# Pressure Sensor Accuracy

When using a pressure sensor to measure pressure, one of the first questions that comes up is accuracy: How closely does the value reported by the pressure sensor come to the actual pressure?  We use the term accuracy, but what we really want to know is the error of the pressure measurement.  This article will describe standard specifications and methods for calculating sensor accuracy.

The definition of sensor accuracy actually defines error, and the two terms are used more or less interchangeably:  The accuracy of the sensor is the simply the difference between the pressure reported by the sensor and the actual pressure, expressed as a percent of the sensor full scale.  For example, if a pressure sensor with a full scale range of 100 psi reports a pressure of 76 psi – and the actual pressure is 75 psi, then the error is 1 psi, and when we divide this by the full scale and express it as a percentage, we say that accuracy (or error) of the sensor is 1%.  Most industrial sensors are better than that, with specified accuracies of +/-0.25% or +/-0.1% of full scale (FS).  So the error of a 100 psi FS sensor with an accuracy of +/-0.1% FS will not exceed +0.1 psi or -0.1 psi – at any point in the measurement range of the sensor.

Sensor accuracy is comprised of two error modes: Non-linearity and Hysteresis.  Ideally, pressure sensors are perfectly linear – the output signal or reading is directly proportional to the applied pressure.  Because sensors are mechanical devices, however, they are not perfectly linear, and this error is called non-linearity.  Non-linearity is determined by the five-point calibration method.  A pressure standard (a device know to be at least five times more accurate that the pressure sensor) is used to apply pressures at 0%, 50%, 100%, 50% and 0% of the sensor full scale.  A best-fit straight line is fitted to these points, and the maximum deviation of any of the five points from the value predicted by the best-fit line, is defined as the non-linearity error.

Hysteresis is simply the difference in the value of the readings at the same pressure along these five calibration points, up-scale and down-scale.  Hysteresis can be measured from the readings at the 50% points and at 0% FS pressure.  The greatest difference in any of these is defined as the hysteresis.

The accuracy of the sensor is defined to be the sum of the non-linearity error and the hysteresis error.

Note that the sensor accuracy calculation is pretty much the worst case error that can be determined from the calibration data, and it may not occur at every point along the pressure sensor FS range.  But this is considered to be a conservative method for calculating the error present at any pressure over the range of the sensor.

For differential pressure sensors – those having a plus and minus full scale range, the same techniques are used to calculate accuracy with the addition of -50%, -100%, -50% and 0% calibration points required (9 all together).

Spreadsheets that automatically calculate non-linearity, hysteresis and accuracy are available from Validyne for gage, absolute and differential pressure sensors.  Factory calibration sheets showing these errors are shipped with most sensor models.

# Differential Pressure Transmitters for Kiln and Furnace Applications

Many industries rely on large furnaces. For example, the gas and oil industry uses a great deal of heat to refine and process crude oil. The steel industry must heat metals as part of heat treatment processes that occur in special furnaces. With new EPA regulations and efficiency considerations, it’s important to keep pressure low and this has led to special draft range differential pressure transmitters like the DR800. This instrument provides several important benefits in furnace operations.

The Importance of Furnace Draft

The flow of air within a furnace is a vital to energy efficiency and heat control. But what is draft? Draft is the difference between atmospheric or room pressure and pressure within the furnace combustion chamber. This affects the flow of airneeded for the combustion process.

Drafts can be natural, forced, induced or balanced. It is important to monitor this airflow at all times, as it can greatly affect temperature and heat transfer. Draft range differential pressure transmitters are able to detect very low amounts of pressure and send their readings to a furnace control system. In fact, the DR800 can measure pressures as low as 0.1 inches H2O full scale, delivering accuracy to within 0.5 percent. The DR800 is extremely stable, even though large ambient temperature ranges.

Thanks to the DR800, pressure calibration is uncomplicated. It is made possible by including a Hi/Lo gain jumper and continuous the span adjust. You also can choose an LCD display for local indication.

The DR800 is very durable, but should you need repairs they can be done right in the field. You can easily remove its entire electronics housing without disassembling the unit. Plus, your maintenance personnel won’t have to carry specialized tools for the job. The NEMA 4 enclosure and standard process industry form factor make the DR800 easy to use.

To check out the differential pressure transmitters we have to offer, visit our home on the Web today at http://validyne.com/ or call 818-886-8488.

# What to Look for in Durable Differential Pressure Transducers?

At Validyne Engineering, we have almost half a century in the industry, providing a range of different low cost, highly accurate pressure sensors for many different industries and applications. Over time, we have provided these sensors to various industries and companies. We have also listened to the needs of our customers to develop the sensors, transducers and transducers to get the job done.

One of the most important aspects of our devices is quality. We produce a durable, rugged line of differential pressure transducers. We believe we have a top selection of these transducers for all industries and applications.

The Validyne Difference

All of our differential pressure transducers are designed to meet performance specifications in highly demanding environments. Most of the transducers we sell can be used for both liquids and gases. They have been extensively used in the field with test vehicles and aircraft as well as in all types of environmental conditions.

