Understanding Common Specifications for MEMS Silicon Dies

To evaluate the quality and performance of a MEMS silicon die, customers must rely on specifications, at least until they can test parts for themselves. This article will discuss the most common specifications related to these pressure-sensor dies.

The primary thing to understand about MEMS dies is that when they are exposed to either pressure or temperature, they will produce a corresponding output, which will be in millivolts, provided that an input voltage, or excitation voltage, has been supplied. The millivolt output from the MEMS die is essentially the pressure value. Therefore, the general characteristic to look for in any MEMS die is a stable and repeatable output when the die is tested under various conditions.

This article discusses common specifications used to characterize a pressure-sensor die’s performance under different operating conditions.

The first groups of specifications we will discuss are commonly used to characterize how the MEMS die will perform at room temperature (25 °C).

Bridge resistance (or impedance): This indicates the resistance (from Ohm’s Law the voltage divided by the current) measured across the bridge. Due to our Wheatstone bridge design along with our Sentium® and MeritUltra™ processes, the input resistance (+E to -E) and the output resistance (+O to -O) on all of our dies are the same.

Offset (or zero-pressure output voltage): This indicates the difference, at zero pressure, between zero output and the actual output of the MEMS die. With absolutely no offset, at zero pressure the output would be 0 mV/V. However, with an offset of ±10 mV/V, the difference with 5 volts of excitation could be ±50 mV. Refer to the image of the transfer function below.

Sensitivity (or span): Sensitivity and span are, in general, synonymous. The two terms are used to indicate the electrical output, or the response, of the MEMS die to an applied pressure and supply voltage. It is typically represented by the slope of a line on a graph with output on one axis and pressure (for a given supply voltage) on the other axis. Refer to the image of the transfer function below. Sensitivity is generally stated in terms of microvolts per volt per psi (µV/V/psi).

Transfer Function Graph for S Series 15 psi MEMS Die

Non-linearity (or linearity): This shows how linear/non-linear the output is. The ideal output is perfectly linear. For example, at a constant 5-volt supply, for every pound per square inch that the pressure were to increase, the output in millivolts would increase linearly, as shown in the image of the transfer function above. The pressure non-linearity is calculated by measuring—at the mid-point of the pressure range—either of two differences: One is between the actual output and the best-fit straight line (BFSL) or the other is between the actual output and the invisible line that connects the two endpoints of the actual output. This line is called the end-point line or terminal base. Refer to the image below. The actual output shown in this image has been exaggerated for illustration. Whether the pressure non-linearity is based on the BFSL or end-point line, it is expressed as a percentage of the full-scale output (FSO).

MEMS Die Pressure Non-Linearity Example

Pressure hysteresis: This shows the delta, or difference, of the output at zero pressure and then up to full-scale pressure and back to zero pressure. It would be ideal to have no pressure hysteresis, meaning the output would be the exact same every time the pressure returned to zero. This specification will give you one indication of the die’s repeatability. Pressure hysteresis is expressed as a percentage of full-scale output (FSO).

The next three specifications indicate how a part will behave over a specified temperature range. At Merit Sensor all MEMS dies are tested over a temperature range from -40 to 150 °C.  These three specifications are first-order effects.

Temperature coefficient of offset (TCO): This is also known as temperature coefficient at zero pressure (TCZ). This indicates the offset changes at zero pressure as temperature changes.

Temperature coefficient of resistance (TCR): This indicates how the resistance changes at zero pressure as temperature changes. The bridge resistance does change significantly over temperature.

Temperature coefficient of sensitivity (TCS): This is also known as temperature coefficient of span. It indicates the deviation in full-scale output as temperature changes. As the temperature increases, sensitivity decreases. So at room temperature you might get a 100 mV output, but at 150 °C the output will decrease to around 75 mV.

The great news is that all the errors listed above are repeatable and consistent, which means they respond well to compensation. In addition to manufacturing MEMS dies, Merit Sensor also builds pressure-sensor packages and performs calibration over various temperature ranges.

