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GE Measurement & Control

Glossary of Common Sensor and Terms

Introduction Like every industry, the & calibration segment has its own language, acronyms and unique abbreviations. Similarly some of the ideas and concepts behind calibration and accuracy can be confusing at first, especially to young engineers learning their trade. GE’s been at the forefront of pressure instrumentation and sensors with its Druck product range for over 40 years. So we’ve produced this short guide to common calibration and pressure terms to help. It’s not intended to be a complete guide to calibration but a simple reference sheet. We hope it’s useful. Common terms

4 -20 mA Loops 4-20 mA loops are a 2 wire system for reading a value from a sensor. Its advantages are: a) Only 2 wires are required b) In a fault mode the is less than 4 mA thus indicating a fault c) Robust to electrical noise d) Transmitter does not need to be calibrated with its cabling e)Supports the use of long cable lengths Absolute These sensors measure pressure relative to a . pressure sensor Accuracy A measure of how close a value or distribution of values are to a specific point. The shots in the example below have a very low degree of precision but have a high accuracy because they are equally spread out around the bulls eye. The average value is therefore close to the bulls eye

Best Straight Best Straight Line is a technique for producing a straight line from a range of Line (BSL) different data points. Instead of joining the first and last points, BSL is “fitted” through all the points. The position of the line of best fit is calculated by minimising the sum of squares of all the errors, expressed as a percentage of the full scale value. Capacitance Capacitance is the ability of a body to store an electrical charge - all things have some capacitance. Charging a takes time and energy and so reduces how quickly a sensor can be turned on and off. Cold junction When measuring a , a meter is usually connected. This compensation connection results in another junction with two new , each adding/subtracting mVs to the measured value. This is called the cold junction. Originally the junction was contained in a bath of ice, hence the name cold junction. This kept the of the junction at 0˚C to eliminate ambient temperature influences and allow the temperature at the “hot” end to be determined. The modern method is to measure the temperature where the thermocouple is connected to the copper connectors in the instrument using an accurate RTD. Any temperature change can quickly be measured and compensated for. Differential Differential pressure sensors measure the pressure difference between two pressure sensor points, one connected to each side of the sensor. FS Full Scale. This is the maximum deviation from zero. For example for a range of -1 to +1, the full scale is 1. For a range of 0 to +1, the full scale is 1 Most Instruments quote precision is a percentage of Full scale (FS), percentage of span or percentage of reading (RDG). Precision and accuracy are not the same as resolution. If the precision is quoted as a percentage of span the same error will be a smaller number than if its quoted as a percentage of full scale (FS). For example, say the error is +/-1 mbar and the unit range is -1 to +1 bar. In this scenario the error as a % of full scale is 0.1% and as a % of span is 0.05%

Gauge pressure These sensors measure pressure difference between two points. If the negative sensor port is left open to the atmosphere (which varies according to the location and the weather), the is then relative to . Hysteresis Hysteresis is the difference in output of a transducer between when the input is increasing and when the input is decreasing. The input can be either pressure or temperature. The curve below is an example of a hysteresis curve.

Linearity Transducer linearity is a measure of how the electrical output varies in proportion to the pressure applied when compared to the ideal straight line. There are two common versions of the straight line namely Terminal Straight Line (TSL) or Best Straight Line (BSL). Line Pressure Sometimes referred to as . Both of these are terms are used when referring to differential transducers or transmitters. It means the pressure that will be applied equally to the positive and negative pressure parts, LH & RT Linearity, Hysteresis & Repeatability (LHR) is used to describe the room temperature precision of a pressure sensor, excluding all zero & span offsets, temperature errors and long term stability. mV output Pressure sensors giving a mV output are a 4 wire system, driven from a dc Transducers voltage source. Precision This is a measure of the distribution of data. If a process has high precision, then the results will show a low variation. The shots below have a high precision but low accuracy.

