High Temperature Pressure Transmitters and Sensors: Steam, Hot Oil and Protection

Quick answer: most pressure transmitters and sensors have electronics that can only tolerate a limited temperature, often well below the temperature of hot steam or oil. To measure pressure on hot media you keep the hot fluid away from the electronics using a siphon (pigtail) for steam, a cooling element or a long neck, or a remote diaphragm seal with a capillary. Saturated steam at 7 bar gauge is about 170 degrees Celsius, far above the 60 degrees stated for our gauges, which is why steam lines always need protection. This guide explains the problem, the four ways to solve it, how to choose, and how to avoid the errors that heat causes, with a worked example.

Why temperature matters

A pressure sensor has two temperatures to worry about: the temperature of the process fluid and the temperature of the electronics. They are not the same thing, and the data sheet gives limits for both. Heat causes several problems:

• Damage: excessive temperature can destroy the electronics, seals and cable insulation.
• Drift: even within limits, temperature shifts the zero and span, so accuracy falls. Datasheets give a temperature coefficient.
• Seal ageing: hot oil and steam speed up the ageing of elastomer seals.
• Safety: hot surfaces and steam are burn hazards for technicians.

How hot is steam?

Saturated steam temperature rises with pressure. Some typical values (gauge pressure) are:

Compare that with the working temperature limit of up to 60 degrees Celsius stated on our gauge datasheets, with a pigtail recommended for higher temperatures. Superheated steam and some hot oils are hotter still. Always check the exact limit for the sensor you plan to use.

Four ways to protect the sensor

1. Siphon (pigtail) for steam

A coiled tube between the steam line and the sensor holds condensed water. The water forms a barrier, so the pressure reaches the sensor but the steam does not. Fill it with water before start-up where possible. Our guide to steam and boiler pressure gauges explains how it works. The same idea protects transmitters on steam.

2. Cooling element or long neck

A finned or extended section between the process connection and the sensor lets heat dissipate into the air before it reaches the electronics. It suits moderately hot fluids where a siphon is not wanted, such as hot oil.

3. Remote diaphragm seal with capillary

A flexible diaphragm at the process connection is joined by a fluid-filled capillary tube to the sensor, which can be mounted well away from the heat. It also suits corrosive, sticky or hygienic media. The capillary length and the fill fluid affect response time and accuracy, so these are specified with care.

4. Impulse line

A length of tubing from the process to the sensor allows the fluid to cool along the way. It is simple but needs care with slopes, venting and blocking.

Worked example: a steam line at 7 bar

• Saturated steam at 7 bar gauge is about 170 degrees Celsius.
• The transmitter’s electronics are rated well below that, so the sensor cannot be connected directly.
• Fit a pigtail siphon and an isolating valve between the steam line and the transmitter.
• Choose a range of 0 to 16 bar, which puts 7 bar at 44% of the span and leaves room for surges. Our range selection guide explains the sizing.
• Use a 316 stainless steel wetted part, and a 4 to 20 mA output for the control room. See 4 to 20 mA wiring and calibration.
• Commission by filling the siphon, opening the valve slowly and checking the reading against a test gauge.

The WIKA Model A-10 has 316L stainless steel wetted parts, but its temperature limits for the process and electronics must be taken from the datasheet for the version you order, so check them before using it near hot media. Tell us your media and temperature through the contact page and we will confirm suitability.

Temperature errors and how to reduce them

• Thermal zero shift: the output at zero pressure changes with temperature. Zero the sensor at its normal operating temperature.
• Thermal span error: the sensitivity changes with temperature. Check the temperature coefficient in the datasheet, usually quoted per 10 degrees.
• Radiant heat: hot pipes heat nearby electronics. Use shields and distance.
• Ambient swings: outdoor sensors see sun and cold. A shade or enclosure helps.

For how to read the accuracy figures on a datasheet, see pressure gauge accuracy compared.

Cold weather: the other extreme

Siphons and impulse lines full of water can freeze, damaging the sensor. In cold places, insulate and trace-heat the lines, or use a diaphragm seal with a suitable fill fluid. In most of Australia this matters mainly in alpine regions and for winter nights in inland areas.

Other considerations for hot service

• Seals: choose materials suited to the temperature, such as FKM, PTFE or metal seals.
• Cable: use cable rated for the temperature near the sensor.
• Materials: stainless steel resists corrosion at temperature better than brass.
• Hazardous areas: hot, flammable media may need approved equipment. See hazardous area equipment.
• Safety: isolate and cool the line before removing a sensor, and follow lockout procedures.

Common mistakes

• Connecting a standard transmitter directly to a steam line.
• Forgetting to fill the siphon, so hot steam hits the sensor at start-up.
• Blocking the siphon with scale and then trusting a dead reading.
• Zeroing the sensor cold, then using it hot.
• Using a long capillary without allowing for its temperature effect.

Worked example: hot oil at 250 degrees Celsius

A thermal oil heating system runs at about 250 degrees Celsius and low pressure. A standard transmitter cannot sit directly on the line. A remote diaphragm seal with a capillary lets the transmitter sit well away from the heat. The seal and capillary are filled with a fluid chosen for the temperature range. Points to check in the datasheet are the allowable process temperature at the seal, the fill fluid, the capillary length and the effect of temperature on the reading. A longer capillary gives more freedom to mount the transmitter away from the heat, but it slows the response and can add temperature error.

Why capillary systems have errors of their own

The fill liquid inside a capillary expands and contracts with temperature. If the temperature at the diaphragm differs from that along the capillary or at the transmitter, the volume changes and the reading shifts. Makers publish the expected error for a given capillary length and temperature, and it is worth checking before you buy. Mounting the transmitter at the same height as the seal, or at a known offset, keeps the head pressure of the fill liquid constant.

Insulation, trace heating and shields

• Insulate impulse lines on very hot services to protect people from burns, but leave the electronics free to dissipate heat.
• Use heat shields between hot pipes and the transmitter.
• Choose cable and glands rated for the temperature near the sensor.
• In freezing conditions, trace-heat water-filled lines.

Maintenance checks for hot service

• Check the siphon or impulse line for blockage whenever the reading looks dead.
• Look for leaks at hot joints, which can also be a burn hazard.
• Compare the reading with a test gauge on a regular schedule. See pressure gauge calibration.
• Replace seals on schedule, since heat shortens their life.

A selection checklist

1. Process temperature and maximum temperature during upsets.
2. Pressure range with headroom.
3. Fluid: steam, hot oil, hot water or chemicals.
4. Protection method: siphon, cooling element, capillary seal or impulse line.
5. Wetted materials and seals for the temperature.
6. Output signal and cable rating.
7. Calibration and inspection plan.

A final reminder on datasheets

Always separate three temperatures when you read a datasheet: the process temperature at the connection, the ambient temperature around the electronics, and the compensated temperature range in which the stated accuracy applies. A sensor can survive a temperature and still be inaccurate at it, so check all three against your application.

Frequently asked questions

Can a pressure transmitter measure steam?

Yes, with a siphon or other protection so the electronics see only a safe temperature.

What is a pigtail?

A coiled tube that holds condensate and shields the sensor from steam heat.

What is a diaphragm seal?

A flexible metal membrane that isolates the sensor from the process fluid and passes the pressure on through a fill liquid.

How hot can the A-10 go?

Check the datasheet for the version you order. Do not assume.

How do I check a high temperature sensor?

Compare with a test gauge at a safe, cooled condition, and follow your site’s calibration plan. See pressure gauge calibration.

Next steps

Read the sensor, transducer and transmitter guide, then browse the pressure sensor category.