The transmitters that "smart" contain microprocessors have been a breakthrough for industrial instrumentation. These disposivitos have the ability to diganósticos, high precision (due to the digital compensation of sensor non-linearities), and the ability to communicate digitally with host devices to report various parameters.

A simplified diagram of a pressure transmitter "smart" is shown in the figure below:

It is important to realize all the settings in this device, and as you can compare with the simplicity realitva Analog pressure transmitter:

Let us realize that analog transmitters calibration is done by only two settings that are "zero" and "span". Clearly this is not the case of smart transmitters. Not only can we configure the low and high range (LVR and UVR) in a smart transmitter, but it is also possible to calibrate the analog-digital and digital-analog each independently. What this means is that to make a smart transmitter calibration of the instrument requires more work and potentially make a lot of adjustments in the analog transmitters.

A common mistake made by many students and also for experienced instrumentalists to confuse the setting ranges (LVR and URV) to perform an actual calibration. Just for you to enter a value of LRV in a pressure transmitter to 0.00 PSI and PSI 100.00 URV not necessarily mean that record with precision measurements in this range!. The following example will explain this shortcoming.

Suppose we have an intelligent pressure transmitter range from 0 to 100 PSI with a 4-20 mA analog output, but the sensor shows the pressure transmitter sensing accuracy problems, maybe for a long time of use, and when applied to an input of 100 PSI generates a signal that the analog-digital converter performs only as 96 PSI. Assuming everything else in the transmitter is in perfect condition with perfect calibration, the output signal whenever an error Chair.

Here we can see more sophisticated calibration in a digital transmitter could be corrupt despite performing a pefect calibration of analog-digital and digital-like, and seamless range setting on the processor. The microprocessor "thinks" that the applied pressure is 96 PSI, and responds according to this reading and has an output signal 19.36mA. The only way an instrumentalist would know that this transmitter has an incorrect answer to 100 PSI is actually applying a known value of fluid pressure of 100 PSI in the sensor and realize the wrong answer. The lesson here should be clear: setting minimum and maximum ranges in a smart transmitter is NOT a legitimate instrument calibration.

For this reason, smart transmitters always provide a way to carry out the configuration that we call "digital trim" in the ADC and DAC converters to ensure that the microprocessor "look" a correct representation of the stimulus applied and make sure that the processor output signal is converted to DC current accuracy, respectively.

It is very common to see some musicians use the LRV and URV parameters in a way very similar to the zero and span adjustments in the analog transmitters to correct mistakes like these. Following this methodology, we should fix the problems URV transmitter at 96 psi instead of 100 PSI, then for an applied pressure of 100 PSI would give 20mA output signal that we want. In other words, we make the microprocessor "think" that only this "seeing" 96 PSI, then changing the URV will always send the right signal. This solution works to some extent, since if there is any query digital transmitter (for example, using an analog signal protocol such as HART) will result in conflicting values, the current signal represents the entire scale (100 psi) while the digital record in the transmitter display 96 PSI. The only solution for this is "cut" or "trim" the range of the analog to digital microprocessor in the transmitter "knows" the true value of pressure applied to the sensor.

Once you have made the "cut" or "trim" in the input and output converters, of course, the instrument is microprocessor reranguear free as many times as you want without re-calibrate again. This capability is particularly useful when you need to make a re-rangueo for special conditions, such as starting and stopping processes where process parameters are often in unusual settings. In addition, an instrumentalist can use a hand-held digital to communicate with the device and reset the LRV and URV values ​​to desired values ​​by operating area without having to perform a calibration of the instrument by applying a physical stimulus to the instrument. While the trim (trim) the range of the analog-digital-analog is good, the accuracy of the instrument will remain good with the new range. With analog instruments, the only way to change to different ranges of action was to change the zero and span adjustments, which need re-application of physical stimuli on the device (a complete recalibration). Here and here alone we see that the calibration is not necessary for intelligent devices.

If the overall accuracy of the measure shall be verified, we can say that there is no substitute for actual calibration, and this involves both parameter settings for the ADC and DAC.