APPLICATION NOTE AN105
CREATING A SEAL TO A TR SERIES PRESSURE SENSOR
USING AN O-RING
To isolate the media to the wetted materials, i.e. glass, solder, ceramic, and silicon, you need to create a good seal. This will be best
accomplished with an O-ring. The type of O-ring seal depends on the type of TR Series pressure sensor, specifically whether or not it
has a ferrule/pressure port. This application note will discuss the face seal and radial seal. The images below with the O-rings in green
show examples of both types of O-ring seal.

Face Seal

Radial Seal

There are many technical aspects, such as package stress, material compatibility, and O-ring geometry, that need to be evaluatedwhen a seal is created with an O-ring. If you want to ensure that a good design is achieved during the first round of development, youmust consider these aspects. This application note will help you know what should be considered.

Application ConsiderationsFirst and foremost, you should know the pressure, temperature, and media to which the pressure sensor and O-ring will be exposed.
Pressure
In regards to pressure, consider the following questions:
What is the pressure range?
Will pressure be positive at all times, negative at all times, or fluctuating between positive and negative pressure?
Will pressure be constant or intermittent?
Temperature
In regards to temperature, consider the following questions:
What is the storage-temperature range?
What is the operating-temperature range?
Is the operating temperature constant or fluctuating?

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The chart below will help you select an O-ring material based on the operating temperature. Keep in mind that the chart is for generalguidance. Testing should always be performed to ensure that the O-ring will function well in the end application.

MediaIn regards to media, i.e. the environment to which the sensor and O-ring are exposed, consider the following questions:
What are the chemistries of the media?
Will the pressure sensor and O-ring be exposed to the media continuously or intermittently?
How long will the exposure periods last?
How exposed will the pressure sensor and O-ring be to the media?
Refer to https://www.applerubber.com/chemical-compatibility-guide/ for more information regarding media compatibility of O-rings.

O-Ring Considerations
Material
The pressure, temperature, and media of the application will all have an impact on the O-ring. Therefore, you should consider different
O-ring materials. Some common materials are nitrile, silicone, and polyurethane. The following website will help you make yourselection: https://www.applerubber.com/material-selection-guide/.
The media to which the O-ring will be exposed will help you determine the O-ring’s material. An O-ring’s swell and longevity, forexample, will differ depending on the material’s compatibility with the media.
Knowing the maximum pressure will also help you select the O-ring material. One thing to be considered is the softness of thematerial. A soft O-ring provides a very compliant seal, resulting in very low packaging stress. If the ceramic substrate of the TR series
is bent or torqued, as a result of such stress, a shift in offset can be caused. Essentially, this creates a pressure error. Therefore, aradial seal can be the more effective of the two sealing methods, as the stress around the pressure port is less likely to be transferredto the ceramic substrate. But a soft O-ring may not seal well at high pressures. A hard O-ring, on the other hand, seals well at highpressures but may induce high packaging stress.
Another thing to consider is how different O-ring materials handle temperature differently. The glass transition temperature (Tg)
depends on the chemical structure of the O-ring’s polymer. When an O-ring is used in temperatures that are below the Tg, it will beunlikely to conform and seal properly. On the other extreme, it is likely that an O-ring used in temperatures that exceed the Tg will
display excess extrusion and will ultimately fail to seal.

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1600 W. Merit Parkway · South Jordan, Utah · 84095 · USAGeometry
There are several different O-ring geometries (OD and cross section) that can beused to create a seal to the TR Series pressure sensor. Each geometry has
advantages and disadvantages. The most common and cost-effective geometryis the standard circular cross section. This can be used for positive andnegative pressures.
Other geometries to consider are X-shaped and U-shaped
cross sections. The U-shaped O-ring comes in two configurations
that could work to make a seal: One with an inward-facing channel
for positive-pressure applications, and the other with anoutward-facing channel for negative-pressure applications.			

Area for O-ringor Spring
One primary requirement of any O-ring is that the seal remain impervious at minimum and maximum pressures.
The O-ring should mitigate the buildup of package stress during pressure cycles and thermal cycles. To ensure a reliable
seal at high pressure, you can use backing rings. They will help prevent issues related to extrusion.
Gland/Groove
The rectangular gland is the most common type of O-ring gland/groove. It is relatively simple to design and manufacture.
The gland’s depth and width can be customized per the application specifications. The following aspects of an O-ring gland needto be considered:• squeeze• swell• coefficients of thermal expansion

Face Seal – Standard GlandRadial Seal – Standard GlandTR Series Ceramic
Customer’s HousingTR Series Ferrule

Customer’s Housing

The dovetail gland is the most complicated O-ring gland. It is difficult to design and expensive to manufacture. It is notrecommended for small O-rings, and it is unnecessary for a radial seal. The primary benefit of the dovetail gland is that it helps tohold O-rings in place during assembly.
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1600 W. Merit Parkway · South Jordan, Utah · 84095 · USAFace Seal – Dovetail GlandTR Series Ceramic

Customer’s Housing

Engineering Analysis
An optimal O-ring seal should function over the full temperature and pressure ranges of the end application. The efficacy of theseal should also be unaffected by the media to which the O-ring is exposed.
Temperature changes will likely cause dimensional changes. The following dimensions should be calculated at minimum andmaximum temperatures: OD, ID, and cross-section width of the O-ring; width and depth of the O-ring gland; squeeze and swellof the O-ring.
The dimensional changes can result in stress on the O-ring, gland, and ultimately the MEMS sensing element. Differentpressures can also impact the amount of stress.
Because every application involves a unique combination of temperature, pressure, and media, it is critical that verification
testing be carried out by the customer. This will help ensure that the O-ring in combination with TR Series pressure sensor
provides a robust solution in the end application.

We would like to thank Apple Rubber Products, Inc. for the use of the chart and for the valuable information referred toon their website.

Sales +1 801.208.4722 · Customer Service +1 801.208.4700 · Fax +1 801.208.4798 · sales@meritsensor.com · www.MeritSensor.com
1600 W. Merit Parkway · South Jordan, Utah · 84095 · USA