transducers / Transmitters 101

Types of pressure

  • Absolute (psia)

      • Measured relative to perfect vacuum. An example is barometer, which measures atmospheric pressure. Typical reference pressure is 0.029 psia behind diaphragm.

  • Gauge (psig)

      • Measured relative to ambient pressure. An example is a pressure unit with a breathing tube or vent hole. This allows pressure to be equal on the outside of unit and the pressure behind the measuring diaphragm.

  • Sealed Gauge (psisg)

      • Measured relative to a fixed, local atmospheric pressure. The fixed pressure is typically whatever the pressure was on the day the reference cavity was sealed (welded).

  • Vacuum (psiv)

      • Measured relative to a fixed pressure, typically between atmospheric pressure and absolute zero. This pressure can be set to anything other than the above stated reference pressure. Example, reference pressure of 5 psia behind the diaphragm.

  • Differential (psid)

      • Measured between two points. Typically between two points in a tank or pipe.

temperature sense

  • Temperature Output

      • Some units have only temperature output, some have temperature along with pressure. These can have RTD output, either 100 ohm or 1,000 ohm or thermocouple output or any of the same amplified outputs for pressure, for example, a transducer with 4-20mA output for a given temperature range.

  • RTD

      • RTDs may be added to almost any pressure transducer. Either in a probe or located on the sensor. RTDs produce a resistance to determine temperature. They follow the standard 385 platinum curve. Available in class A or class B for accuracy, which is around +/- 0.05 to 0.1C. Available in two wire, three wire or four wire. Ranges from -200C to 400C.

  • Thermocouple

      • Thermocouple use a change in voltage to determine temperature. Temperature ranges from around -200C to 1500C with an accuracy around +/- 0.2 to 0.5C. Available in J, K, T and E Types. Less accurate than RTDs but also cost slightly less.


  • Design Pressure or MEOP (Maximum Expected Operating Pressure)

      • When stating a pressure range, this is the highest pressure given in that range.

  • Proof Pressure or MAWP (Maximum Allowable Working Pressure)

      • Highest level of pressure the transducer can be exposed to under normal operating conditions without affecting calibration. Different technologies have different ranges. This is expressed as either a pressure or can be expressed as a multiple of the design pressure.

      • Going beyond proof pressure will change calibration.

      • Differential Pressure Transducers can have multiple proof pressures depending on your application. One proof pressure for Line Pressure and another for Differential Pressure.

  • Burst Pressure

      • Maximum amount the pressure sensor can see before it has a chance to break/ leak.

  • Span

      • What is read when at the maximum calibrated output.

      • Example on a 4-20mA unit. Span is 20mA.

  • FSO (Full Scale Output)

      • The maximum calibrated output minus the minimum calibrated output.

      • Example on a 4-20mA unit. FSO is 16mA.

  • Line Pressure (aka Base Pressure)

      • When talking about a differential pressure transducer, this is the pressure both the high side and low side will see at the same time. Low side typically only sees up to line pressure while the high side will have line pressure plus the pressure range of the differential transducer.

      • Example, a differential pressure transducer with a range of 50 psid and a line pressure of 500 psia will have a pressure up to 500 psia on the low side and a pressure of 550 psia on the high side. This will make the transducer read its full scale. When both low and high side have 500 psia applied to them, meaning the differential is 0 psid, the transducer will show its zero reading.

  • Bi-Directional

      • When talking about differential pressure, the pressure sensor can measure pressure on both the high side or negative side

      • Example, a differential sensor is calibrated from 0-10 VDC bi-direction. This means when the high side at the design pressure , it will read 10 VDC. When the pressure is equal on the high side and the low side, the unit will read 0. When the low side is at design pressure, it will read -10 VDC.

  • Uni-Directional

      • This is the typical setup for a differential pressure transducer. The low side will always be at the same pressure or below the high side.

  • Hermetically Sealed

      • Transducers with connectors and that are not gauge reference are hermetically sealed. This means there is no air or pressure exchange between the outside environment and the inside of the pressure transducer. Gauge sensors will have a reference port or vent to exchange inside air with the external ambient pressure so they are not hermetically sealed. Come cables will not guarantee a hermetic seal.

  • SmartFit

      • SmartFit technology uses a computer calibration stored onboard ASIC to provide analog outputs. This allows for a highly accurate transducer, calibrated automatically and verified without chance of human error. While we typically calibrate 3 temperatures, we can do up to 15 different temperatures to provide the highest accuracy. Customers may request specific temperature points where accuracy matters most. This may also allow for future re-calibrations of units without need of cutting the product open. Smartfit requires one output pin to be a program pin or the customer may request no program pins on the output.

  • Warm-up Time

      • Time it takes to reach 90% of output. Typically is within milliseconds. May take longer when testing at extreme temperatures or when customer requests low impendence.

