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Load Cells & Force Sensors:
What is a sputtered thin film load cell?
The Wheatstone bridge circuit (now used to measure strain on the surface of a support structure) was improved and popularized by Sir Charles Wheatstone in 1843 is well known but the application of thin-film vacuum deposition to this old tried-and-true circuit is not so well understood. Thin film sputtering deposition process is nothing new to the industry. Many applications, from the manufacture of complex microprocessors to the manufacture of precision resistors for strain gauges, use this technique. In the case of strain gauges, a thin film strain gauge sputtered directly onto the stressed substrate is an option that eliminates many of the problems faced by “bonded strain gauges” which are also known as foil strain gauges, restive strain gauges, and silicon strain gauges.
How are thin film strain gauges better than “bonded strain gauges”?
For bonded strain gauges, the Wheatstone circuit is glued to a load cell structure. The main problem with bonded strain gauges lies with the required adhesive that can contract or grow with changes to humidity and temperature. This physical change induces a “false” load or error on the gage output. Variances in performance can also be observed since part-to-part manufacturing is not consistent. Finally, vibration and other external loads can de-laminate the bonded gage electronics. Thin film strain gauges by comparison use no adhesive between the Wheatstone bridge electronic circuit and the base metal structure which eliminates these sources of adhesive-based errors. The thin film strain gage manufacturing process is highly repeatable and consistent and suitable for high volume manufacturing. Thin-film load cells typically have higher bridge resistance, meaning they draw less power and are well suited for battery-powered applications. Finally, this manufacturing process has the capability of creating the Wheatstone bridge circuit in extremely small space requirements which allows our engineers to create custom miniature force and load sensors for a large range of applications and markets. The result is thin film strain gages offer a long life, are very repeatable, and are highly accurate.
What is meant by overload protection of a load cell?
Every load cell is designed to deflect under load in a controlled manner. Engineers optimize this deflection to maximize the sensitivity of the sensor while ensuring the structure operates within its “Elastic” region. Metal structures that deflect with their elastic region will return to their initial state once the load is removed. Structures that exceed this elastic region are called “Overloaded”. An overloaded sensor experiences “Plastic deformation” where the structure is permanently deformed and will no longer return to its initial state. Once plastically deformed, the sensor will no longer offer linear output proportional to the applied load. In most cases, it is permanently and irreversibly damaged. “Overload Protection” is a design feature that mechanically limits the total sensor deflection below its critical load limits so that the sensor is protected from unexpected high static or dynamic loads that our otherwise cause plastic deformation.
How do you determine the accuracy of a load cell?
A sensor’s accuracy is measured using different operating parameters. For example, if you load a sensor up to its max load and then take the loading off, the ability of the sensor to return to the same zero load output in both cases is a measure of “hysteresis”. Other parameters include “non-linearity”, “repeat-ability”, and “creep”. Each of these parameters is unique and have their own percentage error. We list all of these parameters in our datasheets. Please see our Glossary for a more detailed technical explanation of these accuracy terms.
Do you have other output options other than mV for your load cells and pressure sensors?
Yes, SMD offers small off the shelf Signal Conditioning boards that are powered up to 24 VDC and can offer three types of output options: 4 to 20 mA, 0.5 to 4.5 VDC, or I2C digital. When offering these SC boards, we always provide the sensor soldered to the board and fully calibrated to max load. Custom solutions can be developed for any other output protocol.
What types of pressure sensors do you offer?
SMD offers three styles of sensors: metal diaphragm sensors, noninvasive pressure sensors, and polymer tubing occlusion sensors. The metal diaphragm sensors are “wetted” on one side and need to be welded to a pressure fitting. Non-invasive pressure sensors interface with a plastic disposable pressure fitting that uses a thin silicone membrane to transfer pressure from the fluid to our sensor. The “tube” occlusion sensors are non-invasive and do not require a custom disposable piece, however, it can be difficult to get an accurate pressure reading due to stress relaxation and creep in the viscoelastic tube.
What is the pressure range of each type of pressure sensor?
Our metal diaphragm sensors have a range of 50 psi to 35,000 psi and our polymer tubing sensor reach 100 psi. Custom designs are available for each type for specific custom pressure ratings.
I have many fitting connection requirements. Can your sensors be welded onto multiple configurations?
For the metal diaphragm sensors, we offer just the pressure “cap” sensor as an option. This cap can be welded to any type of fitting or special connector as long as the fitting/connector is machined to prep the weld. Laser welding is recommended to limit temperature levels at the sensor. We can perform this welding or provide you welding guidelines to weld the sensors yourself.
Can the sensors handle harsh environments?
