— A Practical Guide for Engineers, EPCs, and Project Teams
Cryogenic systems—LNG storage and transport, air separation units, superconducting magnets, and aerospace applications—operate at temperatures that push standard instrumentation beyond its limits. At -162°C, the storage temperature of LNG, conventional PVC cables become brittle and crack--52. Standard pressure transmitters freeze. Moisture ingress through unsealed connections instantly forms ice that breaks down insulation-5. Even small contamination can lead to measurement drift that, in LNG custody transfer applications, translates into significant financial losses over time-5.
Selecting instruments and cables for cryogenic service is not about choosing the same products used at ambient temperature and "hoping they work." It requires a systematic approach to materials, sealing, thermal contraction, and signal integrity.
1. The Cryogenic Challenge: What Makes It Different?
Cryogenic applications present a combination of challenges that distinguish them from standard industrial installations:
| Challenge | Impact on Instrumentation |
|---|---|
| Extreme cold (-162°C to -269°C) | Standard materials become brittle; plastics crack; metals shrink; fill fluids freeze |
| Thermal contraction | Metals shrink by approximately 0.3% from room temperature to -200°C, stressing mounting points and electrical connections-5 |
| Moisture ingress | Any moisture entering through unsealed connections freezes instantly, breaking down insulation-5 |
| Thermal cycling | Repeated cooling and warming cycles cause micro-cracks in coverings, allowing contamination and eventual failure-5 |
| Parasitic effects | Parasitic capacitance and lead wire resistance introduce measurement errors at cryogenic temperatures-5 |
| Hazardous atmosphere | LNG and other cryogenic fluids are flammable; instruments require ATEX/IECEx certification-2 |
The key principle: Cryogenic instrumentation must be designed for survival at extreme low temperatures—not just signal transmission. Sensors and cables must be specified, tested, and installed with the understanding that standard industrial products will fail.
2. Sensor Selection: Measuring Temperature in the Extreme Cold
Temperature is the most critical measurement in cryogenic systems. The choice between RTD and thermocouple—and the specific type within each category—depends on the temperature range, accuracy requirements, and installation environment.
2.1 RTDs: The Preferred Choice for Accuracy
Platinum RTDs (Pt100 / Pt1000) are the preferred choice for cryogenic temperature measurement where accuracy is critical-5. They offer excellent long-term stability, reducing the need for frequent recalibration-5.
Key considerations:
| Factor | Details |
|---|---|
| Temperature range | Pt100 RTDs can measure from -200°C to +600°C-; specialised cryogenic RTDs are available down to -269°C (4K) |
| Accuracy | Class A RTDs offer ±0.15°C accuracy at 0°C-; specialised sensors can achieve ±0.01K accuracy- |
| Sensor types | Thin-film RTDs respond to liquid media in less than two seconds-5; wire-wound versions are more stable but respond more slowly-5 |
| IEC compliance | IEC 60751 standards apply to high-purity platinum elements-5 |
RTD sensor construction: Cryogenic RTDs are typically housed in mineral-insulated sheaths (e.g., stainless steel) with vibration-resistant construction. For LNG tank temperature profiling, sensors can connect up to 20 measurement points with total lengths up to 100 metres-2.
Specialised cryogenic sensors: For temperatures below approximately 20K (-253°C), silicon diodes or specialised RTDs such as Cernox®, Germanium, or Rox™ (ruthenium oxide) sensors are required-. Negative Temperature Coefficient RTDs are very effective across the full cryogenic range-.
2.2 Thermocouples: When RTDs Are Not Suitable
Thermocouples are better adapted than RTDs to handling the lowest temperatures, provided an appropriate type is selected-1. Types E, K, N and T have ranges down to -270°C (-450°F), with Type T especially popular in cryogenic applications-1-.
| Thermocouple Type | Low-Temperature Suitability | Key Characteristics |
|---|---|---|
| Type E | Down to -270°C | Highest sensitivity among standard thermocouples at low temperatures; best choice for temperatures down to 40K-; resistant to corrosion at cryogenic temperatures- |
| Type T | Down to -270°C | Especially popular in cryogenic applications-1 |
| Type K | Down to -270°C | Widely available; standard choice for many applications |
| Type N | Down to -270°C | Good stability across wide range |
| Type B | Not suitable | Simply not suitable for low temperatures-1 |
Important: Once below -100°C, most thermocouples begin to lose linearity-1. This is a known characteristic and can be corrected in the transmitter, but not all temperature transmitters or controllers are set up to work at the low end-1. Engineers must verify that any device intended for cryogenic applications has the required capabilities-1.
