Introduction to Pressure Transducers – IOThrifty

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Introduction to Pressure Transducers

 

What are pressure transducers used for?

Pressure transducers have wide industrial applications. Some of the more common pressure measurements are Barometric pressure, Process Air pressure, Hydraulic pressure, Pump Inlet and Outlet pressure, and Industrial Vacuums. They measure both the positive and negative (Vacuum) pressures, of liquids or gasses used in the process industry.

Typical industrial applications include plastic extruder pressure, the vacuum in milking machines, pressure in steam pipes, water pressure from wells, refrigeration, and barometric pressures for meteorological applications. Pressure Transducers are used in off road equipment, aircraft, spaceships, small and large ships including Naval Vessels and in transportation, railroad, trucks and cars.

Types of Pressure

We have referred to vacuum and barometric pressures which are common everyday terms, but what do they really mean, and how do you measure them? There are actually three types of pressure measurements and they differ by their zero pressure reference points.

Absolute Pressure (psia) has a zero referenced to a perfect vacuum

Gage or “Relative” Pressure (psig) has zero referenced to local atmospheric pressure

Sealed Gage Pressure (psis) is referenced to the pressure in a sealed chamber within the gage

Atmospheric pressure measurements are Absolute Pressure “psia”, with a zero reference to a vacuum. Atmospheric pressure changes slightly day to day as weather systems change. It also changes with altitude. At sea level atmospheric pressure is 14.7 psia. At 30,000 ft, the top of Mt. Everest it is 4.4 psia and at the shore of the Dead Sea, -1500 ft it is 15.5 psia.

Gage pressure (psig)  is referenced to the actual atmospheric pressure at the location of measurement. Gage pressure is also called “relative” pressure as the measure is relative to the local atmospheric pressure. What that means is the measurement is the pressure positive above or negative below the local atmospheric pressure. Why does that matter? Atmospheric pressure measurements are Absolute pressure psia, with a zero reference to a vacuum. Atmospheric pressure changes with altitude and with weather systems. At sea level atmospheric pressure is 14.7 psia. At 30,000 ft, the top of Mt. Everest  it is 4.4 psia and at the shore of the Dead Sea -1500 ft it is 15.5 psia. Therefore, a 10 psig measurement at the top of Mt Everest is 14.4 psia, while the same measurement at the shore of the dead sea is 25.5 psia. If you need a repeatable system at multiple locations and are using gage pressures, then you need to take local atmospheric pressure into account.

Seal Gage pressure is a way to solve that issue. The pressure is referenced to the pressure in a sealed chamber inside the gage, normally sealed at sea level atmospheric pressure 14.7 psia. This makes the gage read consistently regardless of elevation or local atmospheric conditions. Since they reference a sealed chamber, there is no vent to the atmosphere which makes them also more immune to moisture and local contamination in the atmosphere.


Pressure Sensor Terminology

The most common terms used to refer to a pressure measuring device are pressure “sensors”, pressure “gauges”, and pressure “transmitters”.

The term Pressure Sensor either refers to the basic pressure sensing element from which higher level devices are built or a small plastic enclosed sensing device with an unamplified output. This is usually a ratiometric mV signal meaning the output varies with the applied voltage and is typically stated as mV/V (of excitation voltage). A pressure sensor with a 3 mV/V output would have a 30 mV output at 100% of its range with a 10 V excitation voltage applied. (Note pressure “sensor” can also be a broadly used term to refer to the bare sensing element all the way up to a fully configured metal pressure transmitter.)

A Pressure Transducer generally refers to a device with a metal enclosed pressure sensing element with some electronics inside. There may be an internal voltage regulator or an amplifier, so the output is not ratiometric to the input voltage. The output can be 0 to 5 Vdc, 0 to 10 Vdc or non-ratiometric 30 mV dc. The housing is typically a metal tube with industrial pipe connections.

