Product type
A pressure sensor turns the pressure in a pipe, a cylinder or a vessel into an electrical signal a machine can read. On a hydraulic power pack, a pneumatic line, a pump skid or a test bench, it is the element that lets the control system know what the fluid is doing, rather than leaving it to an operator reading a dial.
A pressure sensor turns the pressure in a pipe, a cylinder or a vessel into an electrical signal a machine can read. On a hydraulic power pack, a pneumatic line, a pump skid or a test bench, it is the element that lets the control system know what the fluid is doing, rather than leaving it to an operator reading a dial.
The term covers two things that are frequently separated in a catalogue and never separated in an installation. A sensing element on its own produces a small signal that needs conditioning before anything can read it. A transmitter packages the element with the electronics that amplify, linearise and temperature-compensate it, and with an output stage that drives a standard signal. Almost everything sold for machine building is the second kind, which is why the words are used interchangeably in practice.
Selection for a machine turns on four questions: what the sensing technology has to tolerate, which pressure reference the reading is against, what output the control system takes, and what process connection the port already has. The last of those is often the fixed constraint, because a port cannot be changed as easily as a sensor can.
Three technologies account for most machine-mounted pressure sensors, and they fail differently. A piezoresistive element uses a silicon diaphragm with a diffused resistor network; it is sensitive, cheap and well suited to clean, dry gases and to moderate pressures, and it is the technology behind most low-cost transmitters. A thin-film strain gauge deposits the gauge directly onto a steel diaphragm, which gives a robust element for higher pressures and for hydraulic duties, with better resistance to pressure spikes. A capacitive element measures the deflection of a diaphragm against a fixed plate, and is chosen where stability and a wide turndown matter more than cost.
Temperature compensation is what makes the difference between an element and an instrument. Every one of these technologies drifts with ambient and process temperature, and the drift is larger than the accuracy figure over the working range unless it is corrected. The compensation is done in the electronics, which is why a transmitter costs more than the bare element and why the temperature specification of the electronics is as relevant as the temperature limit of the medium.
The mechanical path the pressure takes to reach the element matters as much as the element itself. A direct-mounted transmitter on a tapping point sees the fluid directly. A transmitter behind an impulse line sees a pressure that has been modified by the line: condensate in a gas line or gas in a liquid line changes what arrives, and a pulsating source such as a reciprocating pump imposes a swinging pressure that a direct reading will follow. A damping arrangement at the pressure port, or an impulse line arranged to trap the right phase, is part of making the measurement meaningful.
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
|---|---|---|
| Sensing technology | Piezoresistive, thin-film strain gauge or capacitive | Tolerance for overpressure and pressure spikes, stability over time, and the pressure range at which the technology is at its best. |
| Pressure reference | Gauge, absolute, sealed gauge or differential | What zero means. A gauge sensor reads against atmosphere and moves with the weather; an absolute one does not, which matters on a vacuum system or a sealed vessel. |
| Range and the working point | The full range of the sensor against the pressure the machine normally runs at | The smallest change that can be resolved in service, which is usually what the application actually depends on. |
| Output form | Two-wire current, three-wire voltage, or a digital bus | How the sensor connects to the controller and how it is wired. The pin functions differ between the forms, so the form is settled before the wiring is planned. |
| Supply voltage | The nominal supply available on the machine, and any wide-range option | Whether the sensor runs from the control voltage already present, and whether a separate supply has to be provided. |
| Process connection | Threaded, flanged, clamp or custom | Whether the sensor fits the port that exists. A custom thread is usually cheaper than reworking the manifold. |
| Electrical connection | Plug, sealed cable entry or circular connector | How the sensor is replaced in service, and how well the connection survives coolant, washdown and vibration. |
| Wetted material and media temperature | The body and diaphragm material, and the temperature version | Whether the medium attacks the sensor, and whether the standard temperature range covers the process or the high-temperature version is needed. |
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|---|---|
| Normal working pressure and peak pressure | The range sets the resolution, and the peak sets what the sensor has to survive | Give normal and maximum pressure separately, and say whether the peak is sustained or a spike at start-up. |
| Pressure reference | It defines zero, and a reference chosen wrongly cannot be corrected afterwards by calibration | State gauge, absolute or sealed gauge explicitly rather than inferring it from the machine type. |
| Medium and its temperature | Corrosion and temperature are the two commonest reasons a machine pressure sensor fails early | Name the fluid, its concentration if it is not clean water or oil, and its temperature at the tapping point. |
| Existing process connection | The port is usually the fixed constraint, because reworking a manifold costs more than adapting the sensor | Give the thread, or dimensions if it is a flange or a clamp, and the sealing face. |
| Control system input | The controller input type and the wiring already run decide the output form | Name the input card and whether it takes a current or a voltage, and where the sensor will be wired. |
| Environment at the sensor | Vibration, washdown, coolant and outdoor exposure act on both the body and the connection | Describe where on the machine the sensor sits, and whether it will be hosed or buried in swarf. |
| Dynamic behaviour of the source | A pulsating pump or a fast switching valve imposes a swinging pressure that changes what a reading means | Say whether the pressure is steady or pulsating, and whether a damped or averaged reading is wanted. |
A pressure sensor measures at one point in a system. Where a flow figure is to be inferred from pressure, the result depends on the geometry between the tapping and the reference point, so the same sensor gives different answers on a different installation.