The following two specifications, however, deal with errors that cannot be compensated: thermal hysteresis and long-term drift. Therefore, if you are trying to decide which MEMS die to buy, you will want to find a supplier that produces parts with good specs in these two areas. We, at Merit Sensor, know that our customers do not want their parts, which contain our MEMS dies, to fail in their customers’ applications; therefore, we take pride in producing MEMS dies with excellent thermal hysteresis values and long-term stability.

Thermal hysteresis: This is typically performed at zero pressure and shows the difference between the output when the temperature is at room temperature and then increased to 150 °C and then returned to room temperature and then decreased to -40 °C and then returned again to room temperature and so on. This testing characterizes the repeatability of the die over numerous thermal cycles. It would be ideal to get the same output every time the temperature returned to a given value.

S Series MEMS Die Accuracy with Thermal Hysteresis - Solid White Background

Long-term stability (or long-term drift): This specification indicates how stable the output of the die will remain, or, in other words, how little the offset will drift, over time and sustained temperature. We have tested parts, for example, at 150 °C for 300 hours.

S Series MEMS Die Long-Term Stability - Solid White Background

One thing to watch for is a data sheet advertising a MEMS die with an accuracy of ±0.25 %. Here’s the catch: That accuracy refers only to non-linearity at room temperature; it does not take the other errors that have been discussed into consideration. Hopefully this article has helped you to better understand the different performance characteristics of MEMS silicon dies and the specifications that are used to quantify the dies’ performance.

Finally, if you would like to learn more about the technology and performance of MEMS dies, we invite you to watch our recently broadcast webinar, which is now on demand.

Three Common Types of Pressure-Sensor Packages

At the heart of every MEMS pressure sensor is a MEMS silicon die. Merit Sensor owns and operates a wafer fab, where it produces all of its own MEMS die. Packaging a MEMS die requires specialized equipment and skills to handle the small and sensitive die and to perform delicate wire bonding. Therefore, many customers purchase pressure-sensor packages, in which the die have already been mounted and wire bonded. This article will discuss Merit Sensor’s three types of packages: uncompensated, passively compensated, and fully compensated.

Uncompensated

The most basic pressure-sensor package is uncompensated. In an uncompensated package the MEMS die has been mounted to a ceramic substrate with a special die-bond material, wire bonded to electrical traces on the ceramic, and covered with a protective cap or gel. Since each silicon die is inherently unique, the output for each one will be unique. Fortunately, silicon die have outputs that are very repeatable. This means the output can be compensated.

PMD Series Pressure Sensor Internal Components

PMD Series Pressure Sensor – Uncompensated

To get an accurate output, the customer will need to perform some degree of compensation. Certain applications lend themselves to compensation performed by the customer. Factors that will often determine whether the compensation is performed by Merit Sensor or the customer include the following:

  • Cost
  • Accuracy
  • Size
  • Output signal

Passively Compensated

A more ready-to-use version of a pressure-sensor package, especially for use at room temperature, is one with passive compensation. In this case the pressure-sensor package is basically the same as an uncompensated package; however, the thick-film resistors on the ceramic substrate have been laser trimmed, providing adequate compensation of the die’s output in operating temperatures between 10 °C and 40 °C.

Laser-Trimmed Thick-Film Resistor on AP Series Pressure Sensor

AP Series Pressure Sensor – Passively Compensated

For applications, such as invasive blood-pressure monitoring in a hospital room, compensation in this temperature range is sufficient. Other benefits of passive compensation are the pure analog signal with practically infinite resolution and frequency response times in microseconds.

Fully Compensated

In a fully compensated pressure-sensor package, signal conditioning (an on-board ASIC) is used to compensate the die’s output across a wide temperature span. A MEMS silicon die does not know the difference between pressure and temperature, so this level of compensation is especially critical in applications where the temperature of the sensing environment fluctuates drastically or reaches extremes highs or lows. Compensation through signal conditioning can provide a linear output and make that output as accurate as ±1 percent of the full-scale output (±1 %FS total error band) in operating temperatures between -40 °C and 150 °C.