2.3 Thermocouple vs RTD: When to Choose Which
| If your priority is... | Choose... |
|---|---|
| High accuracy and long-term stability | RTD (Pt100) — the best choice for cryogenic temperature measurement where accuracy is critical-5 |
| Extremely low temperatures (< -200°C) | Specialised cryogenic RTD (Cernox, Germanium, Rox)- |
| Fast response | Thin-film RTD (less than two seconds response)-5 |
| Wide temperature range with good sensitivity | Type E thermocouple (highest sensitivity at low temperatures; best down to 40K)- |
| Corrosive cryogenic environment | Type E thermocouple (resistant to corrosion at cryogenic temperatures)- |
| Custody transfer / fiscal measurement | RTD (Pt100) — contamination and drift must be minimised-5 |
NASA experience: Type E is the most common thermocouple used in cryogenic systems, with best performance above 70–80K-. For temperatures below 20K, RTDs are required-.
3. Cable Selection: Maintaining Signal Integrity at Cryogenic Temperatures
Cable selection for cryogenic systems is as critical as sensor selection. Standard PVC cables become brittle and crack at low temperatures--52. The wrong cable can fail mechanically, introduce signal noise, or compromise the entire measurement chain.
3.1 Cable Materials for Cryogenic Service
| Material | Temperature Range | Key Characteristics | Applications |
|---|---|---|---|
| Polyimide (PI) | -196°C to +260°C | Best choice for extreme cryogenic; no brittleness at -196°C; low noise (<5µV/m); excellent chemical resistance; lightweight-50 | LNG storage, superconducting systems, aerospace-50 |
| FEP (Fluorinated Ethylene Propylene) | -163°C to +200°C | Excellent dielectric properties; chemical resistance; maintains flexibility across extreme temperature span-14 | LNG magnetic level gauges, cryogenic instrumentation-14 |
| PTFE (Polytetrafluoroethylene) | -200°C to +260°C | Exceptional dielectric properties at cryogenic and elevated temperatures-14 | High-performance cryogenic applications |
| XLPE (Cross-linked Polyethylene) | -70°C to +90°C | Better cold resistance than PVC; Tg approximately -70°C to -90°C-52 | Medium-voltage cables, nuclear applications |
| PE (Polyethylene) | -100°C to +80°C | Good cold resistance; LDPE Tg approximately -100°C-52 | General low-temperature applications |
| PVC (Polyvinyl Chloride) | -15°C to +70°C | Not suitable for cryogenic—becomes brittle and cracks below -30°C to -50°C--52 | Not recommended for cryogenic applications |
3.2 Key Cable Selection Criteria
| Criteria | What to Look For | Why |
|---|---|---|
| Temperature rating | Continuous operating temperature below the minimum process temperature | Prevents embrittlement and mechanical failure |
| Flexibility | Small bending radius (e.g., 6× cable diameter for PI cables)-50 | Enables installation in tight spaces and complex routing |
| Signal integrity | Low noise (<5µV/m for PI cables)-50 | Prevents signal distortion at cryogenic temperatures |
| Mechanical strength | High abrasion resistance and fatigue resistance-50 | Withstands vibration and repeated thermal cycling |
| EMI protection | Braided shield (e.g., SCR silver-plated copper)-14 | Protects against electromagnetic interference |
| Chemical resistance | Resistance to LNG, liquid nitrogen, and process chemicals | Prevents degradation in cryogenic fluids |
3.3 Specialised Cryogenic Cables
PI (Polyimide) low-noise sensor cables: These cables, such as the TST-SDJPI-DYT 600V series, are specifically designed for liquid nitrogen temperature (77K) applications-50. Key features include:
Working temperature range: -196°C to +260°C-50
Ultra-low noise: <5µV/m at 1Hz-1kHz-50
Extruded PI insulation (not tape-wrapped) to prevent cracking-50
Minimum bending radius: 6× cable diameter-50
Weight reduction: approximately 20% lighter than conventional cables-50
Electrical safety: 2000V/5min withstand, insulation resistance ≥500MΩ·km-50
Applications: LNG storage tanks and filling stations, superconducting systems, aerospace instrumentation, and extreme environment precision measurement-50.
