Gauge, absolute or differential?
| Type | Measures against | Typical uses |
|---|---|---|
| Gauge | Atmospheric pressure | Pump discharge, compressed air, process lines, open-tank level |
| Absolute | A perfect vacuum | Vacuum processes, condensers, distillation, barometric correction |
| Differential (DP) | A second process pressure | Flow across orifice plates, filter condition, closed-tank level, interface |
Choosing the range
Select a transmitter whose range covers normal operation with margin for upsets, then set the measuring span within it. Most smart transmitters allow turndown, so one model can cover several spans — but accuracy is best when the set span is not too small a fraction of the sensor's maximum range.
Sensor and diaphragm materials
Ceramic sensors resist abrasion and many corrosive media and tolerate vacuum well; metal sensors (stainless steel or special alloys) suit high pressures and a wide range of process conditions. Endress+Hauser's Cerabar and Deltabar ranges offer both, so the sensor can be matched to the process.
When to use a diaphragm seal
A diaphragm seal (remote seal) keeps the process away from the transmitter. Consider one when the medium is:
- hot enough to damage the electronics,
- viscous, crystallising or likely to clog impulse lines,
- corrosive, needing a special diaphragm material,
- hygienic, needing a cleanable flush connection.
Capillary length and fill fluid affect response time and temperature effects, so keep capillaries as short as the installation allows.
Measuring flow and level with DP
A DP transmitter across an orifice plate, venturi or pitot tube gives a signal proportional to the square of flow, which the transmitter or control system linearises. For level, the hydrostatic head of liquid is proportional to level; closed tanks need a DP measurement to cancel the vapour-space pressure.
Installation tips
- For liquids, mount below the tapping so impulse lines stay full; for gases, mount above so condensate drains back.
- Use a manifold for isolation, equalising and zero checks.
- Protect impulse lines from freezing or overheating, and slope them to avoid trapped gas or liquid.
Outputs and diagnostics
4–20 mA with HART remains the most common output, with fieldbus options available. Diagnostics such as plugged-impulse-line detection and electronic verification help maintenance teams find problems before they affect the process.
Frequently asked questions
Can one DP transmitter measure both flow and level?
The same hardware can, but each installation is configured for one measurement.
What is a pressure switch for?
A switch gives an on/off signal at a set point — for alarms or trips — while a transmitter gives a continuous value.
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