Scale Factor Information for MDL Sensors

Introduction
Scale Factor Determination
333 millivolt Module
4-20 milliamp Module
5 Volt and 10 Volt Module
Pulse Count Module

1. Introduction

The Scale Factors of Slope and Offset are used to convert raw data into engineering units suitable for display.

These Scale Factors are very important for user-supplied sensors that do not Auto-ID in the MDL-DataManager™ software. See the Scale Factor Calculator

Sensors and transducers convert a physical input (pressure, flow, etc.) into an electrical signal (volts, milliamps, pulses, etc.) that can be measured by the MicroDataLogger® unit (MDL).

The user supplied Slope and Offset values are used to convert the electrical signal into engineering units.

Slope and Offset values are entered in the Configuration Parameters screen of DataManager™ Software.

This feature permits a wide range of standard industrial sensors and transducers to be used with the MDL.

DataManager™ Software, when the Configure MDL screen is first entered, the MDL-DataManager™ software reads the module's ID number and automatically selects the correct default configuration parameters for the type of module installed in each channel.

However, if the scale button for a given channel is active (black) you can enter the Slope and Offset values as well as Units of measure so the MDL will display and record your readings in the proper engineering units

Clicking on the scale button opens the Configuration Parameters screen.

In the Configuration Parameters screen, Slope and Offset values are entered and used to convert output signals (voltage, current, pulse, etc.) from a sensor or transducer into correct engineering units.

The conversion formula is shown below:

Reading in Engineering Units = ((Sensor or Transducer Output) x (Slope)) + Offset

NOTE: In most situations, the Offset value is a negative number and should be entered in the Configuration Parameters screen with a minus (-) sign.

2. Scale Factor Determination

Examining the equation of the line (y=mx + b) created by comparing the Sensor/Transducer Input (y) against the Sensor/Transducer Output (x), we can determine the Slope (m).

The Slope is then determined by the following formula:

Slope = Full Scale Range of Sensor/Transducer divided by Full Scale Output Signal of Sensor/Transducer

An Offset may be entered if required. It must be entered in the correct engineering unit as determined by the following equation.

To determine the Offset (b), we examine the line when the input is at a minimum. Therefore, when y = y min, we have y min = mx + b. Solving for b yields b = y min - mx. Consequently we have:

Offset = (Minimum Sensor Input Value in Engineering Units) - ((Slope) x (Sensor Output (voltage, etc.) at Minimum Input Value))

Specific examples are now shown that demonstrate how these values are determined based on the sensor output signal and the module used to connect the sensor or transducer to the MicroDataLogger® unit.

Specific Examples

3. 333 millivolt Module

Note that the slope is printed on the label of each Current Transducer and Potential Transformer supplied by AEC. For AC Current Transformers supplied by other companies, here is the formula that you need to follow for determining slope; note that non-AEC Current Transformers must be internally burdened and provide an output voltage of 333 mV maximum.

An AC Current Transformer that has an Input range of 0 - 20 Amps, and an Output range of 0 - 333 mV is used for this example.

Full Scale Input Range of Sensor = 20 A - 0 A = 20 A Full Scale Output Range of Sensor = 333 mV - 0 mV = 333 mV

Slope = 20 A divided by 333 mV = .06006 A/mV

For this current transformer the Minimum Input value is 0 A. When it is at this minimum, the corresponding Minimum Transformer Output is 0 mV.

No offset is necessary. Enter "0" for offset.

4. 4-20 milliamp Module

An ultrasonic flow meter with a Full Scale Input of 2000-5000 GPM and a Full Scale Output of 4-20 mA is connected to the MDL using a 20 mA DC signal conditioning module.

Thus, Full Scale Input Range of Flow Meter = 5000 - 2000 GPM = 3000 GPM Full Scale Output Range of Flow Meter = 20 - 4 mA = 16 mA

Slope = 3000 GPM divided by 16 mA= 187.5 GPM/mA

When the flow meter is at its minimum Input Value of 2000 GPM, the corresponding Minimum Output Value is 4 mA.

Using equation: Offset = (Minimum Sensor Input Value) - ((Slope) x (Sensor Output at Minimum Input Value)) Therefore: Offset = 2000 GPM - ((187.5 GPM/mA) x (4 mA)) = 1250 GPM

5. 5 Volt and 10 Volt Module

The following example can be used with both the 5 Volt and 10 Volt modules.

A Gage Pressure Transducer with an Input range of 0 - 150 psig and a 1 - 5 V dc Output. Full Scale Range of Transducer = 150 psig - 0 psig = 150 psig Full Scale Output Range of Transducer = 5 V - 1 V = 4 V. Slope = 150 psig divided by 4V = 37.5 psig/V.

In this instance, at our Minimum Input Value of 0 psig, the Transducer Output is 1 V.

Offset = (Minimum Sensor Input Value) - ((Slope) x (Sensor Output at Minimum Input Value)) Therefore: Offset = 0 psig - ((37.5 psig/V) x (1 V)) = -37.5 psig.

6. Pulse Count Module

The following example should be used when using the WattNode™ transducer for measuring WattHours. To use the WattNode equipment for kW or kWh readings, reference the MicroDataLogger: WattNode User Guide [360kb

] supplied with the WattNode units.

A three phase, four wire, 277/480 Volt Wye WattNode™ transducer is used with three 200 A CT's. (Note that the calculations are the same for one, two, or three CT's.)

Looking on the back of the WattNode™ transducer, we see that the "watt-hours per pulse per CT Rated Amp = 0.05771." We take this value, and multiply it by the Amperage of the CT(s) being used, which in this case is 200 A; therefore:

Slope = (0.05771) x (200 A) = 11.542 Watt Hours / Pulse. No offset is necessary. Enter "0" for offset.