FEP coaxial cables for LNG level gauges: These custom coaxial cables use solid 316L stainless steel conductors and FEP insulation and jacket, with operating ranges from -163°C to +200°C-14. The SCR silver-plated copper braid provides reliable EMI protection-14. These cables are specifically designed for magnetic liquid level gauges in LNG storage tanks-14.
Mineral-insulated cables: Mineral-insulated cables with hermetic connectors provide reliable signal transmission in cryogenic environments and can pass through vacuum chambers and walls-13. These cables maintain electrical integrity down to the lowest temperatures-13.
3.4 Cable Installation Considerations
| Consideration | Requirement | Why |
|---|---|---|
| Bending radius | Follow manufacturer's minimum bending radius (e.g., 6× OD for PI cables)-50 | Prevents mechanical damage at low temperatures |
| Cold flow | Select cable glands that reduce the effects of cold flow characteristics- | Prevents loss of seal integrity at cryogenic temperatures |
| Sealing | Use proper cable glands and sealing methods | Prevents moisture ingress and immediate freezing |
| Routing | Avoid sharp edges and stress points | Reduces risk of cable damage during thermal cycling |
| Identification | Use durable marking suitable for cryogenic temperatures | Enables maintenance and troubleshooting |
IEC 60079-14 requirements: For installations in explosive atmospheres at extremely low ambient temperatures, the characteristics of insulation materials must be considered-. Precautions must be taken regarding the operating temperature and minimum bending radius of cables at low temperatures-. Cable installation must be carried out in the appropriate ambient temperature range-.
4. Pressure and Level Measurement in Cryogenic Systems
4.1 Differential Pressure Flowmeters for Cryogenic Liquids
DP flowmeters using appropriate DP transmitters are well suited for cryogenic liquids such as LNG-1. However, their installation differs from conventional liquid applications:
Mounting position: For LNG applications, the transmitter should be positioned above the primary element, creating an insulating gas barrier that prevents contact of the cold liquid directly with the transmitter diaphragm-1
Impulse lines: Brief direct contact of LNG with the transmitter diaphragm should not cause failure, but it will slow responsiveness-1
Materials: The transmitter has a stainless-steel body that can handle the cold, but gaskets, O-rings, and bolts must also be compatible with low temperatures-1
Welded impulse lines: DP flowmeters equipped with welded impulse lines reduce the potential for sealing material embrittlement by moving gaskets away from the frost line-1
Impulse tubing diameter: Typically 0.25 inches (6 mm) to help maintain the gas barrier-1
4.2 Diaphragm Seals for Cryogenic Service
Treatment of cryogenic applications uses similar techniques to those employed for handling aggressive fluids. A diaphragm seal can be used for pressure or level measurements, but extreme temperatures call for specialised fill fluids designed to retain their fluidity at the operating temperature without freezing or boiling-1.
