Capillary Diaphragm Seal Differential Pressure Transmitter

A Capillary Diaphragm Seal Differential Pressure Transmitter is an industrial level measurement instrument designed for applications involving high temperature, corrosive media, viscous fluids, crystallizing media, and restricted installation space. It is typically composed of a differential pressure transmitter body, a single flange diaphragm seal assembly, a capillary remote seal system, and internal fill fluid, enabling continuous and stable level measurement under complex process conditions.

Compared with a single flange direct-mount structure, the capillary type separates the measuring diaphragm from the transmitter body. The purpose of this design is usually to mount the flange diaphragm at a high-temperature or space-limited measuring point, while installing the transmitter body at a location more suitable for observation, wiring, and maintenance.

Pressure Type: Gauge Pressure, Absolute Pressure, Differential Pressure, Level
Pressure Range:
-1 bar to 30 bar, 0~300mH2O
Accuracy: ±0.075%, ±0.2%
Output Signal: 4–20 mA / HART
Process Connection: DN20 / DN25 / DN40 / DN50 / DN65 / DN80 / DN100 / DN150 (customized available)
Diaphragm Material: SS316L, HC-276, Tantalum, Gold-plated
Capillary length: 1-10 meters (length can be customized upon request)
Power Supply: 12–36 VDC
Protection Rating: IP65 / IP67

OEM Service: Available. The transmitter can be supplied with your company name and logo on the nameplate.
Packaging: Individual carton box with protective foam.
Weight: Approx. 6 kg per unit.
Port of Shipment: Shanghai or Ningbo Port, China.
Delivery Time: 3-7 working days.
Transportation: Air freight, sea freight, or express delivery.
Documentation: Certificate of Conformity (COC); Inspection / Test Report; Packing List; Commercial Invoice

Accessories

Basic Introduction to Capillary Diaphragm Seal Differential Pressure Transmitter

1. What Is a Capillary Diaphragm Seal Differential Pressure Transmitter?

A single flange capillary differential pressure level transmitter is essentially a differential pressure level measuring instrument with a single-side diaphragm seal and capillary remote seal structure.

It still belongs to the category of differential pressure level measurement solutions, but unlike a standard single flange direct-mount design, the flange diaphragm seal assembly is not mounted directly on the transmitter body. Instead, it is connected to the transmitter through a capillary.

There are three key concepts involved here:

Single Flange
This means that the process connection uses a single-side flange diaphragm seal assembly. The isolation diaphragm is in direct contact with the process medium and is used to sense process pressure.

Capillary
This refers to the remote transmission system that connects the flange diaphragm seal assembly and the transmitter body. The capillary is filled with a special fill fluid, which stably transfers the pressure applied to the diaphragm to the transmitter sensor.

Differential Pressure Level
This means that the liquid level is not measured directly as a height. Instead, based on the hydrostatic pressure principle, the pressure difference is converted into liquid level.

Many users confuse this product with a conventional single flange level transmitter. In fact, the biggest difference between the two is:

In a direct-mount design, the diaphragm and the transmitter body are connected together, while in a capillary type, the two are installed separately.

Therefore, a single flange capillary diaphragm seal differential pressure transmitter is more suitable for applications where direct mounting is inconvenient, temperature is high, vibration is significant, or the transmitter body needs to be installed more flexibly.

2. Structure and Diagram of a single flange capillary diaphragm seal differential pressure transmitter

A single flange capillary diaphragm seal differential pressure transmitter typically consists of the following parts:

Differential Pressure Transmitter Body
This is the core electronic and sensing unit of the product. It receives the pressure signal transmitted through the capillary and outputs a standard industrial signal. The common appearance is usually based on a 3051-style industrial structure, which is convenient for field installation and maintenance.

Single Flange Diaphragm Seal Assembly
Installed at the measuring point on the vessel, this part is in direct contact with the medium. Its function is to isolate the measured medium from the sensitive internal components of the instrument, preventing corrosion, clogging, or contamination of the sensor.