  • Certificate of Calibration

      • A certificate that gives actual data of the calibrated unit. Data includes a Static Accuracy Run at room temperature and the zero and span readings at the low and high calibrated temperatures. Equipment used with calibration dates and serial numbers are also included on this certificate. Each serial number receives its own Certificate of Calibration.

  • Certificate of Conformance

      • ASI confirming that the requirements of certain drawing or document have been met by the serial numbers listed on the certificate.


  • mV/V

        • mV/V output is one of the most common outputs, sometimes called 'un-amplified'. This output has only resistors to compensate it and is a ratio of the voltage input. Typically output is anywhere from 0-20mV to 0-100mV at 10 VDC excitation, also stated as 2mV/V and 10mV/V. Less voltage input can be used for less output. Using our previous example, with 5 VDC excitation, the output would be 0-10mV to 0-50mV.

        • Since there are no electronics, radiation has no effect on these units and are the

        • Smallest units that can be made

        • Less accurate than amplified units

        • 4 wire output includes: +exc, +output, -exc, -output

  • 4-20 mA (Current)

      • Only requiring two electrical pins, current output is a popular option.

      • High Accuracy available

      • Available in both Smartfit or true analog

      • Remote electronics available

  • Voltage

      • Voltage comes in many different outputs. The most popular are 0-5Vdc, 0.5-4.5VDC, 0-10VDC. They also come in 3 wire devices or 4 wire

      • High Accuracy available

      • Available in both Smartfit or true analog

      • Remote electronics available

  • Digital

      • The term Digital covers many outputs including RS485 Modbus, RS485 Canbus, USB, Bluetooth or I2C. I2C only requires 2.7 Vdc and 2.5mA of power to work.

  • Pressure Switch

      • This is a special kind of transducer where the electrical signal changes at a pre-calibrated point and resets at another pressure point

  • Shunt

      • Many units can have a shunt. This simulates either 100% FSO or 80% of FSO without need of pressure. Shunts can be internal or external. Simply short two pre-determined wires together to get an instant calibrated output.

  • Redundant Output

      • This refers to when a unit has multiple sensors and electronics in one unit. They can be completely independent with separate ports, sensors, electronics and connectors or can share the same port or connector. This typically increases size.

  • Protection

      • May include input, output, reverse polarity, lightning, EMI/RFI and ESD

How to hookup mv/v transducer

DMM should be in DC Voltage measuring mode

How to hookup 4-20ma transmitter

DMM should be in Current DC measuring mode

How to hookup 3 wire voltage transducer

DMM should be in DC Voltage measuring mode

How to hookup 4 wire voltage transducer

DMM should be in DC Voltage measuring mode

pressure sensing technologies

  • iQuartz

      • ASI Proprietary technology that delivers 10-20 times higher sensitivity than other technologies.

      • Monolithic design (no welds) down to 25 psi

      • Operates at full scale of 275 micro strain, compared to 2000-3000 of regular bonded foil strain gauge

      • Inorganically bonded

  • Bonded Foil Strain Gauge

  • MEMs bulk piezoresistive

  • Silicon on sapphire piezoresistive

  • Thin film

Transducer Accuracy


  • Deviation from ideal straight line

  • Can be calculated by:

      • Best Fit Straight Line (BFSL). This is the default ASI uses. This takes into account what the actual zero and span of the unit are.

      • Terminal (TERM). Maximum deviation from ideal. This does not account for actual zero and span


  • Sensors ability to give the same output while increasing and then decreasing in pressure


  • Sensors ability to product the same output with consecutive applications of the same pressure

Temperature coefficients of offset

  • Offsets due to temperature

Static Accuracy

  • Is the Root Sum Square(RMS) of the Linearity, Hysteresis, and Repeatability

Total Error Band

  • Sum of the Static Accuracy and Thermal Accuracy

Transducer options

  • Material

        • The materials used may not matter to your application. For other applications, it is important to use the correct material or else the transducer may not function at all or last very long. There are a few areas that need to be considered.

          • Wetted Material - This is the area where pressure media is in contact with. This is the sensor material. Material may be limited by the technology being used

              • Typical material includes 316L SST, Hastelloy C276, 15-5PH, 17-4PH, Inconel 625, Inconel 718, Monel k500, Titanium, Nitronic 50, Vascomax

          • Some sensors use oil behind a passive diaphragm, this oil is usually Fomblin, may be substituted with Dow Corning

          • O-ring Material (if needed) - Needs to compatible with pressure media and temperature range.

          • Case Material - Typically 304 or 316 SST

          • Connector Material - Connectors have their own standards and materials. Most are compatible with case materials

          • Cable Material (if needed) - Needs to be compatible with your environment and temperature. Cables come in many materials and rated for different temperatures. Some have vents.

  • Gauge Impendence

        • Typically impendence is around 2.5k-5k ohm. Customers may request a impendence such as 350 ohm when placing their order. It is important to know this right away as it effects everything from sensing technology to how the unit is compensated and expected thermal performance and warm up time.

