The key advantages of today’s electromechanical devices are reliability, accuracy, and the ability to be monitored remotely. Their primary measurement technology is based on piezoelectric materials or strain gauges. In addition, they are made of stainless steel, and it has a gas seal weld design rather than the O-ring and adhesive seal that usually proves to be the weak point of typical transducers. The article then describes the sources of measurement error and how they can be made very small before demonstrating how the transducers can be applied in commercial refrigeration systems to improve process efficiency.
1. How to work with electromechanical pressure sensors
A strain gauge is a circuit in which the resistance changes when subjected to a strain, where the strain is the ratio of the change in length of the material subjected to the force to its unloaded length (known as). Strain gauges are often classified according to their strain factor (GF), a measure of strain sensitivity. We should use a suitable adhesive to attach the strain gauge to the upper side of this diaphragm Figure 1
2. How to perform strain gauge measurements
We want to measure such small changes in resistance accurately, and we should reduce the effect of noise to a shallow level before integrating the strain gauge of the pressure transducer into one leg of a Wheatstone bridge, which consists of a network of four resistive arms with an applied excitation voltage E at both ends (Figure 2)
A Wheatstone bridge is the electrical equivalent of two voltage divider circuits connected in parallel. RG (assuming negligible resistance of leads RL1 and RL2), R4 comprises one voltage divider circuit, and R2 and R3 form the second voltage divider circuit. The output is measured between the intermediate nodes of the two voltage dividers, and we can calculate it by the following equation.
3. Temperature compensation
When we use a strain gauge, its design challenge is its sensitivity to the effects of temperature. The strain gauge may excite the voltage and become hot, but we can mitigate this considerably by keeping E low. The disadvantage is that this will reduce the system’s sensitivity, but the output voltage of the Wheatstone bridge can be amplified if required. However, we must take special care to avoid strengthening the superimposed noise. One solution is to use a carrier frequency amplifier that converts voltage changes to frequency changes and uses a narrow bandwidth output to keep noise low and reduce out-of-band EMI.
We want to reduce these effects, and modern strain gauges use temperature compensation measures. Strain gauges are usually 55% copper/45% nickel alloy. This material has a very low coefficient of thermal expansion (CTE), which limits the temperature-induced strain. Furthermore, by carefully matching the CTE of the strain gauge to the CTE of the diaphragm material to which it is connected, a degree of self-temperature compensation can be achieved, limiting temperature-induced strains to a few microns per meter per degree Celsius (m/m/C).
When the temperature affects the output of a pressure sensor, there are other sources of error. These sources of error are often referred to as the ideal transfer function, a straight line independent of temperature that passes through a perfect offset in the ideal pressure range with a slope equal to the ideal full-scale range (FSS). The balance is the output signal obtained when a reference pressure is applied. At the same time, the FSS is the difference between the output signal measured at the upper and lower limits of the operating pressure range (Figure 3).
4. Errors in pressure sensors
When a lower quality pressure sensor is shipped from the factory, it will suffer from large offset and FSS errors. The offset error is a substantial pressure deviation compared to the ideal offset. In contrast, the FSS error is the considerable deviation from the FSS measured at the reference temperature relative to the perfect (or target) FSS as determined from the excellent transfer function.
Other errors arise from the accuracy of the pressure sensor itself, which can be affected by pressure non-linearity, pressure hysteresis, and non-repeatability. The combination of thermally induced errors, sensor inaccuracies, and offset and FSS errors determine the total error band (TEB) of the pressure sensor, which is the substantial deviation of the output from the ideal transfer function over the entire compensated temperature and pressure range (Figure 4)
5. Heavy-duty pressure sensors
When we use a pressure sensor, it will be exposed to corrosive liquids and gases and is susceptible to large temperature fluctuations. For example, transducers used in HVAC/R applications are exposed to refrigerants such as butane, propane, ammonia, CO2, ethylene glycol plus water, or a range of synthetic HFC refrigerants, R134A, R407C, R410A, R448A, R32, R1234ze, or R1234yf. Similarly, the temperature range in industrial HVAC/R systems is -40 to +85C and even higher in the industrial temperature range.
To avoid these potential failure modes, designers can use BCST’s (China) MIP series pressure sensors. The heavy-duty pressure transmitter with media isolation eliminates the internal O-ring and adhesive seal. Instead, the transducer is stainless steel and has a gas-welded design rather than an O-ring seal. This design allows the MIP sensor to be compatible with a wide range of being; it includes corrosive liquids, water, and gases, and they have a temperature range of -40 to 125°C and at pressures from 100 kilopascals (kPa) to 6 megapascals (mPa) (Figure 5).
6. Pressure sensors in HVAC applications
Pressure transmitters are crucial in applications such as HVAC systems. When it comes to reducing energy consumption, they also achieve very high efficiency for precise control. For example, consider the HVAC/R cycle used in industrial refrigeration equipment (Figure 6).
The pressure transmitter is at the condenser, where the high-temperature vapor releases its latent heat into the air and then condenses into a hot liquid. The dryer then removes all water from the refrigerant. Then, at the metering unit, the hot liquid from the condenser is pushed through a flow restrictor which lowers its pressure and thus forces the refrigerant to release its heat. Then, inside the evaporator, this cold liquid absorbs heat from the return stream of the condenser and turns into a vapor. This vapor continues to absorb heat until it reaches the compressor, where the cycle repeats. Finally, the cold air from the evaporator is used to lower the temperature of the refrigerated container.
Strain gauge pressure transmitters offer an excellent solution for pressure measurement in industrial processes. Still, system designers who may be exposed to extreme environments need to be aware of the limitations of models that use O-rings.
BCST (China)’s MIP series pressure transducers are designed for applications that may encounter such extremes, with stainless steel fabrication and a hermetically sealed welded design. This construction makes the MIP sensor compatible with a wide range of liquids and gases, ensuring a long service life even at high temperatures and pressures. In addition, BCST (China) pressure sensors also offer high accuracy, fast response, good long-term stability, and excellent EMI immunity.