Although permanently monitoring the vibration of a machine provides a critical piece of information about its condition, a temperature measurement can be easily integrated into your reliability program at the same time to provide a second critical piece of information for trending purposes. There are many complementary technologies in reliability and quite often, vibration monitoring and temperature go hand-in-hand with each other. If we trend the vibration amplitude of a bearing, wouldn't it be nice to also trend the temperature of the bearing? This can be easily accomplished with a dual output sensor that provides a vibration output in acceleration (g's) and a temperature output in centigrade (C) or kelvin (K). Providing a sensor that will output both acceleration and temperature requires a three wire technology.
The temperature trend will actually be a measure of the temperature inside the accelerometer. These dual output models incorporate a small integrated circuit inside the accelerometer that detects the internal temperature of the accelerometer. Although this is not the absolute temperature of the bearing, it is a relative temperature of the bearing casing the accelerometer is mounted on and can be trended over time or alarmed for changes in the bearing temperature. Wear and tear or problems with lubrication will often cause high vibration and high temperature in the bearing. There should be relatively good correlation between the overall vibration trend and the temperature trends providing supporting data on bearing condition and performance.
In the case of a dual output sensor that provides a vibration output (mV/g) and temperature output in centigrade (10 mV/°C), pins "A" (positive) and "B" (common) would require the typical integrated electronic piezoelectric (IEPE) sensor power on the vibration side of the sensor to create the acceleration output. Pins "C" (positive) and "B" (common) would provide a DC voltage output at 10 mV/°C for a typical range of approximately 0 to 120 degrees centigrade (Figure 1).
Figure 1: Dual output circuit for vibration and temperature (centigrade)
In the case of a dual output sensor that provides a vibration output (mV/g) and temperature output using the Kelvin scale (10 mV/K), pins "A" (positive) and "B" (common) would require the typical IEPE sensor power on the vibration side of the sensor to create the acceleration output. Pins "C" (positive) and "B" (common) would also require the IEPE sensor power, using a DC coupled input for the monitoring system on the temperature side of the sensor to create a DC voltage output of 10 mV/K (Figure 2) for a typical range of approximately -40 to 100 degrees centigrade. Centigrade? Yes, we won't be working on the Kelvin scale, but we will have sensor capable of measuring a temperature that is less than zero °C, and because it is a powered circuit, avoid any impedance mismatch between the sensor output and monitoring system input. (An impedance mismatch could cause an error in the temperature measurement by creating a voltage drop much like you would experience if current was passing through a resistor.)
Figure 2: Dual output circuit for vibration and temperature (kelvin)
Kelvin to Centigrade Conversion:
If your application uses the dual output sensor with the Kelvin scale for temperature measurement, you may need a conversion from kelvin to centigrade for the monitoring system and correlation with bearing specifications or temperature limits.
Converting an output sensitivity of 10 mV/K requires compensation for the offset between scales where C = K - 273. If setting the temperature sensitivity of the monitoring system in C, apply 10 mV/°C as the engineering unit to scale the monitoring system in C and then subtract a 2.73 V DC or 273 degree offset from the output of the sensor to get the correct temperature value. The type of offset needed - voltage or temperature - will depend on the monitoring system's software or firmware design and setup.
Although the dual output vibration and temperature sensors use the centigrade (C) or kelvin (K) temperature scale, it is possible to convert these units to the Fahrenheit (F) scale for the monitoring system.
Converting from C to F, where F = 1.8C + 32, would require an engineering unit of 5.556 mV/°F and an offset of +177 mV or + 32 degrees.
Converting from K to F, where F = 1.8K - 459.4, would require an engineering unit of 5.556 mV/°F and an offset of -2.55 V or - 459.4 degrees.
Vibration monitoring relies on trended amplitude over time to determine the condition of the bearing and machine. Typical values for dual output sensors are 10 mV/g, 100 mV/g, and 500 mV/g. The vibration trend is not an absolute measurement due to small errors created by the sensitivity of the sensor, frequency response of the sensor and window filter used in the monitoring system. However, we understand and accept that if the amplitude of the trended vibration increases, the bearing or some part of the machine is experiencing a problem and will require the attention of the vibration analyst to identify the fault and the maintenance department to repair the machine.
Although the dual output vibration and temperature sensor does not provide a direct contact measurement that a thermocouple or resistance temperature detector (RTD) might, it does come in a very durable and reliable stainless steel package that will survive the most severe industrial applications. Trending the temperature will also indicate if the operating condition of the machine is changing. An unexpected change in temperature should be an alert of a pending problem and it can be correlated with the vibration to assess the overall condition or criticality.
A thermocouple or RTD could be set up to monitor absolute bearing temperature, but the mechanical installation, electrical installation and power supply requirements are much more complicated when compared to a dual output vibration and temperature sensor. These complications with the thermocouple and RTD make dual output vibration and temperature sensors a much more cost-effective method for measuring temperature with a permanent vibration monitoring system.
Remember, a dual output vibration and temperature measurement can be achieved at one location, with one sensor reducing the need for multiple connections, cables and monitoring devices. It can be used to provide efficiency and reliability for your condition monitoring program as an integrated technology preventing failures and increasing uptime.
Jack D. Peters is an International Sales Manager for Connection Technology Center, Inc., and a Category IV Vibration Analyst in accordance with ISO 18436-2. www.ctconline.com
Sudipta S. Das is an Electrical Engineer for Connection Technology Center, Inc., and a Category II Vibration Analyst in accordance with ISO 18436-2. www.ctconline.com
Joseph L. Sklepik III is a Mechanical Engineer for Connection Technology Center, Inc., and a Category II Vibration Analyst in accordance with ISO 18436-2. www.ctconline.com