  • Envelop

        • There are projects where size matters, either as small as possible or when larger is better. Going to one extreme or another may change things

        • Any size or weight restrictions should be told upfront as this may limit options, even if your design isn't' set in stone and the size may change

  • Ports

        • There are a number of standard threaded and flanged connections available. Most typical are NPT, SAE, AN, Tri-Clover, API flanges

        • Custom/ proprietary ports may be used in your design. We will custom make ports to your specification.

  • Approvals

        • Transducers used in hazardous locations require certain approvals. This should be told up-front as it limits options to what is already approved. ASI is open to obtaining approvals upon customer request.

  • Pressure Range

        • Deciding your pressure range is very important as it will often decide what technology is required. This includes needing to know proof and burst pressure.

        • We can calibrate and make all documentation to your pressure units. If a datasheet says psi or bar but your application typically uses atm or inches of water column, we can make everything consistent to your nomenclature.

  • Temperature Range

        • There are three temperature ranges to be aware of: compensated temperature range, operating temperature range, storage temperature range. Typically we consider storage and operating temperature range to be the same thing unless the customer requests differently.

          • Compensated temperature range is the range where stated accuracies are met. When we calibrate the unit, we take it to these temperature. If your unit will be used at one specific temperature, you can let us know and we can focus our calibration at that temperature point.

          • Operating temperature range is the same or wider range than the compensated temperature range. In this range, the readings will not be within the stated accuracies but going into this range will not effect the calibration while the unit is powered.

          • Storage temperature range is typically the same or wider than operating temperature range. The unit should not be powered on when outside the operating temperature range or the calibration may shift.

        • Temperature range greatly affects accuracy. Cryogenic temperatures of -320F may be calibrated and temperatures down to -65F to +250F.

        • ASI calibrates at, at least three temperatures: room temperature (typically 70F unless customer requests differently), low compensated temperature, high compensated temperature. If using a digital unit or digitally corrected analog unit, as many as 15 temperatures may be used during calibration providing a unit with nearly no thermal shift.

  • Required Accuracy

        • See the section above about how to calculate accuracy. Customers may request testing with their own methods/ equations and pass/fail criteria. Some customers only care certain aspects of accuracy, such as linearity. Some customers want accuracy calculated terminal and not best fit straight line. Some Customers only want to express in +/- units, such as all mV outputs must be with 5mV of ideal reading. Customers may request what their accuracy requirements are, otherwise they default to the unit drawing or datasheet.

        • Accuracy will vary with temperature

  • Related Paperwork

        • Customers may request additional paperwork to accompany their products. This is typically a Certificate of Calibration, Certificate of Conformance, Origin Certificate, Material Certificates, or installation manuals.

  • Post Testing

        • Usually referred to as Acceptance testing (ATP) or Qualification testing (QTP). This is testing performed after the product is complete. It ensures it will meet the customers environments. Typical testing includes proof pressure testing, insulation resistance, thermal cycling, accuracy testing, leak testing, and sometimes vibration or shock testing. Custom reports are given for each serial number.

  • Delivery and Cost

        • All options have some effect on delivery time and cost of the unit. A unit with non-amplified output, no size requirements, no paperwork or testing required and isn't needed super fast is going to cost less than something that requires 15 calibration tables in an exotic material that is small and light weight that needs all forms of paperwork and testing in a timely manor. ASI can handle all custom orders and projects / programs. Lead times can vary from 2 weeks to 3 months depending on options. Sometimes longer when large amounts of post testing is required. Because ASI is ran by engineers, all units are under extreme scrutiny and are only shipped when the requirements are met.

  • Electrical Connections

        • Connectors and cable can have a large effect on lead times and cost.

        • Units that are gauge require a breather hole, this may be in the side of the unit or in the cable. These should be kept clean or in an enclosure.

        • Cable length needs to be known. 2 feet and 6 feet are standard lengths but any length may be requested. Keep in mind what this will do to packaging and shipping.

        • Connectors can range from a classic Bendix 6 pin to special mini connectors that have specials screening as directed by NASA. If you have an exact connector that you need, we can find it. If you want a connector and connector mate already wired for your use, no problem, just ask and we will ship it with the units.

        • IP rated connectors and cables available

  • Cleaning

        • All units receive basic visual cleaning.

        • Special cleaning may be requested. This may include LOX cleaning performed by a certified third party.

Custom options

  • ASI engineers will work with your companies engineers to create new products, sensors, housings, building methods, tests and reports. If there is no product on the market that can meet your pressure or temperature needs, tell us and we come up with a solution for you. This includes legacy products no longer on the market or discontinued by the manufacturer. If the project is for one transducer or a program that is scheduled to take place over 10 years, we understand customers needs and timelines.

  • All custom transducers receive a dedicated drawing, which is only for that transducer and only for that particular customer. Configuration Control available.

  • 3D models can be created and emailed for your transducer/ transmitter. Typically sent in step file form.

Everything on this page is for reference/learning purposes only