For a turbine oil tank level measurement project, the client specified an RF capacitance level transmitter with a 700mm coaxial rod probe and PTFE insulation. The operating conditions are 200°C and 24 bar, with a 4–20 mA + HART output and a 1.5″ BSP threaded connection.
An assessment based on application experience confirms that an RF capacitance level transmitter configured with a coaxial probe and PTFE insulation is suitable for these 200°C operating conditions. However, the signal output and thread size require final verification before project implementation.
Below, we outline the configuration specifications for the RF capacitance level transmitter, covering the probe, insulation material, temperature and pressure ratings, output, and process connection.
| Item | Recommended Configuration for Oil Tanks |
| Probe Type | Coaxial rod probe (switch to coaxial cable probe for longer ranges) |
| Probe Material | 316L stainless steel |
| Insulation Material | PTFE (temperature resistant up to 250°C) |
| Temperature/Pressure Rating | Probe rated for 250°C; pressure rating based on customer design (24 bar in this case) |
| Transmitter Head | Add heat dissipation section or use remote mounting for high-temperature media |
| Output | 4-20mA; HART optional |
| Not Suitable For | Crude oil, heavy oil, or products containing sludge or prone to heavy buildup (use RF Admittance instead) |
RF capacitive level transmitters measure liquid levels by detecting changes in capacitance between the probe and the tank wall (or a coaxial outer tube); they support continuous measurement and can also output switching signals. This is a contact-based measurement method in which the probe is inserted into the medium, and there are no moving parts.
For information on the basic principles, classifications, and common applications of capacitive level sensors, please refer to “What Are Capacitive Level Sensors”.
| Parameter | Specification |
| Probe temperature resistance | −50~250℃ |
| Pressure rating | −0.1～32 MPa |
| Ambient temperature | −20~60℃ |
| Storage temperature | −55℃～+125℃ |
| Output Signal | 4–20mA, 4–20mA with HART communication, 485 communication, CAN bus communication |
| Power supply voltage | 12–28VDC |
| Mounting Methods | Threaded mounting: M20×1.5, M27×2. Flange mounting: DN25, DN40, DN50. Special specifications can be customized upon request. |
| Probe Type | Coaxial rod, coaxial cable, single rod, cable |
| Wetted materials | 316 stainless steel, 1Gr18Ni9Ti, or PTFE |
| Long-term stability | ≤0.2% FS/year |
| Temperature drift | ≤0.02%FS/℃ (within the range of 0–70℃) |
| Explosion Protection Rating | Intrinsically safe ExiaIIC T6, Flameproof ExdIIC T5 |
| Protection Rating | IP67 |
| Intrinsic Safety Parameters | Ui: 28VDC, Ii: 93mA, Pi: 0.65W, Ci: 0.042μF, Li: 0mH |
An RF capacitive level transmitter treats the probe and the outer electrode (tank wall or coaxial outer tube) as the two plates of a capacitor. The dielectric medium between the plates consists of air and the liquid being measured.
As the liquid level rises, the length of the probe submerged in the liquid increases, causing the capacitance to rise; conversely, as the level falls, the capacitance decreases. The circuitry detects this change in capacitance and, following temperature compensation and linearity correction, outputs a 4–20 mA or digital signal.
Capacitance formula for a coaxial probe:
The coaxial probe recommended for oil tanks consists of a central rod and an outer tube; its capacitance is calculated as follows:
C = 2πε₀εᵣL / ln(D/d)
C: Capacitance
ε₀: Permittivity of free space (8.85 × 10⁻¹² F/m)
εᵣ: Relative permittivity of the medium between the plates
L: Probe length
D: Inner diameter of the outer tube
d: Outer diameter of the central rod
When the submerged height of the probe (total length L) is h:
C = 2πε₀[εᵣ_oil · h + εᵣ_air · (L − h)] / ln(D/d)
Since D and d are fixed by the probe’s structure, the capacitance C is linearly related to the liquid level h.
The change in capacitance resulting from a change in liquid level is proportional to (εᵣ_oil − εᵣ_air); the smaller the difference in permittivity between the oil and air, the weaker the signal.
Relative permittivity of common media
| Medium | Relative Permittivity εᵣ (Approx.) |
| Air | 1.0 |
| Mineral oils (turbine oil, engine oil, etc.) | 2.1–2.4 |
| Water | 80 |
The difference in permittivity between oil and air is only about 1, whereas the difference between water and air is approximately 80-fold; this explains why oil tank measurements impose stricter requirements on probe design.
The dielectric constant of the oil varies slightly with temperature and aging; therefore, it should be calibrated against the specific oil before leaving the factory.