A Pressure Transmitter is typically used to refer to a pressure sensing device that has a 4 to 20 mA output signal. The form factor can be a small metal tube housing up to a large cast iron enclosure.
The 4 to 20 mA output is compatible with most electronic industrial control systems.

Types of Sensor Construction

Originally, the pressure in a railroad steam boiler was measured by wrapping a wire around the boiler and measuring the change in resistance. As the pressure in the boiler increased the wire stretched and the resistance increased. From this technique the BLH strain gage company was founded. BLH was the Baldwin Lema Hamilton Railroad Company. Then the Bourdon Tube dial gages became popular because they were direct reading. A Bourdon tube is a flattened curved tube where the pressure is  applied internally. The inner side of the tube is slightly shorter than the outer side so as pressure stretches it, it tends to straighten slightly. This movement is attached to a gear mechanism which rotates the pointer of the gage. Bourdon tube pressure gages are still very common today but lack the accuracy and ruggedness of electronic pressure sensors based on the strain gage. Today, strain gage are the most common element used to make electronic measurements of pressure. The strain gage has evolved to  either small thick film devices or an even smaller semiconductor element. They are placed on a metal or ceramic diaphragm that deflects a small amount when pressure is applied to it. The deflection of the diaphragm stretches the strain gage and that change in resistance is converted to the electronic output of the device.

Limitations of Pressure Sensors

Wetted Parts
When selecting or using a pressure sensor you have to be aware of its system compatibility. The parts of the Sensor that will come in contact with the working “fluid” (can also be a gas) are called the wetted parts. These typically include the pressure fitting, and the internal path and diaphragm of the transducer. Depending on the internal construction, the diaphragm is either welded or sealed with an O-ring. All these materials must be “compatible” (not corroded or altered) by the working fluid. Viton or Silicon O-Rings have excellent compatibility as well as all stainless steel parts. The outer body that protects the electronics also needs to be compatible with any material that may be splash upon it.


 

Temperature Range and Compensation

The sensor and the electronics must be able to function over the full temperature range that the working fluid and environment may reach.
A note here on temperature compensation: The difference in the cost of similar transducers is often due to the quality of their temperature compensation. A 0.25% accuracy transducer with a 100 mV output and a temperature compensation of 0.1 mV/degree may look good, but that temperature compensation will cause a 10% change in output over 100 degrees. So pay careful attention to the temperature compensation specs when selecting a pressure transducer.  

Electrical Compatibility

If you are replacing or upgrading a system, pay careful attention to supply voltage and transducer output specifications to be sure that the transducer is compatible with your existing equipment. Your system may require 0 to 5 Volts or 100 mV full scale inputs, some transducers are adjustable while others are fixed.

Fittings/Pressure Connections

Make sure that the connections and mounting fittings will work in your space requirements. You can use bushings to change fittings,  but they make the overall envelope of the transducer larger so double check your space requirements.

High Pressure Limitations

Overpressure is the most common reason for a transducer to fail. Make sure that you select a transducer that will withstand the maximum pressure in your system plus a safety margin.

Special Considerations for Oxygen Measurements

Oxygen over a certain pressure is explosive. Even at low pressures it is a highly combustible gas.
Any transducer used on a Oxygen system should be fully rated for Oxygen Service.

Explosive Working Fluids

Explosive fluids and gasses require a pressure transducer with the proper Explosion Proof rating.
Have a knowledgeable person in this area review the specifications to verify the transducer is safe for your application.

Helium

Helium is a very small molecule and can leak where a normal person would think that it is impossible.
Silicon diaphragms and O-ring sealed diaphragms are generally problematic with Helium. A welded stainless steel diaphragm transducer is the best choice with Helium, or use a special extra thick silicon diaphragm if stainless steel is not available.

Selecting a Pressure Transducer Review

All the things you need to consider when making your selection: input voltage, output signal, wetted parts, outer housing, temperature rating, temperature compensation, maximum pressure rating, pressure connections, electrical connections, special requirements for Oxygen, Helium and explosive material.