Engine test stands used with advanced data-acquisition systems require a different type of pressure sensor than those used in the past. Engine hydraulics and pneumatics operate faster and often, with higher pressures and temperatures. Accordingly, the pressure sensors have to be more reliable and rugged in order to reduce downtime.
In choosing the correct pressure sensor for your application, you must consider several specifications. You must decide which type of pressure sensor will give you the best results in your application.
There Are Five Basic Types:
- Gage pressure sensors - With gage sensors, pressure readings are referenced to the atmosphere. That is, zero output is at atmospheric pressure. You use this type of sensor when you need to measure both vacuum (negative output) and pressure (positive output).
- Vacuum pressure sensors - A vacuum sensor's output is zero at atmospheric pressure, like the gage sensor, but the output increases as vacuum increases. You calibrate vacuum sensors so their output becomes more positive as the pressure becomes more negative.
- Differential pressure sensors - This type of sensor has two pressure ports, as shown in Figure 1, and senses the difference in pressure between the two ports. You can use differential pressure sensors to measure the pressure of liquids or gasses.
- Absolute pressure sensors - The reference for this type of sensor is full vacuum. That is, the output is zero at full vacuum. Note that there is no polarity change when the input pressure changes from vacuum to pressure above atmosphere.
- Barometric pressure sensors - Barometric pressure sensors are absolute pressure sensors with a limited range. Usually, the output of these sensors is expressed as "inches of Mercury (Hg)," and the output ranges are 16-32 inch HgA or 26-32 inch Hg with zero output at the low number. A standard "absolute" sensor may be used over 0-30 in HgA range, but the limited range offers more resolution, especially with a voltage output for the typically small barometric pressure changes. You may use several different types of pressure transducers in a typical engine test stand.
Figure 1. Pressure sensors of many different types are used in a typical test stand.
The Applications For Each Sensor Can Be:
Gage Pressure Sensors:
- Engine oil pressure - To ensure lubrication system integrity, you often want to correlate oil pressure with time and crankshaft position.
- Coolant pressure - Coolant pressure is a measure of how well the cooling system is working.
- Fuel pressure - You measure fuel pressure during fuel pump and pressure regulator tests.
- Cylinder compression (cold test) - To measure this parameter, you insert a gage sensor into each cylinder's spark plug hole. By correlating cylinder pressure with cranking torque, crank angle and timing, you can detect piston ring, valve or crank problems.
- Pressure decay - By measuring how quickly the pressure decays inside a pressurized cavity, you can detect damaged or missing gaskets and O-rings, emission valve problems, and other leaks. You also can use a differential pressure sensor to measure pressure decay.
Vacuum Pressure Sensors:
- Manifold vacuum - This is the most common use for vacuum sensors in engine testing. For this measurement, you calibrate the output to be in either inches of mercury, Hg or psiv (psi, vacuum).
Differential Pressure Sensors:
- Fluid flow - Using a precision calibrated orifice or Venturi tube, and measuring the differential pressure across the orifice, you can measure intake airflow or engine coolant flow. The advantage of using a differential pressure sensor for this measurement instead of two gage sensors is that accuracy is always specified as a percentage of the sensor's full-scale reading. By using a differential pressure sensor, the full-scale reading can be much smaller, thereby reducing measurement error.
- Dry airflow - You need to measure dry airflow when testing engine-block oil ports. You can correlate the back pressure with crankshaft position to find missing main bearings or plugged oil cavities.
You also can use a differential pressure sensor as a gage sensor inside a closed test chamber; you must vent the low or negative pressure port to the outside atmosphere rather than vent it inside the test chamber where the ambient pressure may be different or variable. The wet/wet characteristics of the differential pressure sensor offer environmental protection and reliable operation.
Absolute Pressure Sensors:
You use absolute pressure sensors when you have a data acquisition system that can only accept unipolar inputs. In this case, you would use an absolute pressure sensor in place of a gage sensor or vacuum sensor.
Selecting the Right Pressure Sensor
There are many specifications you must consider when choosing a pressure sensor. The most important specifications are electrical output, accuracy, operating environment and mechanical coupling.
Pressure sensors are available with either voltage outputs or current outputs. Strain gauge sensors usually have output ranges of 0-30 mV or 0-100 mV, depending on the type of strain gauge they use. Sensors using bonded metal strain gauges have the lower output, but you can use them over a wide temperature range, often as low as cryogenic temperatures and some up to 450E°F. Sensors using piezo-resistive strain gauges offer higher output voltages, but they have a more limited operating temperature range.
Some sensors have an internal amplifier that provides a 5-VDC or 10-VDC output.
The advantages of using these sensors include output signal levels that are above the noise level, internal zero and span adjustments for a precise setup and three-wire connections instead of the four-wire connections needed for millivolt output sensors. Unfortunately, the internal amplifiers used to boost the output signal limit the frequency response of these sensors to approximately 3 kHz. They have narrower operating temperature ranges than millivolt-output sensors.
Current output sensors are two-wire sensors with a 4-20 mA output.
They offer high noise immunity and can be located further from the data acquisition system than voltage output sensors. Frequency response is in the 2.5-3.0 kHz range, and they have zero and span adjustments for precise setup.
Accuracy is the most important performance specification. Sensors with accuracies of 0.05% full scale are available, as are accuracies of 0.1%, 0.25% and 0.5%. Price and availability are usually inversely proportional to the accuracy of the sensor, so you can save time and money by not specifying a sensor that is more accurate than you really need.
You also need to consider where you will use the sensor. The amount of protection the sensor will need depends on whether the sensor is indoors and dry or outside and exposed to the elements. Obviously, the cost of a sensor designed for use in a harsh environment will be higher than a sensor used in a benign environment.
The operating environment also will determine the type of connectors and cabling you will need. Quick-disconnect electrical connectors make it easy to remove sensors for periodic calibration, but most cannot be used outdoors.
Waterproof connectors rated for outdoor use are costly in comparison.
Some sensors come with an integral cable. A sealed boot on the sensor protects the cable. When equipped with an atmospheric vent tube inside the cable jacket, these sensors are submersible.
You also need to consider how you will couple or install the sensor to the test system. Sensors are available with a variety of threaded options, including English threads, metric threads, SAE tube connections and pipe threads. Most pressure sensors made for test and measurement applications are built with stainless steel, wetted parts. The wetted part of the sensor is the pressure port that will come in contact with the measured media (fluid or gas). Welded stainless steel is one of the most trouble-free materials, as is a combination of stainless steel and hastelloy. A completely welded assembly built without the use of epoxies, sealants or O-rings ensures long-term stability and hassle-free use.
Finally, consider ease of installation and calibration. If you are using sensors that use strain-gauge technology, the best option is to purchase one with a built-in shunt calibration circuit. The shunt calibration circuit provides a calibrated output without having to apply a known pressure source to the sensor. By removing the system pressure and venting the sensor to the atmosphere on a gage-type pressure sensor, you can adjust the sensor's output to zero. Then, by engaging the shunt calibration circuit, you can calibrate the full scale reading.