Reading or RDG Most Instruments quote precision is a percentage of Full scale (FS), percentage of span or percentage of reading (RDG). Precision and accuracy are not the same as resolution. Reading is the indicated value. For example 0.005% rdg + 0.005% FS Note: all percentage reading statements will have an adder otherwise the error will be impossible to achieve at the low end of the scale. i.e. 0.005% of zero = zero error. Alternatively % RDG values are stated as true over a limited range of the instrument e.g. 0.005% RDG between 10% and 100% of FS. Repeatability If the same readings are taken twice in the same circumstances the difference between them is the repeatability. Resistance Resistance temperature detectors (RTDs) are temperature sensors that exploit temperature the predictable change in electrical resistance of some materials with changing detectors (RTD) temperature. Almost always made of platinum due to it’s linear properties with temperature, they are often called platinum resistance thermometers (PRTs). An example is the common Pt100 having a nominal resistance of 100 Ω at 0 °C. A small current is passed through the RTD and voltage is measured across it. From Ohm’s Law (R=V/I), the resistance can be calculated to give a measure of temperature. If the current is too large heat will be generated in the sensor (P=I2R), so good measurement systems typically provide 1 mA or less excitation current, and may use techniques such as pulse power to reduce the heating effect further. Resolution Resolution is the smallest change in a measured value that a device can detect (Resolution is also known as sensitivity). Precision and resolution are linked but just because an instrument has 7 digits of resolution, it does not mean that the instrument has extremely high precision. In order to reliably meet an accuracy specification, an instrument must have sufficient resolution. For example, an instrument with a FS of 1 bar and an accuracy of 0.01% FS can reliably measure down to 0.1 mbar, so the display should have enough digits of resolution to display a change in pressure of 0.1 mbar.

Resolution = ±0.5 psi Resolution = ±1 mbar Sealed Sealed gauge sensors are absolute pressure sensors. These sensors measure sensor pressure relative to a vacuum. This type of sensor is then calibrated to read zero at 1 bar absolute. Span Span is the maximum range between the lowest point and the highest point. For example for a range of -1 to +1 the span is 2; for a range of 0 to +1 the span is 1. If the precision is quoted as a % of span the same error can be a smaller number than if quoted as a % of full scale (FS).

Stability Stability is the change of output over time. It is quoted at reference conditions, so a sensor that is cycled across temperature, or held at a high temperature, may shift more than the stability number due to the effect of temperature on performance over time. • On a gauge device - zero drift requires calibration to correct, this is normally performed by equalising the positive port with the reference port (i.e. Both ports open to atmospheric pressure) and adjusting the reading to zero. • On an absolute device - zero drift requires calibration to correct, this can be performed by using a good reference standard, the device zero reading is adjusted to match the reading of the reference. • On an absolute and a gauge device - span drift requires calibration to correct, this can only be performed by using a pressure/vacuum source and a reference standard, the device span reading is adjusted to match the reading of the reference.

TEB TEB is Temperature Error Band. For Sensors, the output change with temperature is normally expressed in Temperature Error band. TEB is The maximum error due to temperature, over the full pressure and temperature range divided by 2 Total Error In order to calculate the worst case error the maximum error for all factors Calculation (or need to be added up: Error Budgets) Total Error = NLH&R + Span + Zero setting + TEB + Stability + Uncertainty of calibration + Environmental conditions In reality it is extremely unlikely that all these errors will be at full magnitude in the same direction so in most cases the error is calculated as the Square root of the sum of the squares of the factors. Thermocouple In 1821, Thomas Seebeck discovered that when a metal conductor is subjected (T/C) to a thermal gradient, it will generate a voltage. This is known as the Seebeck effect. A thermocouple (T/C) probe is made of two pieces of dissimilar material, welded together to form a junction known as a hot junction. The resulting potential difference between the two ends of the thermocouple can be measured and converted into a temperature using an instrument. TSL Terminal Straight Line. A technique for producing a straight line based on a number of finite data points. In Terminal Straight Line technique, a straight line is drawn between the first and last pressure points Voltage Output Pressure sensors giving a voltage output are essentially based upon the mV Transducers output transducers. Additionally they have further electronics fitted internally within the sensor to amplify the mV signal from the Wheatstone bridge.

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