Wheatstone Bridge with ASIC

Wheatstone Bridge on a MEMS Die with an On-Board ASIC

If we use fuel pumps in airplanes and fuel rails in vehicles for examples, it is typical for pressure sensors to be exposed to extreme temperatures; nevertheless, it is essential that these pressure sensors offer an accurate output. A fully compensated pressure sensor would be the appropriate solution.

TVC Series Pressure Sensor Internal Components

TVC Series Pressure Sensor – Fully Compensated

It is important to emphasize that each pressure sensor will require individual compensation, as each one will have a unique output inherent to its MEMS die. Many customers simply do not have the time or equipment to do this logistically or economically to each unit passing through their assembly line.

Merit Sensor has the experience and equipment to handle this necessary step for the customer. Furthermore, it often, although not always, makes sense for compensation to be done before the part leaves our facility. Nevertheless, we have left options for those customers who choose to do their own compensation. As always, our sales managers and technical team will be happy to answer any related questions.

Four Characteristics of Our Newest MEMS Sensing Element

Merit Sensor has owned and operated a wafer fab from its beginnings. Fabricating our own MEMS (micro-electro-mechanical systems) sensing elements, or die, is something that sets us apart from other pressure sensor manufacturers, many of whom source their MEMS die from foundries or suppliers. Producing our own wafers, which are diced into individual MEMS sensing elements, allows us to control our own technologies, development, and supply chain. To learn more about the advantages, read this AZoSensors interview with our director of engineering.

Since we continue to see interest worldwide in these MEMS sensing elements, we continue to develop MEMS die with superior performance at competitive cost. Our newest MEMS product on the market is the S Series, offering optimal size, sensitivity, and stability. Perhaps best of all is its excellent performance in regards to thermal hysteresis. Each of these characteristics will be discussed below.

Size

One remarkable feature of the S Series is its solid performance at a very small size: 1.5 mm x 1.5 mm x 0.9 mm. This size also makes it is possible to optimize the amount of die produced on each 150 mm (6 inch) wafer. The end result is a lower-cost die for the customer without any loss of superior performance.

S Series MEMS Die Dimensions

S Series MEMS Die Dimensions

Sensitivity

Silicon, which is the raw material of MEMS wafers, has piezoresistive properties, which means when pressure is applied, it is strained and its resistance changes accordingly. An output is based on the changes in resistance. Merit Sensor uses Wheatstone bridge technology to optimize the linearity of the output. It is challenging, however, to obtain an adequate output when the pressure is low. Nevertheless, through Merit Sensor’s proprietary MeritUltra technology the S Series provides a typical output at 5 psi / 34 kPa / 345 mbar of 100 millivolts (mV).

Stability

A stable part will remain accurate, i.e. it will not drift, over time and sustained temperature. The S Series data sheet specifies a long-term stability of ± 0.2 % of the full-scale output (% FSO). The chart below shows how stable and accurate the part has proven to be, demonstrating a typical offset drift of <0.05 % FSO at 300 hours.

S Series MEMS Die Long-Term Stability

Long-Term Stability of the S Series

Thermal Hysteresis

The characteristic we are really talking about here, once again, is accuracy. In addition to remaining accurate over time and sustained temperature, the S Series displays exceptional accuracy when exposed to thermal cycling. A MEMS sensing element is inherently sensitive to temperature. Its resistance and output will change when temperature changes. Fortunately, changes that are consistent are simple to compensate. The S Series die exhibits very consistent, accurate output when it is exposed to extreme temperatures and returned to room temperature. In thermal cycling tests it demonstrated a typical thermal hysteresis offset of <0.05 % FSO.

S Series MEMS Die Accuracy with Thermal Hysteresis

Accuracy of S Series with Thermal Hysteresis

If you have any questions about using the S Series in your application, contact one of our sales managers. You might also find the application note “Handling of Mounting of Pressure Die” useful.

