Absolute (psia)
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)
Differential (psid)
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
4-20 mA (Current)
Voltage
Digital
Pressure Switch
Shunt
Redundant Output
Protection
iQuartz
Bonded Foil Strain Gauge
MEMs bulk piezoresistive
Silicon on sapphire piezoresistive
Thin film
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
Offsets due to temperature
Is the Root Sum Square(RMS) of the Linearity, Hysteresis, and Repeatability
Sum of the Static Accuracy and Thermal Accuracy
Material
Gauge Impendence
Envelop
Ports
Approvals
Pressure Range
Temperature Range
Required Accuracy
Related Paperwork
Post Testing
Delivery and Cost
Electrical Connections
Cleaning
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.
Uncertainties are present on all types of sensors. To quantify the uncertainties of the sensors, it is essential to consider the different factors of each uncertainty. First of all, it is important to distinguish an uncertainty from an error. An error, also misnamed accuracy, is the difference between the exact value and the measured value, but in all cases, the exact value is not known. The uncertainty, however, represents the spread of values that can reasonably be attributed to the measurement.
When accuracy is required, we have to ‘compare apples to apples’. This means that we have to understand what we are talking about regarding uncertainty and to define the confidence interval. A confidence interval is the range of values where our true value is expected to lie. From a statistical point of view: “The larger the confidence range the greater the uncertainty” is what we consider for a “normal” law.
There are 3 confidence interval sizes:
The quantity ‘σ’ represents the standard deviation of the measurement. It is important to understand that a lot of datasheets give accuracy values by mentioning the average value with a variation range equal to ‘σ1′.
In this tutorial we will summarize the different uncertainties that are encountered when calibrating a pressure sensor.
The following diagram shows the different elements that are necessary for a calibration. All calibrations require a reference pressure sensor that allows us to calibrate our sensors against it. A temperature sensor is particularly useful when we make calibrations at a specific temperature. Then, the power supply and the output signal of the elements are managed by an interface card whose components can have an impact on the measurement of the sensors. Furthermore, it is necessary to add to this calibration an acquisition equipment that allows to receive the data from all the sensors, as well as a power source that supplies them.
Each of these elements have uncertainties that have an impact on the tested pressure sensor, it’s the misnamed error/accuracy.
Please notice that this tutorial outlines uncertainties and does not show all the uncertainties taken into account during the calibrations on the Sensorade test bench.
In our case, it is necessary to determine the uncertainties of the above elements such as the pressure sensor, the acquisition system,… In order to determine these uncertainties, we must use the following equation (from the “Evaluation of measurement data —Guide to the expression of uncertainty in measurement”):
For a test bench, it is easy to understand that we must use the elements with the lowest possible uncertainties to reduce the uncertainty on the sensor itself. So we must know/calculate the uncertainties of the different elements of influence. Be advised that if this job is not done, you will not be able to expect any confidence on the tests performed.
Determination of Uncertainties
There are several methods to determine uncertainties. We particularly use two types of uncertainty determination : type A and type B.
Type A :
Type A uncertainty determines the uncertainty of an element by following a certain procedure. This procedure consists in measuring the signal of the element several times in a repeated and identical way. This often makes it possible to determine a much more precise uncertainty, but it is only applicable to one specific element.
Type B:
Type B consists in determining the uncertainty by using the documents / datasheet provided by the supplier. Unfortunately, not all manufacturers specify the uncertainties of their components, or some manufacturers provide the uncertainty on a batch of components, so the uncertainty can often be very large, too large. Therefore, if the uncertainty is not given by the manufacturer, it is mandatory to use the type A method. Moreover, the type A method must also be used if the uncertainty provided by the supplier is too large and covers a batch of the same component.
On this example, we can notice that the resistor R004 has the lowest standard deviation but the highest mean value. So, if we modify – where it is allowed – the value of the resistor from 1000 by 1040 in the calculation, we can decrease the uncertainty by a factor of 5 (type B) by 0.1% (type A). But this uncertainty would only be valid for the resistor R004.
In our case, it is necessary to apply this determination of uncertainty to determine the uncertainty of the elements of influence which are the reference sensor, the temperature sensor, the acquisition card and various components on the interface card.
If several uncertainties occur for an element, it is necessary to apply Equation 1 (General uncertainty) before applying this equation. It is important to check that all the uncertainties are given in the same confidence interval and that they follow a standard law, otherwise the calculation would be wrong.
Once the uncertainties of the different elements of influence are determined, it is necessary to be able to combine them in order to determine the uncertainty of the pressure sensor under test. To quantify the impact of the influence elements on the sensor, it is necessary to know the best fitted linear equation of the pressure sensor.
Pressure sensors are governed by a linear equation that relates the applied pressure to the output and supply voltage.
We can therefore see by Equation 2 that two parameters, a and b, must be determined. To do this, we use a linear model which, with the help of calibration data, allows us to determine them. We will not detail these calculations in this tutorial.
Important remark: Theoretically speaking, a and b are constant, but in fact, a and b are temperature-dependent (at least). It means that you need to have the full control on your environment.
S = (sensor output signal voltage)
The fact of using a model, introduces a new uncertainty which is the uncertainty of the model. This uncertainty changes according to the selected model. This is more or less complex according to the complexity of the model.