Yes, the unique features of sputtered thin film pressure sensors eliminate adhesives used in “bonded strain gauge” pressure sensors. Adhesives limit temperature extremes and can de-laminate under severe vibration or cyclic loads. Our thin film pressure sensors do not use adhesives to bond the strain gages circuit to the diaphragm. Our sputtering thin film process creates an atomic bond directly to the surface of the metal diaphragm which provides a much more robust and longer life sensor. This is why our pressure sensors are preferred in harsh applications like deep-sea drilling rigs and oil and gas exploration applications.
Can you measure pressure in small diameter tubing non-invasively?
Yes, we have a disposable two-part pressure sensor for tubing. Offering 1.5% accuracy up to 30 psi, this sensor is made of two parts: one is a disposable flow module with a silicone diaphragm which isolates the fluid from the second part which is the re-usable sensor base. The disposable module is low cost and snaps to the reusable base. The reusable base is designed for panel mounting.
What is the smallest detectable bubble and how is this verified?
Our ultrasonic bubble sensors can measure down to 1 microliter bubble using our proprietary signal conditioning and sound crystal design. We have designed our own bubble generator that can repeatable and accurately produce bubbles for product verification.
What output options do I have for bubble sensors?
We offer a TTL 5 VDC signal for liquid and 0 VDC for air presence or a PNP/sourcing option for applications that require higher current. Input power options are 5 to 15 or 5 to 24 VDC
Why should I use ultrasonic bubble detectors over optical sensors?
Typically, ultrasonic sensors are superior to optical (infrared) sensors in terms of reliability; most critical applications that we are aware of (e.g. medical devices) use ultrasonic bubble sensors. Optical sensors tend to be sensitive to:
- Ambient light variation
- Sensor degradation
- Contamination in front of the sensor
- System power variation
- Temperature changes
These factors result in a sensor that needs to be calibrated regularly, perhaps even as much as every time the device is powered on. This would mean that you need to have a way of guaranteeing there is no tube in the sensor upon device startup and this may not be possible in every application.
With optical sensors, you need an optically transparent tube that won’t change over time, e.g. due to fouling inside of the tube or plastic degradation. Markings on the outside of the tube can also interfere with the proper performance of optical sensors.
Additionally, optical sensor performance can be extremely dependent to the type of liquid in the tubing to the point that some liquids cause an increase in sensor output compared to a dry tube while other liquids cause a decrease in sensor output compared to air, making bubble detection when using liquids with differing optical properties impossible.
Finally, most optical bubble sensors do not come with integrated control electronics and calibration so the additional cost of associated development, components, and labor need to be considered to make a proper apples-to-apples cost comparison between technologies.
How does an ultrasonic bubble sensor work?
Operating at a high frequency, the sound is transmitted between two crystals and through the tubing and liquid media. The signal strength is monitored and if a bubble of known size or greater flows between the crystal, the sensor signal strength is attenuated and we notify the customer via the output signal. Note a LED light is available also (RED air/gas bubble and GREEN liquid). Technical Note: Bubble Sensor Technology Overview
Why is the standard bubble sensor output “high” for liquid and “low” for air?
When might I want to have the output inverted?
In most situations, you need to consider the “fail-safe” condition of the sensor. If the sensor fails and the output goes to 0V (for example if the power to the sensor gets cut), which state (air or liquid) should it fail to such that it will cause the least amount of harm?
Most applications are fail-safe with a 0V output with air in the sensor. For example, if you have a bubble sensor in a medical device that is used to prevent air from being injected into a patent’s circulatory system, you will want the sensor to fail to the “air” state so that in the event of a failure, the sensor will shut down the pump and not risk delivering air to a patient’s circulatory system.
However, some applications require an inverted output (5V air/0V liquid). For example, if you’re using a bubble sensor to monitor the liquid level at the top of a tank to prevent overfill, you likely want the sensor to fail to “liquid” instead of “air” – you don’t want a false air reading that results in overfilling the tank, which might result in a dangerous situation or loss of valuable product.
Ultimately it is up to the user of the sensor to perform their own analysis to determine which output configuration is correct for a given application. Contact us if you require an inverted output.
What is the reliability of your bubble sensors?
We calculate Mean Time to Failure (MTTF) for our bubble sensors using MIL-HDBK-217F Notice 2. The results of this calculation vary by the application because they are dependent on the maximum operating temperature, thermal cycling profile, and environment in which the sensor is used. Contact us with your application specifics.
What can we do to reduce the power consumption of the bubble sensors?
Our standard bubble sensors without an LED draw around 2 to 3 mA of current without a load on the output. Contact us if your application requires and even lower power consumption.
How does the flow rate affect bubble sensor performance?
Our bubble sensors check for a bubble in intervals of the sensor “response time” (typically 50 microseconds). Use this calculator for a more detailed explanation and to see the maximum distance a bubble can move during this time.