Blood-Pressure Monitoring During the COVID-19 Pandemic

As a result of the COVID-19 pandemic, many people have been in hospitals under critical care this year. These patients require beat-to-beat blood-pressure monitoring, which helps clinicians see important vital signs about the patient in real time over the length of the critical-care treatment and make clinical decisions accordingly. Therefore, continuous, reliable monitoring of a patient’s blood pressure is as important as ever.

Merit Sensor has been a supplier of blood-pressure sensors since 2010. Our parent company, Merit Medical, to whom we supply blood-pressure sensors, is one of the global leaders in blood-pressure transducers. Check out their newest transducer, the Meritrans DTXPlus. Through our experience and that of our parent company, we have learned what matters most to clinicians using invasive blood-pressure devices.

Meritrans DTXPlus Blood Pressure Transducer

Merit Medical’s Meritrans DTXPlus

One important factor is that the fluid column from the needle to the sensor should be free of bubbles. Bubbles dampen the pressure pulses in the fluid column and, therefore, degrade the conversion of the pressure signal. To prevent bubbles, clinicians routinely tap or whack the blood-pressure transducer with a hemostat or forceps. This certainly poses a risk to the integrity of the pressure sensor and can cause even greater issues than just dampened signals. However, due to the insight provided by our parent company and our in-house expertise, we have improved the BP Series design over the years and have produced a robust pressure sensor that is able to withstand this common debubbling practice.

Merit Sensor's BP Series pressure sensor

Merit Sensor’s BP Series

Another common-enough issue that could destroy a pressure sensor is the inadvertent opening of the transducer’s stopcock to an undesired pressure source. For example, when a clinician injects medicine or contrast through the line, there is a pressure spike of around 300 psi in the line. This is considerably greater than the typical pressure of blood pressure, which is around 2 psi. In order that the pressure sensor be accurate at such a low pressure, it must contain a very thin MEMS diaphragm. At Merit Sensor we have designed a blood-pressure-sensor package that provides accuracy at low pressure yet robustness when exposed to overpressure. The BP Series has a typical burst pressure of > 800 psi. As long as the overpressure does not damage the MEMS diaphragm or the sensor package, the sensor will return to its specified performance once it is again within its operating pressure range of −30 to 300 mmHg.

In addition to providing a robust pressure sensor that can withstand forceful tapping and high overpressure, Merit Sensor has complete control over its manufacturing processes and supply chain. We own and operate a wafer fab in South Jordan, Utah (USA). Our on-site wafer fab enables us to monitor production closely and ensure high quality with everything we produce. It also gives us the flexibility to meet unique requirements of our customers, who might have a unique application for a blood-pressure sensor. With Merit Sensor you get reliability as well as flexibility.

To learn more about the advantages of owning and operating a wafer fab, read this interview published by AZoSensors.

Pressure Measurement for Ventilators and Respirators

Merit Sensor has a long history of supplying products to the medical-device industry. We are owned by Merit Medical, a leading manufacturer and marketer of disposable medical devices used in interventional, diagnostic, and therapeutic procedures.

A couple of the products we supply for medical applications are the BP Series for monitoring blood pressure, where accuracy and reliability are extremely important, and the AP Series for angioplasty, where knowing the precise pressure of the catheter balloon is essential.

The current demand for mechanical ventilators and respirators due to the COVID-19 pandemic presents us with another opportunity to fill a need in the medical field. The use of pressure sensors in ventilators and respirators is similar to their use in CPAP and BiPAP. Essentially, when the lungs need assistance taking in air, a machine blows the required amount of air into the lungs. But how does the medical professional monitor and control the required amount of air, or positive airway pressure? This is where pressure sensors play a critical role.

LP Series pressure sensor for mechanical ventilators

Merit Sensor’s LP Series is the ideal pressure sensor for these applications. The LP Series can measure pressure as low as 1 inH₂O (250 Pa) with resolution better than 0.001 inH₂O (<0.1 Pa). It was designed to measure differential or gauge pressure, depending on the application. It contains two pressure ports to which tubing can be connected, one tube directing pressure to the topside of the MEMS sensing element and the other tube directing pressure to the backside of the MEMS sensing element. It comes in several calibrated pressure ranges, functions with a 3.3- or 5-volt supply, and offers I²C or analog output.

