Pressure transmitters are widely used in the process industry. Pressure transmitters have different accuracy components such as the effect of re-ranging, ambient temperature, mounting position, static pressure, long term drift, vibration, power supply, and more.
Typically, a pressure transmitter’s specifications include several accuracy components that need to be taken into account when considering the total accuracy. The transmitter’s accuracy affects the accuracy of your calibration equipment. The calibrator should be more accurate than the transmitter you calibrate with it, but the accuracy ration between the two have varied opinions.
The accuracy components are broken down into several categories. The reason it is critical to look at these components is to understand the different things that have an effect on the total accuracy you can expect in practice.
Reference accuracy is a baseline accuracy specification that does not include process effect that can contribute to transmitter errors such as temperature and static pressure effects. The reference accuracy is established under reference conditions.
Many times, the best reference accuracy is valid only for certain pressure ranges, not for all the ranges available. Also, it may vary on the pressure type, an absolute range may be different than a gauge range. You need to be sure that you know what the accuracy is for the exact pressure ranges/models that you are using.
Re-ranging an instrument means to set the lower and upper range values so it responds with the desired sensitivity to changes in input. Often you can re-range a transmitter with a turndown ratio of 100:1 or even more. Accuracy specifications are commonly given to the full range, or with a limited turndown ration.
Most pressure transmitters are used in varying environmental conditions in the processes. Also, the temperature of the pressure media may vary widely during usage. Pressure transmitters, like so many measurement devices, have some kind of temperature coefficient. Which means there is an accuracy component that depends on the environmental temperature.
If the temperature in your process varies a lot, you should take this into account.
Differential pressure transmitters can be used under static line pressure conditions. This means that both inputs have a certain pressure, and the transmitter is measuring the difference between the two inputs.
Compared to a gauge transmitter that is measuring pressure against the atmospheric pressure or an absolute transmitter that measures pressure against full vacuum.
An ideal differential transmitter would measure only the difference between the inputs, but in practice, the common-mode static line pressure has some effect on the output. Line pressure may also have some effect on the span of the transmitter which makes it far more difficult to handle and to calibrate. It requires a differential pressure standard for the calibration.
Long Term Stability
All measurement devices will slowly lose their accuracy over time. Some pressure transmitters have 1-year stability specified while some have a 5 or 10-year specification. Depending on how often you re-calibrate your pressure transmitters, you should consider the long-term stability effect, as the transmitter may drift that much before the next calibration.
The mounting position typically has some effect on the accuracy of the pressure transmitter. Most pressure transmitters have a specification for the mounting position. The orientation should be considered if you first calibrate the transmitter and then install it into the process, or if you remove the transmitter from the process for recalibration.
Many pressure transmitters have a specification for the effect of vibration. This needs to be considered only if the transmitter is installed in a vibrating location.
Some transmitters have some kind of “total accuracy” specification that includes several of the common accuracy components. These other components can include reference accuracy, the ambient temperature effect, and static line pressure effect. This kind of total accuracy has a more user-friendly value as it gets closer to the real accuracy you can expect from a transmitter.
When a pressure transmitter is used in a process to measure pressure, there is a big risk that the transmitter’s membrane gets contaminated by the pressure media or some dirt. This kind of contamination can have a huge effect on the transmitter’s accuracy. In addition to the transmitter’s long-term drift specification, this should be considered in the risk analysis.