A cryogenic level sensor (also called a liquid nitrogen level sensor or cryogenic liquid level sensor) measures liquid level in tanks of liquid nitrogen (LN₂), liquid oxygen (LOX), liquid argon (LAr), LNG or liquid hydrogen (LH₂), where the wetted parts sit 180–250 K below ambient and the tank is constantly boiling off. Instranova builds three families for that service: a capacitive probe, a differential-pressure (DP) transmitter and a magnetic-flap gauge, in 50 to 3000 mm ranges with 4–20 mA, 0–5 V, RS485 or HART output and 316L or 9% Ni wetted parts. Full specifications and a quote request are below.
Contents
- Why cryogenic level is a different problem
- Which technology fits which cryogen
- Continuous vs point level
- Product specifications (3 instruments)
- How DP level works on a pressurized tank
- Measuring LN₂ level without contacting the liquid
- Applications
- Monitoring, alarms & outputs
- FAQ
Why cryogenic level is a different problem
A cryogenic tank is not a still pot of cold liquid. The gas head is regenerated continuously by boil-off, the liquid stratifies into a warmer top layer and a colder bulk, and the wetted parts of any probe see a temperature step of 200 K or more during a fill. Most ambient-service level technologies cannot survive that.
Three failure modes drive the selection. Glass-filled or plastic probes can cold-shock and crack on a fast fill. Any vent or reference line exposed to ambient humidity can grow an ice plug. And hydrostatic (DP) measurement reads high when the saturated boil-off layer sits between the head-pressure tap and the liquid. A correctly specified instrument answers all three through material choice (austenitic 304/316L, or 9% Ni for LNG), a vacuum-jacketed neck or stilling well, and density-corrected level math. Hydrogen service adds one more rule: any gauge on LH₂ must meet IEC 60079 Group IIC with an internal flame arrestor.
Which technology fits which cryogen
Several technologies measure cryogenic level. The families used in practice are capacitive probes, differential-pressure (DP, hydrostatic) transmitters, magnetic-flap (float) gauges, point-level switches (carbon-resistor, thermistor or RTD), and superconducting-wire sensors for liquid hydrogen and helium; guided-wave radar is used mainly on LNG. Which one works depends on the fluid, because density, dielectric constant and the dominant hazard all change. Instranova builds the first three, which cover most LN₂, LOX, LAr and LNG tanks; the table below maps technology to fluid.
| Cryogen | Boiling point (1 atm) | Liquid density (kg/m³) | Dielectric εr | Dominant hazard | Workable level tech |
|---|---|---|---|---|---|
| Liquid nitrogen (LN₂) | −195.8 °C | 808 | 1.43 | Asphyxiation, cold burn | Capacitive, DP, magnetic flap |
| Liquid oxygen (LOX) | −183.0 °C | 1141 | 1.49 | Oxidiser ignition | DP (O₂-cleaned), capacitive (Cu-free) |
| Liquid argon (LAr) | −185.9 °C | 1394 | 1.50 | Asphyxiation | Capacitive, DP, magnetic flap |
| LNG | ≈ −161.5 °C | 422–470* | 1.66–1.85* | Flammable (IIA) | DP, servo, GWR with PTFE seal |
| Liquid hydrogen (LH₂) | −252.9 °C | 71 | ≈ 1.23* | Flammable (IIC) | Superconducting / resistive probe, DP |
Continuous vs point level
Decide this before you pick a technology. Continuous level gives a live 4–20 mA or digital reading of fill from 0 to 100%, which you need for inventory, custody transfer and auto-fill control. The capacitive probe and the DP transmitter below are continuous. Point level only tells you when liquid crosses one height (a high-fill cut-off or a low-level alarm), and is usually an RTD or thermistor point switch. Many sites run both: a continuous sensor for the reading plus an independent point switch as a hard high-level trip.
Product specifications
Three instruments cover most LN₂, LOX, LAr and LNG tanks. Pick by tank type: capacitive for direct-immersion dewars and small tanks, DP for tall or pressurized bulk tanks, and magnetic-flap when you want a power-free visual indicator. Specifications below; we configure range, connection and output to your tank.
1. Capacitive cryogenic level sensor (continuous)
A linear capacitor of two concentric 316L tubes; the cryogenic liquid between them is the dielectric, so capacitance rises with level. No moving parts, robust to thermal cycling, best for LN₂ and LAr direct immersion.
| Measuring principle | Capacitance (concentric 316L tubes; cryogen as dielectric) |
|---|---|
| Measuring range | 50–3000 mm (custom probe length) |
| Suited media | LN₂, LAr (LOX with copper-free, oxygen-clean build) |
| Resolution | 1 mm on a 1 m probe (LN₂, LAr) |
| Output | 4–20 mA / 0–5 V / RS485 (Modbus RTU) / RS232 |
| Power supply | 24 VDC |
| Wetted material | 316L stainless (Cu-free for LOX) |
| Temperature rating | to −196 °C (−253 °C optional) |
| Process connection | Flange or thread (custom) |
| Protection | IP65 / IP66 |
2. SMT3151LT differential-pressure level transmitter (bulk / pressurized)
A smart DP cell measures the hydrostatic head between a bottom (liquid) tap and a top (gas-head) tap, so the reading is vapor-corrected. With two RTDs it compensates for density drift as the liquid stratifies, which is how you hold accuracy on a tall or pressurized tank.