Cross-section image 2 of LP Series pressure sensor

The LP Series, along with all of Merit Sensor’s other products, is made in our on-site wafer fab and assembly areas in Salt Lake City, Utah, USA. Manufacturing our products in our own facility ensures that we have control over product quality and supply chain. It also allows us to customize products to meet the unique demands of our various customers and markets.

We are proud to be a reliable supplier for life-enhancing and life-saving devices, and we hope that our products can play an important role in helping humanity, especially at this time.

Custom Pressure Sensors for the Aerospace Industry

The aerospace industry is known for having some of the harshest environments on the planet and finding a pressure sensor to withstand those harsh environments can be extremely difficult.  The TR Series pressure sensor and HM Series MEMS sensing element are built to withstand temperatures as low as -40°C and as high as 150°C, making them ideal candidates for the Aerospace industry.  The TR Series and HM series measure pressure via direct media pressure sensing to the backside of the die.  Direct media pressure sensing translates into excellent system design flexibility leading to lower cost and ease of manufacture.

TR Aerospace Image

TR Series – TR Series with Ferrule – HM Series Die

So whether your application is Flight control surface positioning, auto-pilot, jet engine throttle and thrust reverser controls, auto-pilot input, landing gear steering and thrust vector control, turbine guide vane, valve controls, turbine actuators, and engine controls or any other number of Aerospace applications Merit Sensor has the right sensor assembly or MEMS sensing element for you.

Merit Sensor Systems, Inc. has partnered with customers for more than 20 years to design, fabricate, assemble and package reliable, cost-effective piezoresistive pressure sensor solutions.

Merit Sensor offers full-service design capabilities, in-house wafer fabrication, flexible shipping, packaging and assembly, piezoresistive technology (PRT), expansive pressure ranges (0.15 psi to 15,000 psi), complete pressure measurement (absolute, gage, differential and vacuum). Additionally, Merit Sensor is able to provide unparalleled flexibility to customize pressure sensing solutions to fit into our customers’ applications. Most customers in the Aerospace industry require a high level of customization because of the demand of the applications.

Unlike other pressure sensor suppliers, Merit Sensor can provide customers with completely customized pressure sensor designs with large or smaller/limited production runs. Our customers range from pressure sensor transducer manufacturers who are already experts in pressure sensing technology and rely on Merit Sensor for highly stable and sensitive MEMS sensing elements (bare die), to customers who have little to no experience in the pressure sensing world and look to Merit Sensor to assist with a completely custom design and implementation of a pressure sensor that best fits their application.

At Merit Sensor our engineers are application experts. We are ready to help customers design your application to work with a pressure sensor, and/or design a customized pressure sensing solution that works for your application. If you are unsure whether a pressure sensor is right for your application, Merit Sensor can help you make that determination.

More questions? Request a quote for a pressure sensor to meet all your automotive engineering needs.

TR Series for Medical Pressure Sensor Applications

When it comes to your patients, you know what’s best. If you could have everything your way, you would. With Merit Sensor’s piezoresistive pressure sensors, you can.

In 1991 when Merit Medical needed a reliable pressure sensor for one of its devices, their search for the right pressure sensor turned up empty. It was then that Merit Sensor was born and we have been constantly innovating to design and customize industry-specific solutions for all your pressure sensing needs ever since.

Our latest innovation, the TR Series, combines Merit Sensor’s proprietary Sentium MEMS piezoresistive technology with state-of-the-art pressure sensor ASIC signal management for best-in-class performance. The TR Series can be used in air pressure sensors, liquid pressure sensors, and gas pressure sensors. It is designed for harsh media compatibility over extreme and extended temperatures (-40°C to 150°C) with a total error band of less than 1%.

