Many Installations Are Wrong — And It Can Affect Accuracy or Even Damage the Instrument
In refineries, chemical plants, and storage terminals, remote seal differential pressure transmitters are widely used for level measurement in pressurized, vacuum, high-temperature, or corrosive applications.
However, one installation detail is often overlooked:
Where should the transmitter body be mounted relative to the upper and lower diaphragm seals?
Many technicians assume:
“As long as zero suppression or elevation is configured correctly, installation height doesn’t matter.”
This is only partially true.
Although static pressure caused by capillary fill fluid can be compensated by calibration, transmitter elevation still affects:
- Zero shift
- Temperature stability
- Vacuum resistance
- Diaphragm stress
- Long-term reliability
In extreme cases, incorrect installation can even lead to:
- Silicone oil vaporization
- Diaphragm deformation
- Permanent sensor damage
This article compares the three common installation methods and explains which one should be selected for each application.
Basic Principle
A remote seal DP transmitter uses fill fluid inside the capillaries to transfer process pressure from the diaphragm seals to the sensing element.
Any vertical elevation difference between:
- the transmitter body,
- the high-pressure seal,
- and the low-pressure seal,
creates additional hydrostatic pressure from the fill fluid column.
This additional pressure is normally compensated by:
- Zero suppression
- Zero elevation
However, compensation does not eliminate the effects of:
- temperature expansion,
- vacuum conditions,
- or long-term mechanical stress.
Case 1:
Transmitter Mounted Above Both Seals
This means the transmitter body is installed higher than both the upper and lower diaphragm seals.
Advantages
- Easier access for maintenance.
- Instrument stays away from corrosive splashes and accumulated liquids.
- Less chance of mechanical damage during vessel cleaning or maintenance.
Disadvantages
This is the highest-risk arrangement.
Both capillaries create negative pressure on the sensing element.
Under vacuum service:
- fill fluid may vaporize,
- diaphragms may deform outward,
- measurement can become unstable,
- permanent damage may occur.
Additional disadvantages include:
- Large negative zero suppression
- Increased temperature drift
- Larger low-level measurement errors
- Faster diaphragm fatigue
Recommended For
✔ Atmospheric vessels only.
❌ Not recommended for vacuum service.
Case 2:
Transmitter Mounted Between Upper and Lower Seals
Industry Recommended Arrangement
The transmitter body is installed approximately midway between the upper and lower remote seals.
This is considered the preferred installation method by most engineering companies and instrument manufacturers.
Advantages
- Moderate zero suppression.
- Excellent temperature compensation.
- Lowest measurement drift.
- Balanced diaphragm loading.
- Suitable for both atmospheric and slight vacuum conditions.
Because the capillary lengths above and below the transmitter are relatively balanced, thermal expansion effects largely cancel each other.
Typical long-term performance is superior to the other two configurations.
Disadvantages
- The transmitter may be exposed to process splashing or contamination.
- Installation space may be limited around the vessel skirt or piping area.
Recommended For
✅ Most liquid level applications.
⭐ Recommended by engineering contractors and manufacturers.
Case 3:
Transmitter Mounted Below Both Seals
The transmitter body is installed below both remote seals.
Advantages
This arrangement is ideal for vacuum applications.
Both capillaries apply positive static pressure to the sensing element, preventing vaporization of the fill fluid.
Benefits include:
- Excellent vacuum resistance.
- Positive zero elevation instead of negative suppression.
- Reduced risk of diaphragm damage.
- Improved reliability under vacuum conditions.
Disadvantages
- Larger positive zero elevation is required.
- Additional range margin must be considered during sizing.
- Temperature-induced zero drift is slightly higher than Case 2.
- Mechanical protection may be required in busy operating areas.
Recommended For
✅ Vacuum vessels.
✅ Evaporators.
✅ Distillation columns.
✅ Storage tanks operating under negative pressure.
Comparison Table
| Installation Position | Zero Shift | Vacuum Capability | Temperature Stability | Long-Term Reliability | Recommendation |
|---|---|---|---|---|---|
| Above both seals | Large negative suppression | Poor | Lowest | Lowest | ❌ |
| Between seals | Moderate suppression | Good | Best | Best | ⭐⭐⭐⭐⭐ |
| Below both seals | Large positive elevation | Excellent | Good | Very Good | ⭐⭐⭐⭐ |
What Do Industry Standards Say?
Several standards and manufacturer guidelines support these recommendations, including:
- GB 50093-2013
- SH/T 3104-2000
- Rosemount installation guidelines
- Yokogawa recommendations
- Fisher remote seal installation practices
In general:
For standard applications, install the transmitter between the upper and lower seals.
For vacuum service, install the transmitter below the lower seal whenever possible.
Final Recommendation
If you only remember one rule, remember this:
For most applications, mount the transmitter body between the two diaphragm seals.
And for vacuum service:
Install the transmitter below the lower seal to keep the fill fluid under positive pressure.
A remote seal transmitter can compensate for hydrostatic pressure differences.
It cannot compensate for poor installation practices.