Pressure Transmitter Accuracy

There are many aspects that impact a pressure transmitter's accuracy, and each one is applicable to different types of instrumentation and applications. The three most important aspects that apply to pressure transmitters are hysteresis, linearity, and repeatability. Of the three, linearity is the greatest contributor to a transmitter's inaccuracy.

Many transmitter manufacturers merge hysteresis, linearity, and repeatability features together, denoting one value as representative of the total accuracy. Viatran, however, separates linearity from the other two. To specify accuracy in a Viatran transmitter, there is a value for linearity and a second value that signifies a combination of hysteresis and repeatability.

Linearity

Preferably, a graph of the electrical output from a transmitter would form a straight line over the range from zero pressure to full-scale pressure. In actual fact, that is seldom the case. Transmitter output plots have minor curvatures over the full range of inputs. This effect is the result of the transmitter diaphragm geometry, the sensing technology utilized, and basic inconsistencies in the sensor's material.

Linearity performance is described to be the closeness to which the actual transmitter output curve resembles a straight line. Linearity is quantified as the highest deviation of the output curve from some straight line of reference.

One way to quantify the linearity error of a transmitter is to use the Terminal Line technique. A terminal line connects the value of the zero output signal to the value of the full-scale output signal (see figure). Linearity is defined as the maximum deviation of the transmitter output from the terminal reference line and is stated as a percentage of the unit's full-scale range.

Another technique of quantifying linearity, the Best Fit Straight Line (BFSL) technique, has become prevalent in the pressure transmitter sector. The BFSL technique uses a mathematical method known as linear regression to fit a straight line through the transmitter output data points. This method equally weights the points above and below the BFSL. In the BFSL technique, linearity is defined as the highest deviation of the transmitter output curve from the reference BFSL.

For the same transmitter, the linearity performance using the BFSL technique seems to be about twice as good as that obtained from the Terminal Line technique (refer figure). This is just a result of the BFSL technique using a line midway between the two parallel straight lines closest together and enclosing all output vs. measurement values on a calibration curve. The Terminal Line technique using a line connects the extremes of output response.

Viatran has standardized on the BFSL technique as it is the most universally used in the pressure industry. With this technique, customers can compare Viatran products to any others on an equal basis.

To reduce the effect of nonlinear performance, Viatran's engineers use linear compensating analog correction methods and engineered diaphragm geometries that compensate for nonlinear performance. Sensor materials and technologies are chosen based on ideal ranges and applications of each model.

Hysteresis

Hysteresis error occurs when a transmitter delivers different outputs at a specific applied pressure, based on whether the pressure has decreased or increased to that point. For instance, when pressure increases from 0 to 50 psi, a transmitter experiencing hysteresis error has a higher output than when the pressure decreases from 100 to 50 psi (refer figure). Hysteresis error is the difference between the two changeable signal outputs. Viatran state hysteresis error as a percentage of full-scale pressure.

Repeatability

Repeatability refers to a transmitter's ability to offer the same output signal on repeated but autonomous applications of the same input pressure.

The zero repeatability performance of a pressure transmitter is examined by measuring the output at zero pressure after each of two independent full-scale pressure applications (see figure). The transmitter output signal at zero pressure should be the same for both trials. Owing to mechanical and electrical effects, sometimes there is a very minor difference between the two output signals, known as zero repeatability error.

Repeatability error is quantified as the variation between the two varying signal outputs. The error value is then indicated as a percentage of the full-scale pressure.

Repeatability and hysteresis both involve differing output signals that take place for a single pressure value. Therefore, Viatran specifies these two values as single quantity, and lists the specification in product bulletins as "Hysteresis & Repeatability."

The mechanical properties of a transmitter's sensor and diaphragm material are the key factors in hysteresis and repeatability effects. Viatran selects sensor and diaphragm materials with negligible vulnerability to these effects so as to reduce sensor hysteresis and repeatability error.

This information has been sourced, reviewed and adapted from materials provided by Viatran.

For more information on this source, please visit Viatran.