We know there are a lot of unpredictable variables you deal with in the medical industry. Pressure sensors shouldn’t be one of them. Whether used in diagnostic or analysis equipment, our MEMS pressure sensors are designed for unparalleled accuracy and reliability.

Further customization options include:

  • Full-service design capabilities
  • In-house wafer fabrication
  • Flexible shipping, packaging, and assembly
  • Piezoresistive technology (PRT)
  • Expansive pressure ranges (.15 psi to 15,000 psi)
  • Absolute, gage, differential, and vacuum pressure measurement

Explore our pressure sensor customization options further or request a quote on our existing series. Whatever your needs, we are your match. If we can’t build it for you, we’ll find someone who can.

Pressure Sensor For All Your Automotive Engineering Needs?

There’s a reason you are still looking for the right automotive pressure sensor. It’s never been on the market… until now.

At Merit Sensor, we are constantly innovating to find the best design. ‘Best’ might seem like a relative term, but not when it comes to our product. Our latest innovation, the TR-Series Pressure Sensor, is the ideal application for high-performance automotive applications.

When it comes to automotive engineering, there are plenty of pieces to the puzzle. Pressure sensors are one of them. Multiple, to be more accurate. Everything from airbag to oil pressure sensors.

We could talk features all day.

You want to know about PSI? It ranges from 30 to 300 PSI.

Flexibility? The TR-Series is designed with sensitivity, resistance, bridge constraint, and more. But forget features. When it comes down to it, you have to be able to trust a pressure series.

Like you, we never know where your automobiles will end up so we plan for the unexpected and the impossible. From freezing temperatures of -40° C to a boiling 150° C, the TR-Series pressure sensor is designed for optimal performance in almost any temperature for extended periods. The TR-Series is a harsh media sensor, so it’s compatible with any harsh environment the pressure sensor could be exposed do including air, liquid, and gas.

The TR-Series was designed for all your automotive pressure sensor needs.

TR-Series-oil-pressure-sensor

The TR Series pressure sensor is used in the following applications:

  • Transmission pressure sensor
  • Oil pressure sensor
  • Fuel rail pressure sensor
  • EGR/Exhaust pressure sensor
  • Fluid pressure sensor, fuel pressure sensors, and other liquid pressure sensor
  • Fuel tank pressure sensor
  • Fuel vapor pressure sensor
  • Fuel rail pressure sensor

More questions? Request a quote for a pressure sensor to meet all your automotive engineering needs.

How to seal an O-ring to a TR Series Pressure Sensor

Merit Sensor offers a fully calibrated, back side pressure, harsh media, pressure sensor for use with any media which are compatible with Silicon, glass, ceramic and solder. This sensor assembly (TR-Series) was designed to be used with an o-ring, creating a face seal to the back of the sensor.

 

There are many technical considerations that need to be evaluated when designing for an o-ring face seal. To ensure that a good design can be achieved during the first round of development, several factors must be clearly defined.   This information will be critical in subsequent material selections (for both the o-ring and the housing into which it will be inserted) and will be required in the subsequent dimensional and stress analysis.

 

Specifications

Temperature Specification

  • Identify the minimum and maximum end use temperatures for both the operation and the storage conditions.  Will the use temperature will be constant or fluctuating? Will the pressure be changing at the same time?

 

 

 

Pressure Specification

  • Identify the minimum and maximum use pressures. Will the pressures be all positive, all negative or a combination of both positive and negative? Will the pressures be fluctuating or constant? Will the temperature be changing at the same time?

 

 

Media Specification

  • Identify the media that will be in contact with the sensor. What chemistries do they contain? Are they compatible with Silicon, Borosilicate Glass, 96% Alumina Ceramic and Solder?   What will be the exposure conditions (temperature, pressure, duration, concentration, etc.) Be sure to think about both sides of the sensor. The backside will be exposed to the harsh media. The front side will be exposed to some other environmental conditions. Be sure that the “top side” is protected from the harsh media.