The graph here on the right illustrates where this uncertainty comes from. On the ordinate, we see the applied pressure and on the abscissa, the S/V ratio which represents the ratio between the output voltage and the supply voltage. The curve in red represents the linear model of the sensor.
For that reason, it is normal that the use of a model introduces uncertainties because when a model is used, the set of data points does not lie perfectly on the model. It is from there that the uncertainty of the model comes. The determination of this uncertainty is done by calculations given by the selected model, therefore we will not go into the details.
For our Sensorade sensors, the coefficient of determination is close to 0.999. This proves that the model we use is very close to reality.
To determine the uncertainty of the pressure sensor, another equation must be used that combines the different uncertainties of elements of influence (from the “Evaluation of measurement data —Guide to the expression of uncertainty in measurement”):
The last step is to apply Equation 4 since all the terms have been determined in the previous sections. Finally, we obtain the uncertainty value of our pressure sensors.
In conclusion, uncertainties are present in each component of the test bench, from all acquisition devices, elements, connectors… to the sensor you want to calibrate.
Therefore, to determine the uncertainty of our sensors, we must first determine the uncertainties of the elements of influence by applying the method of type A or B, according to the elements and the data provided.
Then, the linear model of the sensor and the uncertainty of the model must be evaluated. We can notice that the main uncertainties come from the elements that are external to the pressure sensor, in particular the acquisition system.
Finally, the uncertainties of the element of influence and of the model should be combined in order to determine the calibration uncertainty of our sensors.
The temperature compensation (TC) method enables SENSORADE sensors to give a pressure measure that is compensated by the temperature. Indeed, the pressure sensor is a wheastone bridge (see Figure 1) where the outpout voltage (Vout span = Vout+ – Vout-) represents the level of pressure and the resistance of the bridge (R = (R1+R2) * (R3+R4)/(R1+R2+R3+R4)) represents the level of temperature.
To access this information, we make various measurements of the voltage’s output using the information provided by the deformation of the membrane. The latter deforms all the 4 resistances at the same time and therefore their values change at the image of the deformation of the membrane, as illustrated in Figure 2. The measure is made in known conditions of pressure and temperature (in a fixed range of temperature – from a minimum to a maximum value).
The objective of this tutorial is to present the possible strategy for measuring and or compensate the effect of the temperature. The following is an overview of the 4 possible options that can be done at your facilities*:
This method is best fitted when the application does not require high accuracy and/or does not have to sustain large temperature variation. In this case, we provide one curve to the user (Figure 3).
Equipment needed: an accurate power supply source, and an accurate Voltmeter.
For this method, we implement on the sensor two more wires that enable the access to a temperature at ±0.75°C value so that it is compensated. Measurements are made at different temperature levels. This solution is the most acurate one, and can additionnally give the temperature measure.
This solution is best illustrated with a Wheatstone bridge circuit diagram (Figure 4). We have 2 wires for positive supply ( Vsup1+ and Vsup2+), 2 wires for negative supply (Vsup1- and Vsup2-) and 2 wires for the output of the sensor (Vout+ and Vout-) directly linked to the level of pressure applied on the sensor.
For this method, we implement on the sensor two more wires that enable the access to a temperature at ±0.75°C value so that it is compensated. Measurements are made at different temperature levels. This solution is the most acurate one, and can additionnally give the temperature measure.
This solution is best illustrated with a Wheatstone bridge circuit diagram (Figure 4). We have 2 wires for positive supply ( Vsup1+ and Vsup2+), 2 wires for negative supply (Vsup1- and Vsup2-) and 2 wires for the output of the sensor (Vout+ and Vout-) directly linked to the level of pressure applied on the sensor.
We inject a known current and measure the supply voltage of the sensor to calculate the resistance of the Wheastone bridge. This resistance grants access to the information about the temperature and enables to compensate its effect. The acquisition scheme is shown below:
We provide two curves to the user:
Equipment needed: an acquisition card with a current injector.
We measure the current injected in the sensor and the supply voltage to calculate resistance of the Wheastone bridge. This resistance grants access to the information about the temperature and enables to compensate its effect. The acquisition scheme is shown below:
We provide two curves to the user:
Equipment needed: an acquisition card with an Amperemeter.
In substitution of the current injector or Amperemeter, SENSORADE proposes the SENSORADE BOX: it is a device to place at the entrance of the acquisition card that allow voltage supply for the sensor. Its output is a voltage wich allows you to know the current through the sensor. With these information and the supply voltage you can calculate the sensor resistance (wheastone bridge). This resistance grants access to the information about the temperature and enables to compensate its effect. The acquisition scheme is shown below:
We provide two curves to the user:
The advantage of having a SENSORADE BOX is the ability to work with an acquisition system without a current injector.
Equipment needed : an acquisition card, a SENSORADE BOX.
The laser-Doppler vibrometer is a precision optical transducer for determining the vibration velocity and displacement at a measurement position. It works by sensing the frequency shift of back scattered light from a moving surface. The object scatters or reflects light from the laser beam and the Doppler frequency shift is used to measure the component of velocity which lies along the axis of the laser beam.
The resonant frequency of the absolute pressure sensors reaches up to 2.7 MHz which is the highest resonant frequency on the market, as can be seen in figure 1.
The resonant frequency of the differential pressure sensors reaches up to 310 KHz which is the highest resonant frequency on the market, as can be seen in figure 2.