What is the “test function” on your bubble sensors?
Test function can optionally be enabled on all of our standard bubble sensors:
“E-Type” sensors (e.g. A430-SLTE) have the test function enabled. This is typically used for medical or other critical applications. The test wire (typically white) must be grounded for normal operation and pulling the test pin high disables the ultrasonic transmitter and, if the sensor is functioning properly, forces an “AIR” output.
“D-Type” sensors (e.g. A430-SLTD) have the test function disabled. This is adequate for most non-medical applications and the test wire (typically White) can be ignored.
Do your bubble sensors need a lid or clamp to hold the tubing in the sensor?
It is generally recommended that the tubing in a bubble sensor be held in place with a clamping mechanism to prevent the tubing from inadvertently being pulled out of the sensor and to reduce the chance for partial tube insertion. Ultimately it is up to the user of the sensor to do a risk analysis and determine if a lid or clamp is needed.
Lids are not featured on our standard sensors but we often work closely with our customers to design lids that are either integral to the sensor or part of our customer’s system.
What types of tubing can I use for bubble detection?
The most common tubes we work with are soft plastics or rubbers with durometer hardness between A20 and A90 such as PVC or silicone. These tubes can be pressed into the sensor and will typically function properly without the need for acoustic coupling materials.
Slightly more rigid tubes on the Shore D hardness scale such as PTFE, FEP, or PE can be used in our standard parts as well, as long as they are sized appropriately and can be inserted into the sensor slot without damaging the sensor. These tubes may be permanently deformed by inserting them into the sensor and may need to be replaced after use because inserting the deformed tube into the sensor may result in sub-optimal performance. Custom elastomeric interfaces can be integrated into the sensor to minimize this effect if necessary.
Extremely rigid tubes such as steel, aluminum, or PEEK require custom sensors and require the use of an acoustic coupling material such as grease or an elastomer
What is an Occlusion?
In the medical device industry, occlusions are full or partial blockages that restrict flow in a tube. In a typical system where the flow is achieved with a positive displacement pump such as a peristaltic pump, there are two main types of occlusions:
- Downstream occlusions are tube blockages that occur downstream of the pump. These occlusions result in a lack of liquid flow and, when using a positive displacement pump, they can cause dangerous pressure buildups that can result in burst tubing or, if the tube is only momentarily occluded, it can send high-pressure liquid into a patient.
- Upstream occlusions are tube blockages that occur upstream of the pump. These occlusions are characterized by a decrease in pressure (vacuum) in the upstream tubing and restrict flow through the pump. Note that an IV bag running dry has the same effect as an upstream occlusion and can also be detected by an occlusion sensor
How do you detect an occlusion/blockage in a tube?
Our thin film load cell technology allows us to miniaturize load cells. We use these small load cells inside the base of our sensor, under a U groove feature. When the tubing is placed in the U groove of the sensor (and on top of this load cell) and clamped in place and the tubing is pressurized with fluid, a pre-load value is measured by our sensor. If the tubing is blocked the pressure in the tubing builds and expands the tubing walls. This in turn creates a rapid increase in load on our load cell, thereby identifying the occlusion (blockage).
Do I need to provide a clamp to hold the tube in place?
SMD frequently works with customers to incorporate their clamping device or door with our sensor, or the sensor can be designed to work with your door clamp. The tube should be clamped in place.
Can an occlusion sensor be integrated with a bubble sensor?
Yes, in fact, we make the only combined occlusion and bubble sensor in the market today. This small integrated device reduces the footprint of the customer system.
What other sensors can be integrated into one occlusion sensor?
We have an integrated proximity sensor to sense door closure status as well as thermo-couple and inferred temperature sensors.
Why don’t you sell ultrasonic occlusion sensors?
While there are some ultrasonic occlusion sensors on the market that use technology similar to that used in our ultrasonic bubble sensors, our research has led us to determine that they just don’t work reliably enough. Sensor output is significantly affected by external factors such as micro-bubbles, tube properties, and non-repeatability of tube loading.
While ultrasonic occlusion sensors may seem like a less expensive option, they may end up being more expensive in the long term when considering the costs of deploying an unreliable sensor. We’ve worked with several companies to replace their existing ultrasonic occlusion sensors with our thin film load cell technology due to due to excessive field failures of the ultrasonic occlusion sensors.
Can you customize an occlusion sensor into my design?
Actually, every occlusion sensor is considered a “custom” design. This is because of the occlusion pressure, tubing size and material type, mounting configuration, media type, and temperatures all vary from customer to customer. Also, the signal output requirements vary as well.
Can you measure occlusion if the line is in a vacuum?
