Pressure Calibration Description
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). Refer to the earlier discussion Percent Full Scale vs. Percent Reading Accuracy.
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:
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.
- 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.
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.
Pressure Calibration Pumps
When calibrating pressure transducers on a bench, actual pressure needs to be generated so that the pressure transducer / transmitter and the pressure calibrator can be connected in a manifold arrangement to see the pressure. For that purpose are hand operated pressure test pumps.
There are overlapping ranges, so select a model that best covers your applications.
- Pneumatic models use air and typically range from vacuum to about 600 psig.
- Hydraulic Test Pumps use oil and achieve much higher pressures of up to 10,000 psig.
Features to consider when selecting pressure calibration pumps
- Kit versions include hoses, fittings, spare filters, and/ or more than one pump to cover wider range at a savings.
- Multi-turn knob for fine adjustment of pressure.
- Adjustable stroke control to allow for fast priming or filling of test systems. This gives the operator the ability to switch as needed to a smaller stroke for easier pumping at high pressure.
- Pump can be easily cleaned without disassembly.
Deadweight Testers (DWT)
Deadweight Testers use traceable weights to apply pressure to a fluid such as air, water, or oil to calibrate pressure gauges, transducers, transmitters, and portable calibrators. Due to their fundamental method of pressure measurement using calibrated piston-cylinders and masses, they offer unmatched measurement stability and reliability. DWT’s are considered primary standards.
Deadweight testers also inherently regulate a stable test pressure once the piston is floated, solving a problem that operators of some manual pressure calibrators encounter.
Deadweight testers also measure accurately over a wide range of pressure. The uncertainty of deadweight tester measurements is a percent of the measured value (% of reading). By including multiple piston-cylinders, a single Fluke Pressurements Deadweight Tester can calibrate units under test with full scale ranges that vary by a 100:1 ratio or more.
If you think deadweight testers are too complicated or expensive, then this whitepaper from Fluke Calibration is a must read: Performing Precise Pressure Calibrations May Cost Less Thank You Think
Deadweight Tester Selection Considerations
- Pressure range. Consider the highest and lowest pressure that needs to be generated. DWT’s cover from vacuum to 60,000 psi (400 MPa). The performance is less than ideal when the pressure being generated is below 10% of the full scale. Above 10% of full scale accuracy is percent of reading. Applications that require both low and high hydraulic pressures can be handled using dual piston units. These units are provided with both a low range and high range piston/cylinder. Switching between the low range and high range on a Pressurements DWT is as simple as removing the masses from one piston and placing them on the other. No valves need to be switched.
- Pressure media. Pneumatic or hydraulic (water or oil). Pneumatic, or gas, instruments are ideal for lower pressure ranges. Gas is preferred whenever cleanliness is required. In addition, using gas reduces the impact of head height corrections. However, at higher pressures it is necessary to use water or oil.
The usefulness of gas as a media is limited to approximately 2 000 psi (14 MPa). There are two reasons for this. First, there is more risk of explosion at high gas pressures and that is a safety concern. Second, generating high gas pressure will require expensive intensifiers or gas boosters. Using oil or water eliminates these issues. Since water is not a very good lubricant, oil is preferable when allowed.
One advantage of liquids is they are incompressible. This allows a small change in the volume of the system (through a screw pump) to result in large changes in pressure. The most common approach is to use a mineral oil or water as the media. Oil is ideal in that it assists in lubricating the piston and cylinder. The downside to using oil as a media is it now introduces the device under test to possible contamination. An option is a liquid-to-liquid separator, which uses one liquid in the device and another in the DWT.
- Pressure generation options. On-board hand pumps are offered on Fluke Calibration Pressurements deadweight testers to generate vacuum or air pressure or to prime higher pressure hydraulic systems.
- Weight increments. Purchase the necessary weights for the desired ranges. The calibration weights need to be trimmed for the local gravity, so providing the final location of the instrument is critical for accuracy.
- Accuracy. DWT are inherently a percent of reading device. There is a lower breakpoint, normally 10% of full scale, where the specification ceases to be percent of reading.
Process Pressure Gauges
Process Pressure Gauges are analog or digital gauges with threaded connections for mounting on a gas, liquid, or steam line. Digital pressure gauges are either battery powered or 24 VDC powered. Pressure gauges come in a variety of types and ranges as explained above in Pressure Modules. Some models have datalogging capability.
Reference class pressure gauges are available. They are supplied with NIST calibration certificates and are high accuracy for use in pressure calibration work.
Pressure Transmitters are pressure gauges that output an analog signal, such as 4-20 mA, 0-5 VDC, or 0-10 VDC.
5 things to consider when selecting a Pressure Transmitter
- Pressure Range. 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). Refer to the earlier discussion Percent Full Scale vs. Percent Reading Accuracy.
- Process Connection. Pressure Transmitters are available with many plumbing connections for the process, such as 1/2 inch NPT thread or 1/4 inch NPT thread.
- Material Compatibility. Standard wetted parts are 316 SS or 316L SS that would be in contact with the process. Stainless is suitable for many most applications, but not all.
- Output and connection type. Select 4-20 mA for long runs. For shorter lengths, voltage output is an option to consider. The wiring varies too. Some transmitters can be as simple as potted wires in the transducer and a short length of unterminated wires and others have special connectors.