 

http://www.applerubber.com/src/pdf/chemical-compatibility.pdf

 

O-Ring Options

Material Options

  • The o-ring material should be selected based on the information specified above. The o-ring softness should be selected base on the maximum use pressure and the resulting packaging stresses. A soft o-ring will provide a very compliant seal which will result in very low induced packaging stresses but may not be able to seal well at high pressures. A hard o-ring conversely would seal well at high pressures but may also induce high packaging stresses. Different o-ring materials have different temperature handling capabilities. The glass transition temperature of the polymer will limit the lower functional operating temperature of the o-ring. The temperature at which the polymer begins to decompose or soften will limit the upper functional temperature of the o-ring. It is also important to look at the media compatibility of the different o-ring polymers. The longevity of the o-ring and the amount of swell that the o-ring will experience will be different depending on the o-ring material and the media. It may be difficult to find the exact right material to match all of the specification requirements.

 

http://www.applerubber.com/src/pdf/general-properties-of-orings.pdf

 

 

 

Geometry Options

  • After the material selection, the determination of the o-ring size (OD and cross-section) is the next thing to consider. The o-ring should accomplish several different goals. The o-ring must ensure that the media will not leak at minimum and maximum pressures. The o-ring must ensure that the media does not leak at minimum and maximum temperatures. The o-ring should be chosen to minimize package stress buildup during pressure and thermal cycles.
  • There are several different o-ring geometries that can be used for face sealing. Each of them has advantages and disadvantages. The most common and cost effective o-ring geometry is the standard circular cross-section. This geometry can be used for both positive and negative pressures. To assist with high pressure sealing, backer rings can be used to prevent issues with squeeze-out. In addition to the circular cross-section, there are “X” and “U” shaped o-ring cross-sections. The “U” shaped o-ring comes in two configurations that could work as a face seal (inward facing channel for positive pressure applications, outward facing channel for negative pressure applications). The “X” cross-section will work in either application.

 

 

O-Ring Gland Options

Counter Boar Gland

  • The counter bore gland is the most common o-ring gland. It is relatively simple to design and manufacture. The gland depth and width can be tailored to work with the specific application specifications. Items that need to be considered are the squeeze percentage, the swell and the coefficients of thermal expansion.

 

 

Dovetail Gland

  • The dovetail gland is the most complicated o-ring gland. It is difficult to design and is expensive to manufacture. The primary benefit of this gland design is that it will assist in holding the o-rings in place during assembly. It is not recommended for small o-rings. This design is even more sensitive to the squeeze percentage, the swell and the coefficients of thermal expansion.

 

Suggested Engineering Analysis and Verification

To ensure that the o-ring will seal properly over the full temperature and pressure use ranges, several different analyses should be carried out. It is important to look at static forces, dynamic forces and the effects of temperature on each.

 

Static and Dynamic Analysis

  • It is important to calculate the dimensional changes that will happen with temperature. The OD, ID and cross section diameters of the o-ring should be calculated at the Min and Max temperatures. The width and depth of the gland should be calculated for Min and Max temperatures.   The o-ring squeeze should be calculated at each of these extremes to ensure that the gland dimensions are adequate. Be sure to take into consideration the swell for the o-ring material base on the media in contact with the o-ring. Based on these dimensions, the zero pressure stresses on the package can be estimated.
  • The static model should then be used to evaluate the stresses during changes in both temperature and pressure. Based on the output of this analysis, a suitable combination of o-ring size, o-ring material and gland dimension can be selected to provide the optimal solution.

 

Because each application is a very unique combination of temperature, pressure and media, it is recommended that verification testing be carried out by the customer to ensure that the o-ring material, o-ring cross section and the gland dimensions will provide a robust solution in the final application.