The RF capacitive type measures only the capacitance (C), whereas the RF admittance type measures the resistive component alongside the capacitance—that is, the admittance Y = G + jωC.
The primary difference lies in the probe structure: the RF admittance probe features an additional equipotential shield electrode outside the measuring electrode, which allows for the elimination of parasitic capacitance caused by material buildup on the probe.
| Comparison Item | RF Capacitance | RF Admittance |
| Measured Quantity | Capacitance (C) | Admittance (Y) (Conductance G + Capacitance C) |
| Probe Structure | Single measuring electrode: central measuring rod + PTFE insulation layer; tank wall or outer pipe serves as the other electrode | Three-electrode composite: measuring electrode + equipotential shielding electrode + outer insulation; tank body serves as the reference electrode |
| Insulation Layer | Single PTFE layer; serves both as medium isolation and as the capacitive dielectric | Two layers: one layer between the measuring and shielding electrodes, plus an outer PTFE wrap over the shielding electrode |
| Measurement Accuracy | ±2–3% FS | ±1% FS |
| Resistance to Material Buildup | Poor; buildup is interpreted as liquid level | Strong; shielding electrode cancels out the effect of buildup |
| Temperature & Pressure Rating | Probe: −50 to 250°C | −100 to 500°C; 20 MPa |
| Suitable Media | Clean liquids with stable dielectric constants: turbine oil, hydraulic oil, refined oil products | High-viscosity, buildup-prone, or sludge-laden media: crude oil, heavy oil, slurries, powders |
| Cost | Lower | Higher |
Therefore, RF capacitance probes should be selected for clean oil products, whereas RF admittance probes RF Admittance Level Switch should be chosen when the oil forms a film on the probe, contains debris, or generates foam.
Oil is a low-dielectric-constant medium (εᵣ ≈ 2), resulting in a weak capacitance-change signal. Standard single-rod probes utilize the tank wall as the counter-electrode; however, since the electrode spacing varies with the tank diameter and the electric field diverges, measurements in oil tanks can easily become unstable.
Coaxial probes feature an outer tube surrounding the central rod, with the outer tube serving as the counter-electrode:
Application Constraints: The annular gap between the coaxial outer tube and the central rod is narrow. Media that are highly viscous, contain particulates, or are prone to coking can clog this gap. Coaxial probes are suitable for clean oil products such as turbine oil, hydraulic oil, and refined petroleum products. They are unsuitable for crude oil, heavy oil, or residue-laden products; for these, an RF admittance single-rod probe should be used instead.
Probe Material: Select 316L stainless steel for turbine oil and refined petroleum products. 316L is the preferred choice for sulfur-containing or corrosive oils (it contains molybdenum, offering superior corrosion resistance compared to 1Cr18Ni9Ti). In cases of severe corrosion, a full PTFE coating may be selected.
The insulation layer encasing the central measuring rod acts as the dielectric medium between the capacitor plates.
Its temperature resistance and dimensional stability directly impact measurement stability. Selecting the wrong insulation material is a common source of measurement error.
(1) PTFE: Suitable for 200°C, 24-bar turbine oil applications
PTFE supports long-term operating temperatures up to 250°C; with a customer requirement of 200°C, this provides a safety margin of approximately 50°C. It is chemically stable and oil-resistant.
A weakness of PTFE is cold flow (creep). This effect becomes more pronounced at higher temperatures and pressures; therefore, PTFE sealing sections in high-temperature, high-pressure probes require a reinforced structural design.
(2) Insulation materials unsuitable for high-temperature oil tanks
| Material | Long-term operating temp. (approx.) | Suitability for 200°C oil tank |
| PP (Polypropylene) | 100°C | Unsuitable; softens and deforms |
| PE (Polyethylene) | 80°C | Unsuitable |
| PVC | 60°C | Unsuitable; decomposes at high temperatures |
| FEP | 200°C | No safety margin; not recommended |
| PFA | 260℃ | Suitable |
| PTFE | 250～260℃ | Recommended |
(3) PFA vs. PTFE
PFA offers temperature resistance comparable to PTFE but allows for melt processing and results in a denser coating; it is frequently used to fully coat cable-style probes but is more expensive than PTFE. For rod-style probes used in clean turbine oil applications, PTFE meets all requirements and offers better cost-effectiveness.
(1) Temperature resistance of the wetted probe tip
Customer specifications: Medium temperature 200°C, pressure 24 bar. Configuration: PTFE insulation + 316L stainless steel probe; temperature rating 250°C.
The 200°C figure in this case represents the specification provided by the customer. Since the normal operating temperature of the turbine oil tank is typically lower than this value, the selected model accounts for this by incorporating a safety margin based on the design value.