These are the types an enquiry for a pressure sensor usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a pressure sensor |
|---|---|
| Force sensors | A force sensor measures a load applied directly to it, where a pressure sensor measures the force per unit area of a fluid. The distinction matters for the mechanical arrangement rather than the electronics: a force sensor takes the load through its own body, and cannot be substituted by a pressure reading unless the area is known and constant. |
| Level sensors | A level measurement is often derived from pressure at the bottom of a vessel, which works only while the density is known and constant. Where density varies, or where the vessel is open to the atmosphere at the top, a level sensor that measures the interface directly is the more dependable answer. |
| Temperature sensors | A different quantity measured the same way, in the sense that both are read by the control system as an analogue value. They are frequently needed together on the same skid, and are specified independently because the pressure and temperature ranges rarely coincide. |
| Pressure gauges | A local indication with no electrical output. Where an operator reads the pressure at the point of use and nothing has to travel to a controller, a gauge is simpler and more robust; the sensor exists so that the value can be acted on somewhere else. |
| Pressure switches | A switch changes state at a set point and produces a contact, so it reports that a limit was crossed rather than what the pressure is. Where the controller needs the value, a switch cannot supply it, and where only a limit matters, a switch is the simpler answer. |
| Pressure transmitters | The same instrument specified for a process plant rather than a machine, where the emphasis falls on process connection standards, wetted-part compatibility, remote seals and hazardous-area certification. On a compact machine the housing, the connection and the electrical interface are usually the deciding features instead. |
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
Available products
TFD-801 · Pressure Sensors
Diffusion silicon pressure transmitters with a 304 stainless steel body, IP65 sealing and ranges from -100 kPa to 100 MPa, in two-wire 4-20 mA and three-wire voltage forms.
Also in Industrial Sensors
Every type we quote for under Industrial Sensors. A type with no link yet is still quoted for - it is described on the Industrial Sensors overview, and gets a page of its own as soon as a model is published against it.
Frequently asked
A thin-film strain gauge is usually the better fit for hydraulic duties, because it is built on a steel diaphragm and tolerates the pressure spikes that come with a pump start or a valve closing better than a silicon element of the same rating. A piezoresistive element is well suited to lower pressures, to dry gases and to applications where cost is the constraint. The choice is therefore made from the pressure range and from how often the sensor will see a spike, not from accuracy alone.
Enough to cover the highest pressure the line will see, including a pump start or a valve closure, and no more than that. A range far above the working point costs resolution exactly where the readings are taken, because the accuracy figure applies to the full scale of the range selected. Where the peaks are short, the overpressure tolerance of the sensor covers them, and the range can be chosen for the working point rather than for the peak.
It changes what the sensor sees, which is not the same thing. A line arranged for a gas duty traps condensate, and one arranged for a liquid traps gas; either changes the pressure at the transmitter relative to the process. A long line also damps a pulsating pressure, which may be what is wanted or may hide a fault. The arrangement is therefore part of the measurement and worth describing when the sensor is selected rather than being left to installation.
Not on its own. A flow inferred from pressure depends on the geometry between the pressure tapping and the reference, on the fluid density and on whether the flow is laminar or turbulent, so the same reading corresponds to different flows on different installations. Where flow has to be established, a flow sensor or a differential pressure arrangement with a known restriction is the measurement to make, and the pressure reading is a separate one.
The pressure range with the working point as well as the maximum, the pressure reference, the fluid and its temperature, the process connection already on the machine, and the output the control system takes. Where any of those is open, saying so is more useful than leaving it out, because the connection and the reference in particular are difficult to change once the sensor is made.
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