| Measuring principle | Hydrostatic differential pressure + density compensation |
|---|---|
| Sensing element | Piezoresistive cell with capillary remote seal |
| Accuracy | ±0.1% of span (with density compensation) |
| Span / range | Configured to tank height (typ. 0–6 m and up) |
| Max storage pressure | to 14 bar (LOX / LAr service) |
| Output | 4–20 mA + HART / RS485 (Modbus RTU) |
| Power supply | 24 VDC (2-wire) |
| Wetted material | 316L / Inconel diaphragm; Monel 400 / 316L oxygen-clean for LOX |
| Temperature rating | remote seal to −196 °C (−253 °C options) |
| Hazardous area | ATEX / IECEx Ex d or Ex ia, Zone 1/0; Group IIC for LH₂ |
3. Magnetic-flap (bypass) level gauge (visual + alarm)
A magnetic float in a bypass chamber drives a flap/roller column you can read with no power. Add a reed-chain 4–20 mA transmitter or switch contacts when you also want a remote signal or alarms. The vacuum-jacketed chamber is what makes it work on a cryogenic dewar.
| Measuring principle | Magnetic float in bypass chamber; flap/roller indicator |
|---|---|
| Indication | Local visual (no power) + optional 4–20 mA transmitter + reed-switch alarms |
| Measuring range | To 12 m (sectional chambers for longer) |
| Accuracy | ±10 mm visual; ±0.5 mm with magnetostrictive transmitter |
| Power supply | None for visual; 24 VDC for transmitter / switches |
| Wetted material | 304 / 316L; vacuum-jacketed chamber for cryogenic service |
| Temperature rating | to −196 °C (vacuum-jacketed cryogenic version) |
| Best for | Portable LN₂ dewars, local read, power-free sites |
Need a hard high- or low-level trip as well? Add an RTD or thermistor point-level switch alongside any of the three.
How DP level works on a pressurized tank
On a pressurized bulk tank a smart DP transmitter measures level as the difference between a bottom (liquid) tap and a top (gas-head) tap: L = (Pbottom − Ptop) / (ρ · g). Because both cells share the same gas head, the reading is vapor-corrected automatically; two RTDs then compensate for density as the liquid stratifies.
The numbers make the case. On one LOX project an industrial-gas producer specified five DP level transmitters for storage tanks of roughly 75 in × 20 ft (about 6,000 L) running at 17–20 bar, all on 4–20 mA. At ±0.1% of span on a 6 m (20 ft) tank, that is about ±6 mm of level error, close enough to schedule deliveries against and far better than a sight glass on a pressurized vessel. For LOX the wetted parts are oxygen-cleaned and copper-free; for LH₂ the same approach is used with IIC-rated, copper-free hardware.
Measuring LN₂ level without contacting the liquid
A common request is to read tank level without putting a probe into the cryogen. Two methods do this. The first is the DP transmitter above, with both taps on the tank wall and a capillary remote seal, so nothing electronic touches the liquid. The second is gas-pressure plus temperature compensation: read the saturated head pressure and correct for temperature to infer level. Both keep the sensor electronics warm and serviceable, which matters on LN₂ dewars that cycle hard. The trade-off is that wall-mounted methods need the boil-off layer accounted for, or they read slightly high right after a fast fill.
Applications
Typical applications for these instruments:
- Air-separation plants (LN₂, LOX and LAr storage tanks)
- Industrial and medical gas dewars
- IVF and biobank LN₂ freezers (auto-fill control plus a low-level alarm to protect samples)
- MRI magnet cooling
- Cryogenic food freezing
- LNG fuelling and bunkering
- Hydrogen refuelling and research dewars
When the same tank also needs pressure measurement, pair the level instrument with our cryogenic pressure transducers; for vapor and boil-off metering see our liquid nitrogen flow meters. The selection logic is covered in the cryogenic flow and level guide, and the full range sits under level instruments and gas & cryogenic.
Monitoring, alarms & outputs
The continuous sensors output 4–20 mA into a PLC or DCS, or RS485 Modbus RTU for a multi-tank network, giving a digital tank-level readout you can log and alarm on. From there you can set high- and low-level alarms, drive an auto-fill solenoid, and push readings to a SCADA or remote-monitoring dashboard. For unattended dewars (IVF, biobank, MRI), a low-level alarm and a sudden-drop alert are what catch a failing tank before product is lost, which manual dip-stick checks cannot do.
FAQ
How do you measure liquid nitrogen level?
You measure it without putting an ordinary float or bare electronics in the cold. The common methods are differential pressure, which reads the head of liquid from the bottom of the tank, and capacitance, where the liquid and gas change the capacitance along a probe. For a working tank a differential-pressure or capacitance sensor rated for cryogenic temperature gives a continuous, remote reading, which beats dipping a manual measuring stick and lets the control room trend boil-off.
What are the different types of level sensors?
The main families are float, differential pressure (hydrostatic), capacitance, ultrasonic, radar, and guided-wave radar, plus point-level switches. Not all survive -196 C: contact methods must use cryogenic-rated materials, and ultrasonic struggles in the cold vapour. For liquid nitrogen and similar media, differential-pressure and capacitance sensors are the proven choices, which is what this cryogenic level sensor uses.
What is the difference between a level sensor and a level switch?
A level sensor measures level continuously and reports it as a 4-20 mA or digital signal across the whole range. A level switch only tells you when the liquid reaches one point, opening or closing a contact for a high or low alarm. On a cryogenic tank you use a continuous sensor when you need to track how full it is and trend boil-off, and a switch when you only need a fill or low-level trip. Many tanks use both.