Capillary Remote Seal System
This connects the flange diaphragm seal assembly to the transmitter body and is the key part of the single flange capillary structure. The capillary is filled with fill fluid, which transmits the pressure signal and also allows the transmitter body to be installed away from a high-temperature or confined location.

Internal Fill Fluid
The fill fluid is used to transfer the pressure sensed by the diaphragm stably to the sensor. Different operating conditions require different types of fill fluid, especially in high-temperature or low-temperature service.

Flanged Process Connection
This is used to install the diaphragm seal assembly onto the tank, vessel, or equipment connection. Flange size, standard, pressure rating, and sealing face can usually be customized according to project requirements.

Mounting Bracket
Used to secure the transmitter body, making the installation more stable and facilitating later inspection and wiring.

Display Head and Electrical Connection
Some models can be equipped with an LCD display head for direct on-site reading of level, percentage, current, or pressure parameters. Electrical connection options commonly include M20×1.5 and 1/2-14 NPT.

3. Capillary diaphragm seal differential pressure transmitters Working Principle

The working principle of a single flange capillary diaphragm seal differential pressure transmitter is still based on the hydrostatic pressure principle.

As the liquid level rises, the flange diaphragm seal assembly installed at the lower side or lower section of the vessel senses a greater hydrostatic pressure.

This pressure first acts on the isolation diaphragm, then is transferred through the fill fluid inside the capillary to the differential pressure transmitter body, and finally is converted by the electronics module into a standard output signal, such as 4–20 mA or HART communication.

In other words, the pressure transmission path of this type of product is usually:

Process pressure → isolation diaphragm → fill fluid → capillary → transmitter sensor → electrical output

One important point must be noted: the capillary does not transmit an electrical signal. It transmits pressure through the fill fluid.

Therefore, capillary length, fill fluid type, installation position, and ambient temperature can all affect measurement performance.

In actual level measurement applications, liquid level is not directly entered into the transmitter. Instead, it must be calculated based on the following parameters:

Medium density
Liquid level height
Installation position
Process pressure condition
Capillary structure

That is why, when requesting a quotation for this type of product, it is not enough to provide only “how many meters of level.” The medium, temperature, density, and installation conditions must also be confirmed.

4. Applications of capillary diaphragm seal differential pressure level transmitter

A single flange capillary diaphragm seal differential pressure transmitter is commonly used in the following industries:

Chemical industry
Petrochemical industry
Pharmaceutical industry
Food and beverage industry
Fermentation systems
Water treatment
Environmental protection
Tank farms and storage areas
Various process vessel level measurement systems

It is especially widely used where process conditions are complex, media properties are challenging, and installation environments are less than ideal.

5. Why Choose a Capillary Remote Seal Structure?

Many users ask the same question when they first encounter this type of product:

If a single flange direct-mount transmitter can also measure level, why use a capillary type?

The answer usually lies in the site conditions.

To Prevent High Temperature from Reaching the Transmitter Body Directly
When the flange measuring point temperature is high, directly mounting the transmitter body there may affect the long-term stability and life of the electronics module. The capillary structure allows the body to be installed in a lower-temperature location.

To Improve Installation and Maintenance Convenience
The flange diaphragm can be installed at the vessel measuring point, while the transmitter body can be installed at a location more convenient for observation, wiring, and maintenance.

To Increase Installation Flexibility
For narrow spaces, restricted orientations, or structurally complex equipment, the capillary structure is easier to adapt to field installation requirements.

To Suit More Complex Process Environments
In applications involving vibration, heat tracing, radiant heat, or structural interference, a capillary remote seal is usually a more reliable choice.

So the capillary type is not intended to make the product “more complicated.” It is designed to solve problems that are difficult to address with a direct-mount type.