Considerations for capillary lines:
If capillary line length allows the fill fluid to come to ambient temperature, it can slow response or cut it off entirely-1
Some stand-off mounts use two fill fluids: one optimised for the process temperature and the other for the ambient temperature-1
Each fill fluid is tailored to the application and environment, eliminating the need for heat tracing or other protection methods-1
4.3 Level Measurement in Cryogenic Tanks
LNG storage tanks require level measurement systems that operate reliably at -162°C. Key technologies include:
| Technology | Cryogenic Suitability | Key Considerations |
|---|---|---|
| Radar level | Good | Non-contact; no moving parts; must be cryogenic-rated |
| Magnetic level gauges | Excellent (with cryogenic coaxial cable)-14 | Requires specialised FEP or PTFE cables-14 |
| DP level with remote seals | Good (with specialised fill fluids)-1 | Fill fluid must remain fluid at cryogenic temperatures |
5. Common Mistakes to Avoid
| Mistake | Consequence | Prevention |
|---|---|---|
| Using PVC cable in cryogenic service | Brittle failure, cracking- | Use PI, FEP, PTFE, or XLPE cables |
| Ignoring thermal contraction | Stressed connections, micro-cracks, failure-5 | Specify materials and mounting methods that accommodate contraction |
| Inadequate sealing | Moisture ingress, immediate freezing, insulation breakdown-5 | Use proper cable glands and sealing methods |
| Using standard fill fluids | Fill fluid freezes, measurement failure-1 | Specify cryogenic fill fluids for diaphragm seals |
| Incorrect thermocouple type | Loss of linearity, measurement errors-1 | Select appropriate type (E, T, K, N; avoid B) |
| Mounting DP transmitter below pipe | Direct LNG contact, slow response-1 | Mount transmitter above the primary element for LNG-1 |
| Not verifying transmitter low-end capability | Transmitter cannot correct for non-linearity-1 | Verify transmitter is configured for cryogenic applications |
| No ATEX/IECEx certification | Safety incident, regulatory violation-2 | Specify Ex-certified instruments for LNG applications |
6. Why Choose Anhui Tiankang for Cryogenic Instrumentation?
Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial instruments and cables for nearly five decades. Our cryogenic instrumentation solutions are designed for the most demanding low-temperature applications.
Cryogenic-capable products:
| Category | Products | Cryogenic Features |
|---|---|---|
| Temperature sensors | Pt100 RTDs (Class A/B), Type E/T thermocouples | Mineral-insulated sheaths; vibration-resistant; down to -200°C |
| Pressure transmitters | TK1151/3051 GP/DP transmitters | Remote seal with cryogenic fill fluids; stainless steel bodies |
| Level instruments | Radar level transmitters, magnetic level gauges | Cryogenic-rated; Ex ia/Ex d certification |
| Instrumentation cables | PI, FEP, and XLPE insulated cables | -196°C to +260°C; low noise; small bending radius |
| Ex cable glands | Ex d/Ex e certified glands | Cryogenic-compatible materials; cold flow mitigation |
Core advantages:
Complete certifications: CCC Ex, ATEX, IECEx, SIL
CNAS-accredited laboratory for full low-temperature performance testing
Customisable cable constructions: conductor materials, insulation types, shielding configurations
Proven track record: long-term supplier to LNG terminals, air separation plants, and cryogenic research facilities
One-stop supply: from sensors to cables to glands
7. Conclusion
Selecting sensors and cables for cryogenic systems requires a systematic approach that addresses the unique challenges of extreme cold:
| Decision Point | Recommendation |
|---|---|
| Temperature sensor for accuracy | Pt100 RTD (Class A) with mineral-insulated sheath |
| Temperature sensor for very low temperatures | Specialised cryogenic RTD (Cernox, Germanium) for <20K |
| Temperature sensor for wide range/sensitivity | Type E thermocouple (highest sensitivity at low temperatures) |
| Temperature sensor for LNG applications | Type E or T thermocouple-1 |
| Cable insulation | PI (best), FEP, or XLPE—never PVC |
| Cable shield | Braided copper or silver-plated copper for EMI protection |
| Pressure measurement | DP transmitter mounted above the primary element; welded impulse lines-1 |
| Fill fluid | Cryogenic fluid—must remain fluid at operating temperature-1 |
| Sealing | Proper cable glands; moisture ingress prevention- |
| Ex certification | ATEX/IECEx for flammable cryogenic fluids-2 |
Remember: In cryogenic systems, the cost of the wrong sensor or cable is measured not just in replacement cost, but in lost production, safety incidents, and—in custody transfer applications—significant financial losses. Specify the right materials, the right protection, and the right installation practices.
Contact Us
For cryogenic instrumentation selection advice, technical documentation, or project quotations, please contact:
Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Your partner for reliable cryogenic instrumentation solutions.
