Mud Pulse Telemetry: MWD Data Transmission with Pressure Sensors

Mud Pulse Telemetry: MWD Data Transmission with Pressure Sensors

Hydraulic data transmission requires sensitive pressure sensors capable of enduring high pressures. This is particularly true when used in measurement while drilling (MWD) applications.

MWD has become a standard application, especially for offshore directional drilling. Real-time data collection is essential for measuring the trajectory of the hole as it is drilled. For this purpose, various sensors are mounted on the drill head to provide information about the drilling environment in real time. Inclination, temperature, ultrasound and also radiation sensors are used. These various sensors are physically or digitally connected to a logic unit that converts the information into binary digits. The downhole data are transmitted to the surface via mud pulse telemetry. In addition to monitoring and controlling the drilling process, the data are used for further aspects, including:

  • Information about the condition of the drill bit
  • Records of the geological formations penetrated by the borehole
  • Creation of performance statistics to identify possible improvements
  • Risk analysis for future drilling

Mud pulse telemetry is a binary coding transmission system used with liquids. This is achieved by a valve that varies the pressure of the drilling mud within the drill string and thus converts the recordings of the sensors mounted on the drill head into pressure pulses. The pulsations reach the surface via the drilling mud. The pressure pulses are measured on the surface by a pressure transmitter and converted into an electrical signal. This signal is transmitted to a computer and digitized.

STS provides offshore directional drilling companies with analog pressure transmitters optimized for mud pulse telemetry. The sensors have to meet high demands: They must be extremely sensitive in order to reliably register even the smallest pressure differences. At the same time, the sensors must withstand pressures of up to 1,000 bar. Very high pressures are required to power the drill head in very deep drill holes. The pressure transmitters used for mud pulse telemetry on the surface are also exposed to these forces.

In addition to the high sensitivity, very fast response times are required to ensure good data communication in real time. In order to exclude falsified measurement results, the measuring instrument should be low-noise. The mud pumps in particular can cause the most signal noise in drilling applications. The drive of the drill is another source of interference. For this reason, analogue sensors with a 4 – 20 mA output signal are the best solution for mud pulse telemetry.

The Right Leak Testing Equipment

The Right Leak Testing Equipment

Many applications have components that must be absolutely leakproof to ensure proper operation. Leak tests are commonly carried out with pressure transducers that have to meet high requirements.

Applications and components that have to be leakproof are, amongst others:

  • Engines, brake systems, air conditioning systems, cylinder heads, valves, filters, fuel and injection systems
  • Packaging in the food industry or medical technology
  • Electric appliances
  • Refrigeration systems
  • Hydraulic systems

Components that have to be tight are usually sealed before installation. The equipment used for leak testing must therefore work very reliably during production.

Usually, leak testing is carried out by means of a pressure measurement. Pressure is applied to the component. The pressure is measured again after a rest period. If a pressure drop has occurred between both measurements, the component can be considered leaking.

The stable and precise function of the pressure sensor used for the test is crucial for the detection of leaks. In particular, the requirements regarding stability and the adverse effects of atmospheric noise are very high. Even minimal pressure losses must be detected reliably.

For example, accuracy values should not exceed 10 … 20 Pa or 0.001% … 0.002% of the full scale for a 10 bar sensor.

STS has been manufacturing leak detection sensors for years, including the ATM series analog pressure transmitters with a 4 … 20 mA output signal. The high-precision measuring element detects even low pressure losses in the mbar range and thus meets the high requirements of leak testing applications.

The mechanical design (process connection and electrical connection) does not affect the behavior of the sensor and can be configured thanks to the modular design principle employed by STS.

The pressure sensors of the ATM series are available with different output signals. In this application, however, it is important to use 4 … 20 mA as this robust output signal is largely unaffected by atmospheric noise.

Read more about leak testing here.

Hydrogen embrittlement in steel

Hydrogen embrittlement in steel

The sensor chip of piezoresistive pressure transducers is usually surrounded by a steel membrane. For the housings of these measuring instruments, stainless steel is also used in most applications. But should contact with hydrogen occur, this material can become brittle and then crack.

Hydrogen embrittlement affects not only steel, but also other metals. This is why the use of titanium offers no alternative in regard to hydrogen applications.

What is meant by embrittlement?

Hydrogen embrittlement refers to a loss of ductility in the material. Ductility describes the property of materials to plastically deform under stress before they finally fail. Depending on its type, steel can deform by more than 25 percent. Materials that do not have this ability are termed brittle.

But ductile materials can also become brittle, or frail. When this embrittlement of the material is the result of hydrogen absorption, this is then termed hydrogen embrittlement.

Hydrogen embrittlement occurs when atomar hydrogen diffuses into the material. The prerequisite for hydrogen embrittlement itself is usually hydrogen corrosion.

Hydrogen corrosion, also known as acid corrosion, always takes place whenever oxygen deficiency exists and metal comes into contact with water. The end product remaining from this redox reaction is pure hydrogen, which then oxidizes the metal. The metal goes into solution as ions and causes the material to be evenly degraded.

The hydrogen released by this redox reaction diffuses into the steel due to its small atomic size of only about 0.1 nanometer. The hydrogen directly occupies the metal lattice of the material as atomic interstitials. Lattice imperfections arising here then increase the absorption capacity. This leads to a chemical fatigue in the material, which can ultimately cause cracks from the inside to the outside, even at low loads.