TR Series Pressure Sensor Based Inches of Water Pressure Switch

In many situations there is a need to know the level of a liquid in a tank or the pressure inside of an air duct. Both of these cases are quite low pressure and can be difficult to measure. This can be accomplished in several ways. The simplest is a sight glass or sight tube, as shown below. This works on the premise that liquid in the tank will force liquid up the sight tube to the same level as what is in the tank, or the air pressure being measured will raise the liquid level equal to the pressure applied. A monometer is a commonly used device for measuring low air pressure/vacuum. This is particularly useful in a tank that is not transparent, or at least translucent. In the case of an air duct, there is nothing visible so an external device of some sort is required. Although this is a simple approach, it is not particularly convenient because it needs to be located at the tank or close to the duct being measured. This is not useful if remote monitoring is required, and even less useful if any sort of feedback is desired as it is completely manual.

Tank with Sight Glass

Tank with Sight Glass

If remote monitoring of the level is required, there are several more options. A common example is a float type resistive (potentiometer) sensor, as typically found in an automotive fuel tank level sending unit. These sensors work well, but have some drawbacks.

  • Located in the tank
  • Displace some volume in the tank
  • Moving parts
Tank with Float Type Level Sensor

Tank with Float Type Level Sensor

Tank with float type level sensor

Depending on the media being measured, and the design of the components, this type of sensor can fall victim to malfunctions caused be the media itself. A common issue is the float absorbing the media it is submerged in, which would result in an artificially low level reading because the float will lose some buoyancy.

Tank 3

In order to provide a reliable level sensor, one with no moving parts is very desirable. To accomplish this, a sensor such as Merit Sensor Systems’ TR series could be utilized. The TR series pressure sensor is a piezoresistive MEMS pressure sensing element paired with an ASIC on a ceramic substrate. The sensor is available in many pressure ranges, gage or absolute pressure measurement as well as custom calibration and output.

In order to realize the most accurate level reading possible a gage part should be used. This is preferred because the atmospheric pressure acting on the fluid in the tank will also act on the reference side of the MEMS pressure sensor providing the most accurate reading even while atmospheric pressure changes. In the case of a differential air pressure measurement, the reference can be atmospheric or another space. Some examples of this are:

  • Building duct static pressure measurement (atmospheric to pressurized duct (inches of water typically))
  • Building high duct static pressure (similar to regular duct static, but typically wired into the fan controller to turn off the fans if the pressure exceeds a safe level for the building and air supply system (inches of water typically))
  • Building air filter status (differential pressure across filter – larger differential as filters become blocked (inches of water typically))
  • Building space to space pressure (differential pressure between two spaces to ensure air flow is going in the right direction – common in clean rooms (tenths of an inch of water typically))

Graph 1

Measuring levels, for media such as water or air, is difficult as one inch of water measured at 39°F is a mere ≈ 0.0360911906567 PSI. Merit’s TR series is offered in a high sensitivity (low pressure) configuration that, when calibrated to 5 PSIG, could resolve to 1” of water. It is possible to achieve better resolution with a different calibration and/or custom MEMS device with higher sensitivity.

Below is a plot showing the difference between a 5PSIG calibration and a 1 PSIG calibration. There is a significant difference in the output of the sensor, giving much better resolution.

Once the sensor with an acceptable resolution has been selected there are options for the sensor interface. The TR series sensor provides a linear voltage output of 0.5 to 4.5V from minimum pressure to maximum pressure, and it is temperature compensated. This voltage can be monitored by a system controller or simply connected to a circuit, such as the one below, which would provide a variable level threshold via VR1. This could be used as a low level indicator/alarm or a overfill indicator alarm, depending on the configuration of the circuit.

Basic circuit for variable pressure level switch/indicator

Basic circuit for variable pressure level switch/indicator

Below is an example of the operation of the above circuit. This is an ideal, theoretical, example. With VR1 set to a calculated voltage for the desired level (10” W.C. in this case, or ~1.94V), the output of U1 will go high when the sensor voltage reaches the setpoint of VR1.

Graph 4

Whether a variable signal is desired, or a simple ON/OFF will suffice, a low pressure TR series sensor can be used. Several of the applications discussed here are currently handled by other sensor technologies, but could be handled very well by a properly designed, and implemented, MEMS based sensor.