(2) Transmitter head (electrical end) temperature
The ambient temperature range for the transmitter head is −20 to 60°C. When the medium temperature is high, heat transfers from the tank and probe to the transmitter head; therefore, a heat-dissipating section (extended neck) or a remote mounting configuration must be used to prevent the circuitry from overheating.
(3) Pressure resistance
The coaxial PTFE probe is selected based on the customer’s design pressure—24 bar in this case. The probe sealing method and pressure rating must be confirmed at the time of quotation.
(1) Output Signal
The standard output is a 4–20 mA two-wire signal. Select the 4–20 mA + HART option if remote range configuration or diagnostic data retrieval is required. HART communication requires a 250Ω load resistor in the loop; the power supply voltage should be calculated based on the load.
(2) Mounting Interface
Top mounting via thread or flange is common for oil tanks. Standard threads include M20×1.5 and M27×2; standard flanges include DN25, DN40, and DN50. The 1.5″ BSP (G1½) male thread mentioned in the example is a custom interface that can be machined upon request.
(3) Parameters Required for Inquiry
Unit 1: Turbine Oil Tank
| Project | Client Specifications |
| Medium | Turbine oil |
| Probe | Coaxial rod type, 700 mm length |
| Insulation | PTFE |
| Temperature / Pressure | 200°C / 24 bar |
| Output | 4-20 mA + HART |
| Mounting | 1.5” BSP male thread |
The client’s initial inquiry specified only four parameters: probe type, insulation, temperature, and pressure. We followed up to clarify the signal output and mounting thread specifications; once confirmed, we provided the configuration, and the client subsequently requested technical documentation including dimensional drawings.
Unit 2: Conductive Liquid Storage Tank
Two weeks later, the same client submitted an inquiry for a conductive liquid storage tank:
| Project | Client Specifications |
| Medium | Conductive liquid |
| Probe | Cable type; measuring length 5000 mm, non-measuring section 200 mm |
| Insulation | PFA / FEP; coating over full length or measuring section only |
| Probe Material | 316 / 304 stainless steel; cable diameter 4–6 mm |
| Output / Power Supply | 4-20 mA + HART; 2-wire, 24 VDC |
| Accuracy | ±0.5% or better; factory calibrated for 0–100% range using the conductive liquid |
| Other | IP65 or higher rating; digital display; top-mounted; 1.5” BSP male thread; quantity: 8 units |
Why were the two transmitters configured differently?
| Comparison Point | Turbine Oil Tank | Conductive Liquid Tank |
| Medium Conductivity | Non-conductive; the oil itself acts as the capacitor’s dielectric | Conductive; the liquid acts as the grounded counter-electrode, while the insulation layer serves as the capacitor’s dielectric |
| Probe Type | Coaxial rod: Oil has a low dielectric constant (approx. 2), requiring a coaxial structure with fixed spacing | Cable type: Conductive liquid provides a strong signal, so a coaxial design isn’t needed; 5 m range makes cable type easier to transport and install |
| Function of the Insulation Layer | Isolation medium and temperature resistance | Determines the measurement itself; the entire length must be free of damage. |
| Insulation material | PTFE (200°C) | PFA / FEP (ambient temperature; cable coating is denser) |
A US company developing overflow monitoring equipment for foodservice grease traps inquired about RF Admittance level switches:
RF Capacitance technology is unsuitable for this application for the following reasons:
Comparison with the turbine oil tank in Case Study 1: RF Capacitance is selected for clean oil, whereas RF Admittance is chosen for applications involving buildup, debris, or foaming.
RF capacitance level transmitters feature a simple, cost-effective design suitable for continuous level measurement in clean oil tanks.
Key selection criteria include: a coaxial probe, PTFE insulation, design margins for temperature and pressure, and a heat-dissipating housing for high-temperature applications.
The anti-buildup design characteristic of RF admittance technology is unnecessary for clean turbine oil applications and would only increase procurement costs; however, RF admittance is the preferred choice for crude oil and heavy oil storage tanks.
For selection assistance or a quote regarding capacitive level transmitters Universal Intelligent Capacitive Level Transmitter for oil tanks, please provide details on the medium, probe length, temperature and pressure ratings, output signal, and mounting connection.
Zhang Wei, possesses 20 years of experience as an automation instrumentation engineer, specializing in the research, design, installation, commissioning, and maintenance of automation instruments.
Face to various instrument communication protocols (such as Modbus, Profibus, etc.), with solid hardware circuit design and software programming skills (proficient in C language and PLC programming). Has extensive project experience; projects he has led and participated in have all achieved outstanding results, improving product accuracy, reducing costs, and increasing production efficiency.
Possesses excellent communication and coordination skills and a strong team spirit, enabling him to quickly respond to customer needs and provide high-quality automation instrumentation solutions.