Hydrogen and pressure transmitters

Because of its very tiny dimension, hydrogen can not only penetrate the material, but can actually penetrate it completely. For this reason, not only an embrittlement of the material can occur. The metal membranes of piezoresistive pressure sensors are very thin – the thinner they are, the more sensitive and accurate the sensor becomes. If hydrogen diffuses into and through the membrane (permeation), it can then react with the transfer fluid surrounding the sensor chip. As a result, changes in the metrological properties of the measuring bridge occur due to hydrogen adsorption. At the same time, an increase in pressure can also occur as a result of these deposits, with outcomes ranging from a buckling of the sensor membrane through to its complete destruction.

Besides using a thicker but somewhat more inaccurate membrane, this process can be greatly retarded by using a gold alloy and the unit lifespan thus optimized. You can read more about this here.

High Accuracy Pressure Measurement at High Temperatures

High Accuracy Pressure Measurement at High Temperatures

In some applications, pressure transmitters have to work reliably when exposed to very high temperatures. Autoclaves used to sterilize equipment and supplies in the chemical and food industries are certainly one of these demanding applications.

An autoclave is a pressure chamber used in a wide range of industries for a variety of applications. They are characterized by high temperatures and pressure different from ambient air pressure. Medical autoclaves, for example, are used to sterilize equipment by destroying bacteria, viruses and fungi at 134 °C. Air trapped in the pressure chamber is removed and replaced by hot steam. The most common method for achieving this is called downward displacement: steam enters the chamber and fills the upper areas by pushing the cooler air to the bottom. There, it is evacuated through a drain that is equipped with a temperature sensor. This process stops once all air has been evacuated and the temperature inside the autoclave is 134 °C.

Very accurate measuring at high temperatures

Pressure transmitters are used in autoclaves for monitoring and validation. Since standard pressure sensors are usually calibrated at room temperature, they cannot deliver the best accuracy under the hot and wet conditions encountered in autoclaves. However, STS has recently been approached by a client in the pharmaceutical industry that requires a total error of 0,1 percent at 134 °C measuring -1 to 5 bar.

Piezoresistive pressure sensors are rather sensitive to temperature. However, temperature errors can be compensated so that the devices can be optimized for the temperatures encountered in individual applications. For example, if you use a standard pressure transmitter that achieves 0,1 percent accuracy at room temperature, the device would not be able to deliver the same degree of accuracy when used in an autoclave with temperatures of up to 134 °C.

Users who know that they require a pressure sensor that achieves a high degree of accuracy at high temperatures hence need a device that is calibrated accordingly. Calibrating a pressure sensor for certain temperature ranges is one thing. However, the client who inquired about the autoclave application with very high accuracy demands had another challenge for us that was even trickier to realize than a properly calibrated sensor: not only the sensor element was to be in the autoclave at 134 °C, but the complete transmitter including all electronics had to go in there, too. Unfortunately, we cannot go into specifics as to how we were able to assemble a digital transmitter that both delivers the desired accuracy of less than 0,1 percent total error at 134 °C but whose other components can handle the hot and moist conditions as well.

In short: Piezoresistive pressure sensors are sensitive to temperature changes. However, with the right know-how, they can be optimized for the requirements of individual applications. Moreover, not only the sensor element can be calibrated accordingly, the whole transmitter can be assembled in a way that even hot and wet conditions can be managed.

Basics of flow measurement

Basics of flow measurement

The flow of a gas or liquid is measured for a variety of reasons, certainly including commercial considerations as part of a contract and also in various production processes. The flow or volume flow (volume/time) can be recorded, among other things, by the measured value of pressure.

Volume flow can be measured using various methods. In addition to ultrasonic flow sensors, these include magnetic-inductive flow sensors and sensors that work according to the differential pressure method, among these being the orifice plate, Venturi nozzle and the Prandtl pitot tube. When evaluating the measured values, the Bernoulli equation is used for all sensors operating on the differential pressure method:

Q = V/t = VmA

Q = volume flow
Vm = median velocity
t = time
A = area
V = volume

We will now take the measurement of volume flow using an orifice plate as the example. By attaching the plate to a pipe, this then becomes narrowed at one point.

Image 1: Orifice plate

With a smooth flow, the same pressure should prevail both before and after the orifice plate:

p1 + ½ ρv12=p2+ ½ ρv22

p = pressure
ρ = density
v = velocity

This assumption is based on the continuity equation, which states that everything flowing into a pipe eventually also comes out:

v1A1 = v2A2

v = velocity

A = area

Image 2: Flow measurment

Under realistic conditions, however, friction occurs, which then leads to a pressure drop:

p + ½ ρv2 + wR = constant

p = pressure
ρ = density
v = velocity
wR = rate of frictional force by volume

Image 3: Pressure drop 

This pressure drop is important in determining the volume flow. The friction effect itself, however, depends upon many factors. For this reason, an empirical formula is used, which in turn relies on empirical values. The volume flow now ultimately results from the root of the pressure differential:

Q = 4000 αεd2√∆p/ρ

Q = volume flow
α = empirical flow coefficient
ε = expansion factor
d = internal orifice diameter
∆p = pressure differential
ρ = density

To make this formula a little easier for users, all of the constant values from the measuring system and the measuring medium can be summed up as the constant ‘c’. The result for a fluid, for example, then offers the equation:

Q = c √∆p

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