capillary diaphragm seal differential pressure transmitter Product Technology

1. Technical Specifications of capillary diaphragm seal differential pressure transmitter

SIY3051C Diaphragm Seal Differential Pressure Level Transmitter Selection Table
10	Transmitter type
	D	Differential pressure transmitter
	G	Gauge pressure transmitter
	A	Absolute pressure transmitter
20	Accuracy
	-H	Basic error ±0.075%
	-N	Basic Error ±0.2%
30	Pressure Range
	3051CD Differential pressure
	3	-6kPa～6kPa（-60mbar～60mbar）
	4	-40kPa～40kPa（-400mbar～400mbar）
	5	-250kPa～250kPa（-2.5bar～2.5bar）
	6	-1MPa～1MPa（-10bar～10bar）
	7	-3MPa～3MPa（-30bar～30bar）
	9	Other agreed pressure ranges
	3051CG Gauge pressure
	3	-6kPa～6kPa（-60mbar～60mbar）
	4	-40kPa～40kPa（-400mbar～400mbar）
	5	-100kPa～250kPa（-1bar～2.5bar）
	6	-0.1MPa～1MPa（-1bar～10bar）
	7	-0.1MPa～3MPa（-1bar～30bar）
	8	-0.1MPa～10MPa（-1bar～100bar）
	9	-0.1MPa～21MPa（-1bar～210bar）
	10	-0.1MPa～40MPa（-1bar～400bar）
	3051CA Absolute pressure
	4	0～40kPa（0～0.4bar）
	5	0～250kPa（0～2.5bar）
	6	0～3MPa（0～30bar）
	9	Other agreed pressure ranges
	Note: 3051CD static pressure: 16 MPa / 25 MPa / 40 MPa
40	Communication Protocol
	H	4–20 mA + HART5/HART6/HART7 communication protocol
	M	RS485 Modbus communication protocol
50	Diaphragm material
	2	316 stainless steel
	3	Hastelloy C-276
	4	Monel
	5	Tantalum (only for 3051CD & CG, range 4-9, not for 3051CA)
	6	Gold-plated
	0	Customer-specified diaphragm material
60	Process connection
	S1	One remote flange
	S2	Two remote flanges
70	Wetted seal material
	N	Nitrile rubber (NBR)
	F	Fluoroelastomer (FKM)
	P	PTFE (Polytetrafluoroethylene)
80	Fill fluid
	1	Silicone oil
	2	Fluorinated oil
90	Display
	N	None
	L	LCD display (-20°C)
	O	OLED display (-40°C)
100	Housing material & electrical connection
	B	Aluminum alloy housing, electrical connection M20×1.5
	S	Aluminum alloy housing, electrical connection 1/2 NPT
	C	Stainless steel housing, electrical connection M20×1.5
	T	Stainless steel housing, electrical connection 1/2 NPT
110	Connection accessories
	3M	3-valve manifold
	5M	5-valve manifold
	2F	1/2 NPT oval flange
120	Mounting bracket option
	N	None
	1	Galvanized carbon steel bracket
	2	Stainless steel bracket
130	Hazardous area certification
	E5	Intrinsic safety ia IIC T4-T6
	K5	Explosion-proof d IIC T4-T6
140	Other options
	Q4	Factory calibration certificate
	Q3	Third-party calibration certificate
	V6	Suitable for full vacuum service
	J1	NAMUR NE43 standard
	J3	Built-in lightning protection module
	T1	Oil-free degreasing
	C1	SIL3 certification (for HART communication type only)
150	Remote seal assembly options for diaphragm pressure transmitter (Note: -1199 option)
	RFW	Remote flange type seal
	EFW	Extended flange type seal
	FFW	Flush flange type seal
	PFW	Flat type seal
	SCW	Sanitary type seal
	RTW	Remote threaded type seal
160	Flange rating & material
	A21
	B21
	A31
	B31
	A41
	B41
	A22
	B22
	A32
	B32
	A42
	B42
	A24
	B24
	A34
	B34
	A44
	B44
	YY
170	Diaphragm material
	D4	316L
	D6	Hastelloy
	E0	Tantalum
	F0	Monel
	P	With PTFE/PFA coating
	G	Gold-plated coating
	Z	Other agreed special requirements
180	Insertion tube (for EFW extended flange type seal only)

	EC0
	ES1
	ES2
	ES3
	EP1
	EP2
	EP3
	EH1
	EH2
	EH3
190	Pressure connection port (for RTW remote threaded type seal only)
	A	1/4–18 NPT
	C	3/8–18 NPT
	D	1/2–14 NPT
	H	1–111/2 NPT
	H	11/2–111/2 NPT
200	Seal fill fluid
	A
	C
	D
	H
210	Capillary length
	□□	Agreed capillary length, from 1–15 m (e.g. 2 m: 02)
220	Capillary armor material
	P	304+PVC
230	Other options
	T1	Oil and grease removal
	V6	Vacuum-resistant treatment
	Z	Other agreed special requirements
Note: When selecting a dual-flange DP level transmitter, if the high-pressure and low-pressure side seals are different, select from the high-pressure side first.

