Pressure transmitters: When you need one and how to choose the right one
Your pressure gauge suddenly starts showing incorrect readings – or fails completely. The reason: the medium being measured has corroded or blocked the sensitive Bourdon tube. A pressure transmitter would have prevented this. But when is it really worth using one, and which design is right for your application?
What does a pressure transmitter do – and why is it so important?
The function of a pressure transmitter can be summarised in a single sentence: it isolates the pressure gauge from the process medium – whilst still reliably transmitting the pressure. It sounds simple, but in many industrial plants it is the crucial difference between a measurement that works for years and a total failure after just a few weeks.
The principle behind it: an elastic diaphragm absorbs the process pressure and transmits it via a pressurised fluid – usually a special silicone oil – to the attached measuring instrument. The process medium never comes into contact with the Bourdon tube or the sensor; only the pressurised fluid does.
How a pressure transmitter works in detail
- Diaphragm: Absorbs the process pressure directly – available in 316L stainless steel, Hastelloy, Monel or tantalum, as well as elastomers
- Pressure chamber: Completely filled with transmission fluid – no air, no compression
- Transmission fluid: Depending on the temperature range or application, silicone oil, paraffin oil, glycerine, halocarbon or others. For food applications, a suitable transmission fluid must be selected (e.g. with FDA approval)
- Connection: Mounted directly onto the measuring instrument or connected via a capillary tube
Diaphragm pressure transmitters are particularly well suited to precise measurements at pressures ranging from 600 mbar to 250 bar and temperatures from –90 °C to +400 °C.
When do you need a pressure transmitter? The five key situations
Not every pressure measurement requires a pressure transmitter. However, if any of the following conditions apply to your application, a pressure transmitter is virtually indispensable:
1. Aggressive or corrosive media
Acids, alkalis, chlorides or highly acidic condensates attack the bourdon tube of a pressure gauge within a short space of time. The pressure transmitter, with its chemically resistant diaphragm – a stainless steel diaphragm coated with PTFE or PFA, or a diaphragm made of Hastelloy or tantalum – provides long-term protection for your measuring instrument.
2. Highly viscous or crystallising media
Chocolate mass, polymers, adhesives: highly viscous substances clog the Bourdon tube or measuring cell of conventional pressure gauges. Crystallising media such as sugar solutions settle and block the system. The pressure transmitter, with its large-area, flush-mounted diaphragm, prevents precisely this.
3. Hygienic requirements (pharmaceutical, food)
In the food and pharmaceutical industries, no dead space must be allowed to form in which germs can grow. Pressure transmitters with a flush-mounted diaphragm and process connections such as Tri-Clamp, DIN 11851 or Varivent meet the strict hygiene standards set by EHEDG and FDA 21 CFR. It is important to choose the correct transmission fluid; this must be approved for use with foodstuffs.
4. Abrasive or solids-laden media
Sand in water, abrasives in suspensions, cement slurry – abrasive particles destroy conventional pressure transducers in no time at all. The robust piston of a piston-type pressure transmitter or a low-pressure transmitter with an internal rubber diaphragm can withstand mechanical wear.
Rule of thumb: If your process medium could clog, corrode, overheat or cause hygienic contamination of the measuring instrument – use a pressure transmitter.
5. Extreme temperatures
With steam above 200 °C or cryogenic media below –40 °C, a directly connected measuring instrument would be damaged. A pressure transmitter with a capillary tube provides the necessary thermal separation. Choosing the right transmission fluid is crucial here: silicone oil for standard temperatures, halocarbon for low temperatures or applications involving oxygen, and special heat-resistant oils for high-temperature applications.
A comparison of pressure transmitter types: Which one is right for your application?
Not all pressure transmitters are the same. Different designs are used depending on the installation situation, the medium and the pressure range. The following table provides a quick guide:
| Type of pressure transmitter | Connection type | Typical application | Pressure range |
|---|---|---|---|
| Diaphragm pressure transducer | Screw-in thread, flange connection, paper flange, TRI-CLAMP, DIN 11851/11887, SMS, Varivent, Neumo Biocontrol, homogeniser connection | Viscous, crystallising and hygienic fluids; large nominal diameters; aggressive and corrosive fluids; pharmaceutical, food and chemical applications | 0 – 1000 bar |
| Piston-type pressure transmitter | Screw-in thread with robust pistons | For extremely heavy-duty use with abrasive media, sludge, concrete and plaster | 0 – 600 bar |
| Small pressure transmitters | Threaded connection with a flush-mounted diaphragm or an internal rubber diaphragm | For simple applications in industry and the chemical sector involving contaminated media | 0 – 1000 bar |
| Pressure transmitter with capillary tube | For use with diaphragm or tube pressure transmitters | Extreme temperatures, a measurement point located at a distance, and severe vibration at the installation site | On request |
| Tubular pressure transmitters | Cylindrical tubular diaphragm with TRI-CLAMP, DIN 11851/11887 or as an intermediate flange version | In-line measurement in pipework, compact design, for the food and pharmaceutical industries | 0 – 250 bar |
Practical tip: The choice of design is primarily determined by three factors: the medium (viscous, hygienic, corrosive?), the pressure range and the installation situation (tank, pipework, retrofit). TRI-MATIC supplies pressure transmitters for all standard designs and process connections.
