This is the third post in a multi-part series on how to make successful measurements, i.e., how to get “good data”. In the first post we made the case that good data is about time, money, performance, results, and reputation. In the second post we discussed how to make good measurements of a fluctuating phenomenon such as shear stress, pressure, vibration, or sound. In this post we will discuss how to make good measurements of a static phenomenon such as mean shear stress or static pressure.
When we acquire static data from a sensor, such as a pressure sensor or a floating element shear stress sensor, we want to measure the DC component of the physical quantity that corresponds to the mean displacement of the diaphragm or floating element upon which any AC (fluctuating) component is superimposed. As we discussed in the previous article in this series, to get the best quality fluctuating measurement, we AC couple the input to remove the DC (static) offset.
To make a high-quality static measurement we want to DC couple the input signal and use additional techniques to remove any effects due to the fluctuating signal or other possible electronic interference such as line voltage noise.
Static data does not have frequency content. It may vary slowly over time, or even change in a short period of time due to a rapid change in conditions. For example, rapidly changing the flow speed of a wind tunnel has a rapid effect on the static pressure and static temperature of the air within the tunnel circuit. However, at any point in time, it has a value that is independent of oscillatory behavior.
Accuracy Requirements Drive DC Voltage Measurement System Requirements
The accuracy requirement for a DC voltage measurement determines how good your measurement system needs to be, what parameters of that system need to be used, and how those parameters need to be set. You need both enough digits of resolution in your measurement and sufficient accuracy such that the displayed digits are meaningful. If there is too much noise in the measurement, some of the displayed digits will not be meaningful.
The amount of noise in a measurement may be able to be mitigated/reduced by using certain features of your DC voltage measurement device.
These may include such features as: Auto Zero, to compensate for instrument offsets; Analog-to-Digital Converter (ADC) Calibration, to remove ADC gain drift; Settling Time, to allow for any transients that may occur due to switching of signals upstream of the DC voltage measurement device; Aperture Time, to control for how long the ADC is reading (and averaging) the signal; and DC Offset Nulling, to eliminate offset errors that may be present due to your measurement setup/connections. Note that if you are using a Data Acquisition System (DAQ) to acquire a time series from which the DC voltage will be computed, you have to sample at a low enough rate to ensure statistical independence of the samples.
The combination of these aforementioned features, along with the inherent measurement quality capabilities of the instrument itself, go into determining the accuracy of the data being read. When deciding between various DC voltage measurement systems, it is important to assess whether these features exist, their implications, and ultimately, how accurate of a measurement you can achieve.
There will be trade-offs between how long the measurement takes and how accurate the measurement is, but there are also typically situations where adding additional time to the measurement won’t make a significant difference in the accuracy of the measurement.
DC offset nulling requires a separate (additional) measurement as described in the following section.
Account for DC Offset in the Sensor System
DC offset nulling is a process by where a separate measurement is made to account for measurement bias that is attributable to causes external to the DC voltage measurement device. This bias is typically due to the sensor system and/or how the sensor system is connected to the measurement system. For example, a sensor system that has inherent offsets caused by sensitivity to temperature and/or the cabling that is used to connect the sensor system to the data acquisition system, would require offset nulling to be applied to remove measurement bias that is due to those causes.
The DC offset measurement is made with the full measurement system connected just as it will be when making the primary measurement, but under ambient conditions.
For example, when making shear stress measurements in a wind tunnel, the DC offset measurement might be made just prior to starting up the wind tunnel. The DC offset measurement needs to be of the same level of accuracy as the primary measurement so that it does not contribute to additional errors in the final measurement value. The final measurement value is the primary measurement with the DC offset subtracted.
Use Power Line Cycle Averaging
An additional capability included in high-quality Digital Multimeter (DMM) products, commonly used to attain higher quality DC voltage measurements, is called Power Line Cycle (PLC) Averaging.
PLC Averaging removes errors associated with power line noise (typically 50 Hz or 60 Hz).
By setting the aperture time such that it is a multiple of one PLC, the noise from that source is cancelled (averaged out). Specifying aperture time in this way is typically done by specifying the power line frequency and the number of power line cycles (NPLCs) over which to average. For example, 15 PLCs at 60 Hz would correspond to a 0.25 second aperture. It is interesting to note that the 50 Hz power line frequency may also be used in onboard airplane applications, where 400 Hz power is being used, since one 50 Hz PLC is equal to eight 400 Hz PLCs.
Sanity Check: Are My Sensors Working?
We can sort out all of the above information to determine what specifications will provide the accuracy that we require, but before we start making measurements, we need to know that our sensors and supporting instrumentation are working properly. As we did in the previous article in this series (for fluctuating measurements), we assume that all of the sensors have been calibrated and we have the calibration information properly associated with the data acquisition channels to which the sensors are connected so that the voltages that we measure with the data acquisition system can be properly converted into engineering units. We also assume that steps have been taken to ensure that the correct sensors have been connected to the correct data acquisition system inputs. This means that we just need a way to monitor the health of the instrumentation on an ongoing basis, ideally, acquisition event to acquisition event, so we know that the measurement system is continuing to function as it did when it was initially set up.
A simple digital DC voltage readout display that is providing real-time sensor output information is adequate for this purpose. If several DC readings are being made, it can be very helpful for the operator to have a graphic display that includes side-by-side level meters for each DC reading. Graphical meters provide a much quicker/easier way to inspect multiple sensors at the same time without having to read through a list of numerical values. It will be instantly obvious if a sensor is “dead” or if a sensor output is overloading the input of the ADC. It is handy, along with these graphic meters, to include a digital readout alongside each meter so more detailed inspection can be done on individual sensors when needed.
As described in the previous article in this series, the ADCs of your data acquisition system have a voltage range over which the digitization is performed. If you don’t use the full voltage range, you sacrifice the dynamic range of your measurement. The real-time displays, described in the previous section for sanity checking your sensors, will also provide the needed information to optimize range settings. It can also be helpful to add a real-time overload/overrange indicator for each channel. The overload indicator is just a Boolean indicator that tells you whether the previous reading from the DC voltage meter exceeded the range that was being used for the measurement. The overload indicator for each channel should be placed right next to the graphical meter for each channel so that all of the information for a given channel can be seen together, at a glance.
If the overload indicator is on, the range needs to be increased, one step at a time, until the overload indicator is off.
So, How Do You Know You Are Getting Good Data?
For static data acquisition, we can now answer the question that is the subject of this series. You know you are getting good data because:
1) you know your accuracy requirements and you have chosen the appropriate settings for your DC voltage measurement device to ensure that the needed number of digits of precision are not in the noise;
2) you have made a DC offset measurement that can be subtracted from subsequent measurements to remove any measurement bias that might be present due to the sensor system and how it is connected to the data acquisition system;
3) you know the health of your sensors since you can see what the sensors are seeing in your real-time displays; and
4) you have set the input ranges of your ADCs optimally such that you are using as much of the ADC input voltage range as possible while not registering any overloads.
For the next post in this series, we will discuss traceability of acquired data. You can take all the quality fluctuating and static data that you want, but you need to protect the integrity of that data to ensure its usefulness.