2. How to Select a Suitable Single Flange Capillary Diaphragm Seal Differential Pressure Transmitter？

First Determine Whether a Capillary Structure Is Necessary
If the measuring point is hot, installation space is limited, wiring is inconvenient, or the transmitter body must be located away from a heat source, a capillary type is usually more suitable.

Convert Liquid Height and Density into Differential Pressure Range
Liquid level height is only the surface parameter. In actual selection, it still has to be converted into a differential pressure value.

Select Fill Fluid and Structure According to Process Temperature
The capillary type is better suited for high-temperature service, but only if the fill fluid and structural design are selected correctly.

Choose Diaphragm Material According to Corrosiveness
316L is suitable for most general media, but strongly corrosive applications often require higher-grade materials.

Determine Capillary Length According to Installation Distance
The length should meet field installation requirements, but longer is not always better.

Choose Output Functions According to System Requirements
If remote configuration and diagnostics are needed, HART communication can be considered.

3. Relationship Between Capillary Length and Measurement Performance

Capillary length is a critical factor for single flange capillary products.

Many people assume that as long as the site allows installation, a longer capillary always means greater flexibility. In fact, this is not the case.

As capillary length increases, it usually brings the following effects:

Slower response speed
Greater sensitivity to ambient temperature
More complicated installation layout
Greater dependence of zero stability on installation quality

Therefore, as long as installation and maintenance requirements are met, it is usually recommended to keep the capillary length within a reasonable range rather than making it unnecessarily long.

4. Common Diaphragm Materials for Single Flange Differential Pressure Level Transmitters

316L Stainless Steel

316L is the most common standard wetted material used in single flange differential pressure level transmitters. It is suitable for most conventional liquid media and general industrial operating conditions. For non-strongly corrosive media such as water, oil, and general chemical liquids, 316L is usually sufficient, while also offering good cost-effectiveness and shorter lead times.

Hastelloy

Hastelloy is suitable for more corrosive chemical media. In particular, it provides better corrosion resistance than 316L in certain acidic, chloride-containing, or otherwise complex corrosive environments. When 316L cannot provide long-term compatibility, a Hastelloy diaphragm should be considered first, although the final selection still needs to be confirmed according to the medium composition, concentration, and temperature.

Tantalum Diaphragm

Tantalum offers excellent corrosion resistance and is commonly used in highly corrosive service conditions, especially where very high diaphragm corrosion resistance is required. However, tantalum is not suitable for all media. It is generally not recommended for strong alkalis and some special corrosive systems, so the final selection should always be based on the specific process conditions.

Monel

Monel provides good corrosion resistance in certain special chemical media, especially in some fluorine-containing media, hydrofluoric acid, and other demanding service conditions. It is not usually the first choice for conventional media, but is typically used in applications with clearly defined corrosion-resistance requirements.

PTFE Corrosion-Resistant Structure

A PTFE corrosion-resistant structure is suitable for a wide range of acidic, alkaline, and highly corrosive media. It offers clear advantages in applications where conventional metal diaphragms cannot provide sufficient corrosion resistance. For strongly corrosive media, a PTFE-based protection solution is often a more reliable choice, although temperature, pressure, and installation design must also be taken into account.

Gold-Plated Diaphragm

Gold-plated diaphragms are commonly used for special media such as hydrogen service. They can effectively reduce hydrogen permeation and the risk of hydrogen embrittlement, thereby improving long-term operating reliability. This type of solution is generally used as a special option for specific operating conditions rather than as a standard configuration.

Diamond Diaphragm

Diamond diaphragms are typically used in service conditions involving strong corrosion, solid particles, or severe wear in the process medium. They have already been proven suitable for black water applications.