Our in-house team of designers and engineers, all with extensive experience in sensor component development and design, has the ability to incorporate the needs of our customers into specialized solutions for their applications. We are also able to take this knowledge and design transducers for general use that are superior to other designs on the market today.

Up to the Test

One of the biggest complaints we hear from people looking for transducers is their inability to survive difficult working conditions. Each of our components has been thoroughly tested to provide our end-users with the quality part to stand up to real-world use. We also provide support and assistance in helping you to choose the right component for any application.

Our transducers are able to handle changes in the environmental and ambient temperatures with minimal impact on the accuracy of the pressure readings. They are designed to provide precise readings with fast response, giving test engineers the data they require. Fully compatible with data management systems, they are easy to integrate into field or laboratory testing situations or where and as they are needed.

Easy to mount with pre-drilled holes, they have a small, compact size and low weight make these differential pressure transducers the right component for the job. They are able to stand up to spray and moisture and also capable of wet-wet operations. Our transducers are available in differential or absolute pressure, as well as the option for a CE approved model if so required.

# Reducing Glove Box Filter Costs

Handling radioactive materials must be done in glove box. A glove box is a clear plastic enclosure with rubber gloves attached to the sides so that an operator can handle material inside the glove box, but air and radioactive dust are not allowed to escape. The air pressure inside the glove box must be controlled so that it is always less than the ambient atmosphere. A series of risers, ducts, fans and filters connected to the glove box keeps the pressure inside the glove box lower than the outside atmosphere so that no dust can escape.

If the airflow velocity up the riser from the glove box is too high, radioactive dust is carried into the exhaust system, and these particles are trapped by a series of special filters. When a filter needs to be changed, it is very expensive to dispose of the dirty filter because it is charged with radioactive dust and dirt. The key to reducing filter disposal costs is to limit the exhaust airflow velocity so that dust is not carried out of the glove box and into the filters, while at the same time insuring that the pressure inside the glove box is always less than the ambient atmosphere.

Measurement of the air velocity in the glove box risers is accomplished with a pitot tube and a sensitive differential pressure transducer. As air velocity increases, the differential pressure across the pitot tube increases. The very low velocity needed to exhaust air from the glove box, without raising dust translates into a very low pressure drop across the pitot tube. This pressure drop is as little as 0.02 In H2O under normal operating conditions. A very sensitive differential pressure sensor, combined with a high level DC signal output, is needed to provide the air velocity signal to the system controller.

A special version of the Validyne P532 provides the low pressure measurement required for such a control system and these are used at a major North American nuclear fuel rod processing facility.

# Furnace Draft Measurement

Introduction:
One of the most difficult process measurements to make accurately is furnace draft pressure. This is the pressure inside the furnace that optimizes the air/fuel mixture for the most efficient combustion. The air is drawn into the furnace by the slightly lower pressure that results from heat rising up through the stack, the same way a chimney draws air into a fireplace. In an industrial boiler the draft pressures are very low – typically less than 0.25 In H2O – and are controlled by large vents that open or close depending on the furnace pressure to be maintained. The combination of low draft pressures and the extreme environment of heat around the furnace make accurate draft range pressure measurements difficult. The Validyne DR800 is designed especially to meet these requirements and this application note will describe the features that make the DR800 the best choice for industrial furnace draft measurement.

Low Range:
The DR800 is available in a full scale pressure range of just +/-0.25 In H2O. This is the full scale of the sensor and is not the result of over-amplifying of a higher pressure sensor. The variable reluctance sensing technology of the DR800 produces ten times as much signal as a strain gage sensor, further reducing the need for amplification and resulting in much better performance through temperature.

Rugged Housing:
The DR800 has a NEMA 4 housing that includes an industry-standard junction box with 1/2” NPT conduit connections and externally accessible zero and span adjustments. The mass of the DR800 body acts to absorb rapid changes in ambient temperature and is solidly built.

Offset Calibrations and Turn Down:
Most furnace draft measurements can be both positive and negative with respect to the ambient atmosphere. The DR800 signal can be offset from zero to take these pressure changes into account. A typical calibration of -0.05 In H2O = 4 mA and +0.15 In H2O = 20 mA is possible using the zero adjustment and offset jumpers on the circuit board. The span can be turned down 2.5 times so that a span of just 0.1 In H2O is possible on the most sensitive DR800. This small span can start at -0.25 In H2O, for example, up to +0.15 In H2O using the zero elevation and suppression feature.

Selectable Damping:
Furnace air flows are often noisy and the DR800 has selectable damping with time constants from 0.25 seconds to 8 seconds using jumpers on the circuit board. This smooths the signal allowing for better pressure control.

Standard Mounting and Process Connections:
The DR800 pressure connections are 1/4” female NPT with adapters available for 1/2” NPT. The plus and minus ports are mounted on 2-1/8” centers for easy connection to industry standard 3-valve manifolds. A 2” pipe mounting bracket is also available.

FM Approval:
The DR800 is FM-approved for use in hazards locations for Class I, Div 2, Groups B, C and D.