Diaphragm material and filling fluid – the two critical decisions
With a pressure transmitter, two factors determine the service life of your measuring point: the material of the diaphragm and the transmission fluid inside. If you make the wrong choice here, even the best design will be of no use.
Diaphragm materials: Which one for which medium?
- 316L stainless steel: Standard material for most industrial media – water, steam, oils, mild chemicals. Good value for money
- PTFE/PFA coating: Cost-effective alternative for moderate corrosion exposure. The coating protects a stainless steel diaphragm from chemical attack
- Hastelloy: The first choice for highly corrosive media – hydrochloric acid, sulphuric acid, chlorides. Indispensable in the chemical and petrochemical industries
- Monel 400: Proven for hydrofluoric acid and alkaline media. Also used in seawater desalination and offshore technology
- Tantalum: The ‘precious metal’ among membrane materials – for maximum corrosion resistance in hot, concentrated acids
Transmission fluids: tried-and-tested system fillings for pressure transmitters
The transmission fluid determines the usable temperature range and suitability for the respective medium. Typical examples:
- Silicone oil: standard filling for approximately –40 °C to +200 °C, covering most industrial applications.
- High-temperature oil: for steam and thermal oil systems up to +400 °C.
- Halocarbon oil: for low-temperature applications and oxygen-carrying systems.
- Paraffin oil (FM 09): FDA-listed, for food and beverage processes.
- Glycerine/water: another FDA-compliant option for hygienic applications.
When making a selection, in addition to temperature, compatibility with the measured medium is crucial in the event of a diaphragm rupture. In the pharmaceutical and food industries, only physiologically safe, approved fillings are permitted.
TRI-MATIC supplies measuring units (pressure gauges, sensors or pressure switches) fully assembled onto the appropriate pressure transmitter. Everything is tested, documented and ready for installation.
The four most common mistakes when selecting a pressure transmitter
In our day-to-day consultancy work, we come across the same stumbling blocks time and time again. These four mistakes cost plant operators time, money – and sometimes the entire measuring point:
316L stainless steel is not suitable for every medium. With chloride concentrations of 50 ppm or higher, or concentrated acids, pitting corrosion is only a matter of time. Always check chemical resistance based on specific medium concentrations and temperatures – not just on the medium group.
The design, diaphragm material, process connection and filling fluid must all be suitable for the specific medium and installation conditions. Hygienic processes require a dead-space-free, flush-mounted diaphragm with a clamp (ISO 2852, DIN 32676), VARIVENT or milk pipe connection (DIN 11851) and an FDA-compliant filling; abrasive media require a piston or low-pressure diaphragm seal; highly corrosive media require a PTFE/PFA-coated stainless steel diaphragm or special materials such as Hastelloy C276 or tantalum. Failure to clarify the application properly risks inaccurate measurements and premature failure of the measuring point.
Silicone oil at 300 °C? The thermal limit is quickly exceeded. The filling fluid decomposes and the measurement drifts. The same applies the other way round: for cryogenic applications below –50 °C, you must use halocarbon oil. For food and pharmaceutical processes, the filling fluid must be physiologically safe and approved – for example, an FDA-listed white/paraffin oil or a glycerine-water mixture, not standard silicone oil.
In systems with vacuum phases (e.g. distillation or drying), the pressure transmitter must be designed to withstand vacuum – with a vacuum-compatible filling (vacuum oil) and a vacuum-resistant diaphragm. Otherwise, the transmission fluid will escape or the diaphragm will deform, rendering the measurement unusable.
Conclusion: The pressure transmitter protects your measurement – provided you choose the right one
A pressure transmitter is not just a nice-to-have; in many applications, it is essential for reliable pressure measurement. Whether dealing with corrosive chemicals, viscous foodstuffs or contaminated media – without the protective diaphragm between the process and the measuring instrument, you risk inaccurate readings, failures and unplanned downtime.
When making your selection, ask yourself these three questions:
What medium am I measuring – and how does it corrode standard materials?
What are the temperatures at the installation site?
Are there any hygiene requirements (FDA, EHEDG)?
The TRI-MATIC sales team will support you with the design – from advice on materials and the choice of filling fluid right through to the fully tested assembly. Whether you need a single unit or a complete range of pressure measurement accessories: you’ll get everything from a single source.
Frequently Asked Questions
Pressure transmitters themselves are low-maintenance components. The diaphragm is not subject to mechanical wear in the traditional sense. Nevertheless, we recommend an annual visual inspection for leaks and a check of the measured values. In critical applications – such as in pharmaceutical production – validation often stipulates calibration intervals for the entire system (pressure transmitter + measuring instrument). Pay particular attention to signs of diaphragm fatigue: zero-point drift and sluggish response behaviour.
The accuracy depends on the entire system. A pressure transmitter itself typically introduces an additional measurement error of 0.1 to 0.5 per cent of the measurement range – depending on the design, fill volume and the effect of temperature. When fitted directly and with correct temperature compensation, modern pressure transmitter systems with pressure sensors achieve an overall accuracy of less than 0.5 per cent of the measurement range.
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