Material Selection Recommendation

Diaphragm material selection should not be based on price alone. More importantly, the material must be compatible with the actual process medium and operating conditions. A proper selection should take into account the medium composition, concentration, temperature, pressure, and corrosion characteristics. On this basis, cost, lead time, and long-term operating stability should then be evaluated together.

Installation and Maintenance of capillary differential pressure transmitter

1. How to Install a capillary differential pressure transmitter?

Correct installation directly affects the measurement accuracy, stability, and service life of a diaphragm seal pressure transmitter.

1) Check before installation

Confirm whether the model is correct
Confirm whether the measuring range matches the application
Confirm whether the flange specification is compatible
Confirm whether the diaphragm surface is intact

2) Avoid diaphragm damage

The isolating diaphragm is the key pressure-sensing component. During transportation, unpacking, and installation, it must not be dented, squeezed, or scratched by hard objects. It is best to gently press the diaphragm with a finger to check whether it has elasticity. If the diaphragm does not move when pressed, contact the manufacturer directly.

3) Position the gasket correctly

The gasket provides sealing and helps prevent medium leakage, so it must be installed correctly. At the same time, care must be taken to ensure that the gasket does not press against the diaphragm, otherwise zero shift may occur. PTFE gaskets are commonly used, but if the process temperature is too high, a spiral wound gasket should be used instead.

4) Tighten the fasteners properly

The transmitter flange is usually not welded to the mating flange on the equipment. In most cases, the two flanges are connected by bolts and other fasteners. Therefore, whether the bolts are tightened properly has a direct effect on both measurement performance and whether the medium may leak.

5) Fix the transmitter head properly

Use a flat mounting bracket or an L-shaped bracket to secure the transmitter head to a 2-inch pipe. The installation position should be convenient for on-site personnel to observe and commission the transmitter.

6) Wire correctly

In most cases, only the positive and negative power wires need to be connected to the corresponding terminals of the transmitter. Incorrect wiring may lead to unexpected problems.

7) Check zero before commissioning

After installation, the zero point should be checked whenever conditions allow.
In practice, it is common for customers to position the gasket incorrectly during installation, causing it to press against the diaphragm and resulting in zero shift. In this case, the transmitter can be removed and installed again correctly. If removal is not possible, the zero can be recalibrated. However, this is only a temporary corrective measure and is not suitable for long-term measurement.

2. Common Faults and Troubleshooting of a Capillary Differential Pressure Transmitter

No Output Signal

If the capillary differential pressure transmitter has no output signal, first check whether the power supply is normal, whether the wiring is correct, whether the polarity is reversed, and whether the signal loop is complete.
After confirming that the external electrical conditions are normal, the next step is to inspect the electronic module, wiring terminals, and the capillary differential pressure transmitter itself for any abnormalities.

Output Too High or Too Low

If the output value of the capillary differential pressure transmitter is obviously too high or too low, the first items to check are the range setting, zero suppression or elevation, and overall parameter configuration.
In addition, incorrect medium density settings, changes in installation position, or differences between the actual field conditions and the configured conditions may also cause significant measurement deviation.

Signal Fluctuation

Unstable or fluctuating signals from a capillary differential pressure transmitter are often caused by mechanical vibration, ambient temperature changes, electromagnetic interference, insufficient mounting support, or poor capillary fixing.
When this happens, the installation condition, nearby equipment operation, and cable routing environment should all be checked together.

Zero Drift

Zero drift in a capillary differential pressure transmitter is usually related to long-term exposure to high temperature, improper fill fluid selection, or performance changes after extended operation.
If the operating temperature varies significantly, or if the fill fluid is not suitable for the application, zero stability may be affected. For this reason, these factors should be considered carefully during the selection and installation stages.

Slow Response

If the response of the capillary differential pressure transmitter becomes noticeably slow, possible causes include excessive capillary length, improper fill fluid selection, or low ambient temperature.
Especially in low-temperature service or with long capillaries, the pressure transmission speed of the fill fluid may be affected, which can result in a slower overall response.