Layer daily thermal cycling on top, and delicate sensor elements fatigue faster than the rest of the assembly.
Matching Potting to Sensor Type
RTDs and thermistors need low thermal conductivity (0.2–0.5 W/m·K) so potting doesn’t shunt heat away from the sensing element, plus low CTE to limit mechanical loading on the resistance element. Pressure sensors and load cells are more demanding still — a diaphragm or strain gauge translates CTE mismatch directly into output error, so very low-CTE potting (20–30 ppm/°C) with strong moisture resistance is the baseline. Capacitive sensors (humidity, displacement, pressure) are sensitive to the potting’s own dielectric constant, which needs to stay within ±5% across the operating range and demands moisture absorption under 0.2%. Integrated sensor-plus-electronics modules split the difference, needing moderate thermal conductivity (1–2 W/m·K) to manage IC heat alongside low CTE for the sensor itself.
Standard epoxy — CTE 50–70 ppm/°C, often 1–3% moisture absorption — is generally inadequate for anything but the least precise sensors. Low-CTE potting (25–35 ppm/°C, <0.2% absorption, roughly 2x the cost) handles pressure, displacement, and load-cell duty well. Silicone, at 3–5x standard epoxy cost, adds superior thermal-cycling tolerance and hydrophobic moisture resistance for the most demanding precision applications. Thermally conductive low-CTE blends (2–3 W/m·K, 30–40 ppm/°C) suit intelligent sensor modules combining sensing and signal conditioning on one board.
Isolating the Sensor Element Mechanically
Where possible, don’t pot directly over the sensing element. Pot the signal-conditioning electronics separately, connect to the sensor through thin wires that act as a stress-relief conduit, and encapsulate only the junction area — leaving the element itself unpotted or minimally covered. Multi-element sensors (thermocouples, differential pressure) benefit from a layered approach: compliant potting on the base wiring, a stiffer structural layer above it, and minimal potting thickness directly over the elements themselves.
For temperature sensors specifically, avoid thermally conductive potting near the element — heat shunted in from external sources through a conductive path is exactly what you’re trying to prevent. Always verify sensor output after potting: cure for the full 48 hours, then calibration-check at room temperature and full operating temperature, accepting only drift under 0.5% from pre-potting baseline.
Moisture Control Before and After Potting
Moisture is the dominant failure mechanism for potted sensors. Bake the sensor and connecting wires at 80°C for two hours before potting to drive off absorbed moisture, cool in low-humidity air, and pot within an hour. A thin acrylic or urethane conformal coating applied before the main pour adds an initial moisture barrier while still letting the sensor breathe. For the potting itself, moisture absorption under 0.3% per ASTM D570 — the standard test for water absorption of plastics — is a reasonable qualification bar for precision sensors, validated with post-moisture testing at 85°C/85% RH for 500+ hours. Store potted sensors below 40% RH before deployment, and vacuum-bag with desiccant for storage beyond six months.
Two Field Failures and Their Fixes
A pressure sensor drifted 2–3% per 50°C thermal cycle, worsening over 100 cycles — standard epoxy’s CTE mismatch was stressing the diaphragm. Switching to 25–35 ppm/°C potting cut drift to under 0.3% per cycle. Separately, a humidity sensor’s output offset drifted 1–2% per month in a high-humidity deployment because the potting had absorbed 2–3% moisture, shifting its dielectric properties; specifying moisture-resistant potting (<0.2% absorption) plus a conformal-coating pre-layer held output stable for over a year.
Specification Checklist
For precision sensors (pressure, load cell, strain gauge): CTE 25–35 ppm/°C, moisture absorption <0.3%, Tg ≥200°C, elongation at break >5%, thermal conductivity 0.2–0.5 W/m·K, and mandatory post-potting calibration validation. For temperature sensors (RTD, thermocouple): CTE 30–40 ppm/°C, thermal conductivity 0.2–0.5 W/m·K, Tg ≥200°C, moisture absorption <0.5%, with mechanical isolation built into the design. IPC-CC-830, the industry reference for qualification and performance of electrical insulating compounds for printed board assemblies, is a useful baseline when writing these specs into a supplier agreement.
At scale, sensor-grade potting costs 2–3x standard potting but changes the failure math: for a 100,000-unit annual deployment, standard potting’s 5–10% failure rate can generate $500,000–1 million in warranty cost, while sensor-grade potting’s sub-1% failure rate keeps that exposure under $100,000 — a net saving that outweighs the higher material spend. The same logic applies to vibration-heavy deployments; see Incure’s notes on whether potting compound prevents vibration damage and the broader engineering guide to potting electronics for extreme heat.
Incure formulates low-CTE, moisture-resistant potting for sensors and sensor assemblies, validated through post-potting calibration testing across thermal cycling and long-term exposure. Email Us with your sensor type and accuracy requirements for a formulation recommendation. If your assembly also needs to choose between conductive and insulating potting for a mixed sensor/power design, our comparison guide covers that trade-off directly, and our potting compound vs. epoxy comparison is worth reviewing before finalizing material family.
Contact Our Team to specify sensor-grade potting and hold calibration stability across your operating temperature range.
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