Data Acquisition System

Sensors and Data Acquisition

It is generally accepted in the field of measurement that sensors which give electrical signals are termed as “transducers”, while transducers which give 4-20 mA signal ranges are called “transmitters”. All sensors used in this study have electrical analog signal outputs. The four temperature sensors used in the water test give voltage signals in the range of 0-1 V. All other sensors are standard transmitters and give current output signals of 4-20 mA.

Outputs from the sensors need to be converted into a meaningful quantity, for example, the output from the turbine flowmeter, which is a current signal, needs to be converted into volumetric flow rate. In most cases, measuring transmitters are linearized devices; the conversion rate is given in the manufacturer’s instruction, along with its accuracy specifications. For higher accuracy, a calibration process needs to be carried out with a properly designed procedure.

The data acquisition system used in the present study is the National Instruments’ NI PCI-6224 DAQ system along with the operating software LabVIEW (version 8.0). Most data acquisition systems read voltage signals only, including the NI PCI-6224 system. Therefore, it is necessary to make an arrangement in electrical connection which can give voltage signals. Because current output is the same at any point in the circuit, it can be converted to a voltage output for measurement purposes at any point in the circuit by adding a load resistor in series. The basic

circuit for measuring the output of a transmitter is then to connect a power supply, a precision load resistor and the transmitter in series. The analogue signal is measured indirectly by connecting a voltmeter (built-in with the data acquisition system) across the load resistor which produces a voltage drop and is proportional to the 4 to 20 mA current loop. This is shown in Figure 3.12. The load resistor brings extra uncertainty to the measurement accuracy. However, a calibration procedure can compensate for this uncertainty if the measurement channel, including the sensor and all connections, is calibrated as one integrated unit.

General Features of the Data Acquisition System Used

The NI PCI-6224 data acquisition system is one of the M series products from National Instruments. Voltage signals are the only analogue signals that can be read by the device. Three types of analogue input ground-reference settings are supported, namely: Differential mode, Referenced single-ended mode, Non- referenced single-ended mode. Depending on the ground-reference mode, 16 or 32 channels in total are supported (16 channels for differential mode, 32 for other modes). The Differential mode reduces noise pickup and should be used whenever possible; this was used in these tests.

It is recommended by NI that the M-Series device should be self-calibrated after installation and whenever the ambient temperature changes. Self-calibration should be performed after the device has warmed up for the recommended time period (15 minutes for NI 6224). This function measures the onboard reference voltage of the device and adjusts the self-calibration constants to account for any errors caused by short-term fluctuations in the environment. In the process of self-calibration all external signals need to be disconnected.

Multichannel Scanning Considerations

In multichannel scanning applications, accuracy is affected by settling time. Settling time refers to the time the device takes to amplify the input signal to the desired stability before it is sampled.

Several factors can increase the settling time which decrease the accuracy of measurements. To ensure fast settling times, the following should be considered (in order of importance):

• Use low impedance sources

Large source impedances increase the settling time. To ensure fast settling times, the signal sources should have an impedance of less than 1kΩ. If the source impedance is high, the scan rate can be decreased to allow more time to settle. The NI 6224 has a default settling time of 14 µs, but this was manually adjusted to 1000 µs in the current study. For sensors that have a current signal output, the signal source is actually a 47 ohm resistor and will thus not cause a problem.

• Use short high-quality cabling

Short high-quality cables can minimize several effects including crosstalk, transmission line effects, and noise. It is recommended by NI to use individually shielded, twisted-pair wires that are 2m long or less to connect analogue signals to the device.

• Carefully choose the channel scanning order

a. Avoid switching from a large to a small input range. Switching from a channel with a large input range to a channel with a small input range can greatly increase the settling time. In the current study, all channels have an input range of 1 V.

b. Minimize the voltage step between adjacent channels. When scanning between channels that have the same input range, the settling time increases with the voltage step between channels. If the expected input range of the signals is known, then similar expected ranges should be scanned together in groups.

• Avoid scanning faster than necessary

Scanning slower gives the system more time to settle to a more accurate level. There are two cases to consider:

a. Averaging large number of samples can increase the accuracy by reducing noise effects but it also decreases the required settling time. b. If the time relationship between channels is not critical, it is preferable

to scan the same channel multiple times and scan less frequently.

Technical Specifications and Analogue Input Circuitry

For more detailed information on technical specifications and analogue input circuitry of the data acquisition system, refer to Appendix D.

Scanning Settings and Channel Assignments

The data acquisition system was set to have a sampling rate of 100 Hz for each channel, and the samples were filtered by a built-in third order Butterworth low- pass filter, the filtered signals were then further averaged over every 50 sample. Finally two readings per second were obtained. This setting was maintained throughout all the calibrations and tests.

Channel assignments for the water and refrigeration system tests are given in Tables 3.2 and 3.3, respectively. The channel-sensor pairs must be kept unchanged for all calibration and measurement procedures.

Table 3.2: Channel assignment in water test

Physical

AI 0 Temperature sensor

No.1 Hot inlet

AI 1 Temperature sensor

No.2 Hot outlet

AI 2 Temperature sensor

No.3 Cold outlet

AI 3 Temperature sensor

No.4 Cold inlet

AI 4 Water flowmeter No.1 Hot water stream (at outlet) AI 5 Water flowmeter No.2 Cold water stream (at outlet) AI 6 Diff. pressure

transmitter

Cold water (between outlet and inlet)

Table 3.3: Channel assignment in refrigeration system test

Physical

channel Sensor Measuring quantity

AI 16 RTD No.1 Water outlet

AI 17 RTD No.2 Water inlet

AI 18 RTD No.3 Refrigerant inlet

AI 19 RTD No.4 Refrigerant outlet

AI 20 Refrigerant flowmeter Refrigerant flow rate AI 21 Pressure transmitter

No.1 Refrigerant outlet

AI 22 Pressure transmitter

No.2 Refrigerant inlet

AI 4 Water flowmeter No.1 Water flow rate AI 6 Diff. pressure

transmitter

Refrigerant (between outlet and inlet)