Pressure Calibrators

Pressure Calibrators

Pressure Calibrators are used to troubleshoot and calibrate pressure transducers, transmitters and gauges. Unlike other calibrators, one pressure calibrator cannot cover all pressure ranges. At time of order, the pressure range must be specified. Selecting one very wide range is not recommended because pressure sensors are typically percent full scale accuracy (or have a percent full scale component).

The good news is that most designs have interchangeable optional pressure modules for alternate ranges available for purchase with the instrument or later.

Understanding Pressure Modules and Ranges?

 

The above graphic best explains the difference between all the pressure related terms. Key to understanding is where the reference point is located. To further clarify, Differential has to do with measuring the difference between any two pressures and is performed by differential pressure transmitters. Differential pressure applications include measuring flow using orifice plates and laminar flow elements (both available from TEquipment).

Pressure measurement does not have very wide ranges with good accuracy, just inherent in the nature of the available technologies. It is especially true when you consider the full scale accuracy problem. A pressure transducer measuring 1000 psig +/-1% Full Scale will be +/- 10 psig. It is not practical to use it to measure 8 psig, for example. Manufacturers' solution has been to offer more sensors with different ranges. In the previous example then, you would obtain a model with a range 0-10 psig or even 0-30 psig. Gauge pressure is not the only type of pressure measurement, as explained in the graphic.

Available calibrator pressure modules types:

• Absolute Pressure. Pressure module measurements are referenced to zero pressure absolute or a perfect vacuum (-14.7 PSIG or 0 PSIA). When these modules are open to atmosphere they will read approximately 14.7 PSIA ( approximately 1 atmosphere at sea level). With absolute pressure starting at a true zero, there cannot be a negative absolute pressure measurement.
• Differential Pressure. Differential pressure transmitters will have two ports, a High and Low side and marked as such. You may see units such as psid.
• Dual or Compound. Pressure modules will read both positive and negative pressures through a single input port.
• Gauge Pressure. Pressure modules read pressure relative to the local atmospheric or ambient pressure, also known as one atmosphere. It’s stated in units of “G” (Gauge), for example if you are measuring PSI (Pounds per Square Inch), the pressure will be listed as PSIG.
• Vacuum. Pressure modules read only negative pressure with atmospheric pressure being your reference.

A pressure calibrator or multifunction calibrator with pressure calibration capability will usually include one or two pressure modules and others can be purchased separately. To summarize, when selecting the modules consider the types of measurements being made and the range. When selecting the range, do not get too large a range past the required. This is one time where too much is not a good thing as it will lead to poor accuracy.

Zero and Span errors and how a calibrator corrects them

A zero error in a sensor is a positive or negative shift when the sensor is at zero. Similarly, a span error occurs at the max end of its measuring range (i.e. at full scale, also called span). A relatable real world example for the zero error is a digital weigh scale. At rest with no load, the display does not always read exactly zero. Scale manufacturers provide a zero or tare button to zero the scale and also serves to zero out an empty container in filling applications.

The following charts graphically represent the effect of a zero offset, a span offset, and if both occur. In these examples, a pressure transducer with range of 0-100 psig is plotted, but it could easily be 0-100% for any variable being measured. The center line in each case is the actual (true) value and possible high and low errors are shown. Y-axis represents the value shown on the sensor display or signal output and the X-axis is the input pressure being seen by the transducer.


Load cells, pressure sensors, and flowmeters are particularly known for having a zero offset, but zero and span errors can occur with any instrument. For pressure, it is inherent in the technology because of a slight loss of memory in the deflection of the thin metal membrane in contact with the process and also because of changes in atmospheric pressure due to elevation above sea level versus where the pressure transducer was originally calibrated. In manual weighing, it is easy to press the zero button, but for process instrumentation, it is not as easy. Better sensors will have zero and span adjustments using a keypad, potentiometers (or pots), or through digital communications using a HART Communicator. Ideally, the process input (also called PV or process variable) to the sensor is set to zero and span. Using the example above, it would be setting the pressure vents of the transducer open to read atmospheric pressure (zero) and using a pressure calibration pump or deadweight tester for the full scale span. The second choice would be to trim the output (also called analog output AO) of the transducer instead of the input. This would mean that the transducer will still believe the pressure is incorrect but output the correct value. This could be a problem for smart instruments, where 4-20 mA and digital signals like HART are both being used.