Yes, we can pre-load the load cell such that we can detect a reduction or an increase in pressure due to a vacuum or a sudden pressure rise.
I need a weight scale with a very low (height) profile. What are my options?
Our thin-film technology strain gauges allow us to create very low profile load cells that are especially well suited for weight scale applications.
What type of options can you offer for a custom precision weight scale for my medical device?
We can offer just the load cell or complete custom design services to integrate our load cells into your weight scale application. Our weight scales are used in many medical diagnostic applications as well as those requiring precision flow rate measurements. We have hanging scale designs specifically for hanging IV bag scales.
How do you prevent overload damage?
By limiting the deflection of our load cells we provide the customer an overload protected device with years of reliable service.
How does the ultrasonic flow sensor calculate flow?
We use two PZT crystals and proprietary algorithms to send ultrasonic waves through the media in the direction of the flow and against the flow alternatively. It takes the ultrasonic waves longer time to travel against the flow and the transit time difference in both directions is proportional to the actual flow velocity.
What other data does your FlowDAQ system provide other than the flow rate?
In addition to the flow rate, we offer totalized flow and a reset using our touch screen display. We also offer “sound speed” of the media, and this could be used to help identify the type of liquid in the tube (providing the sound signatures are different). The last data we offer is “signal strength”. This indicates the received signal amplitudes. It is generally in the 50%-95% range unless the sensor door is not shut. This can also help identify whether the tubing is empty or full of fluid.
Which variables affect flow meter calibration and how is this addressed?
Fluid type, temperature, tubing material, tubing thickness, etc.
What type of sensor do you have? Do they both work with the FlowDAQ?
We have clamp-on sensors and in-line sensors. The clamp-on sensors clamp over the tubing and the in-line sensors require the user to attach tubing to each end of the sensor fittings. All sensors require the FlowDAQ display and control module
What are the options for output data?
The FlowDAQ offers +/- 5V analog output as well as UART digital output
Liquid Level Sensors:
I noticed your company has two websites, why?
Yes, we currently maintain www.smdsensors.com and www.fluidswitch.com and each site includes links to each other. Whereas many products are listed on www.smdsensors.com, the www.fluidswitch.com website allows us to focus just on our liquid level switches for a range of industrial markets. This offers the user access to a range of liquid level product solutions in one convenient location. The fluid swich website has generated a large and loyal following.
Can you customize a multi-point level sensor for my chemical tank?
Yes, we have many options for multi-level sensors. We have stainless steel and polymer-based floats with internal reed magnetic reed switches that can be customized easily and quickly. We manufacture in-house and can offer quick turnaround and full technical support. Other non-mechanical multi-level sensor options include our ultrasonic liquid level sensors, our optical sensors, and our capacitance sensors. We offer full support to select the right sensor for your application.
Help me understand the normal positions of fluid switches (Normally Open and Normally Closed)?
For reed-switch type float sensors, the reed switch is made up of two metal components that re normally held apart via spring. When an external magnet (located in the float mechanism) is held close and in-line with the two metal components, they are drawn into contact thus completing or closing the circuit. When the external magnet is moved away (with the rising or falling of liquid level), the circuit is then opened. We can design the system such that the circuit is initially closed and only opened when liquid is present at that point (which is called normally closed). Or the alternative condition which is called normally open.
How does the specific gravity affect my selection of a liquid level switch?
Specific Gravity (SG) is the ratio of an object’s density to that of water. Since water has a specific gravity of 1 at sea level, liquids and substances with a specific gravity lower than 1 will float in water. Therefore it is important to select the proper float switch and float for your application. When using oils, Buna or NBR floats are best. These floats have low specific gravity, around .5, and will float well in most petroleum products ranging from .7 to .86 SG. For example, a float switch with a specific gravity of .8 will float well in water but will sink in alcohol which is around .72 SG.
Another more complete reference can be found at https://www.coleparmer.com/Chemical-Resistance
What are your guidelines for product material selection based on chemical resistance?
Stainless Steel – Ideal for high temperatures, high pressures and corrosive environments such as food equipment, industrial tanks or durable general use.
Brass, Nylon or PBT stems with Buna-N Float – Good for petroleum products, oils and wastewaters. Buna-N floats have superior buoyancy and can be configured for oil-water interface detection.
General Use Plastics – Polypropylene, PVC and Polycarbonate are good to 105 degrees Good choice for general use, acids or food applications. It can be designed economically or custom-molded with additional features for OEM applications.
What do I need to know about power levels to protect my liquid level sensor?
One of the greatest failures of reed switches is caused by over-current conditions. The currents created by many devices at “turn-on” or “shut-down” can be 5 to 10 times the “steady state” current given as part of the power rating.