4–20 mA signals
Pico products for measuring 4–20 mA signals
Pico has several products suitable for measuring and recording 4–20 mA signals, but the input circuit has to be slightly modified.
A simple shunt resistor can be used to convert the current in the loop to a voltage that is suitable for the ADC to measure. A 250 Ω resistor will give a voltage output of 1 to 5 V. This method can be used in systems where the signal can be grounded.
Other resistor values can be calculated using the formula below:
Rb = Vmax / Imax
where Vmax is the maximum input voltage of the ADC, Imax is the maximum measured current and Rb << Rin.
Other information
Pico has four products where this resistor can easily be placed on a terminal board:
Acceleration
Pico products for measuring acceleration
The TA487 IEPE Signal Conditioner: For use with IEPE accelerometers. Powered by USB it can supply a constant current of 4 mA to the sensor. Simply attach the TA487 to the BNC connector on your chosen scope, plug in the BNC accelerometer, and use like a normal scope channel. We can supply a suitable single-axis accelerometer — the TA095 — with a ±50 g measurement range.
When used with the TA487 IEPE Signal Conditioner, the PicoScope 4000A Series and PicoScope 5000D Series oscilloscopes are our recommended ranges of oscilloscopes for measuring acceleration.
However, if you need to use silicon accelerometers, they are often 10 V bridge-type sensors that require a 10 V excitation voltage and produce a millivolt output. An additional precision 10 V power supply is required when using silicon sensors with Pico products.
Types of accelerometer
Piezoresistor
A piezoresistive sensor uses a piece of material whose resistance changes when it is compressed, attached to a weight. When the weight is accelerated, it exerts a force on the piezoresistor. If a constant current is passed through the piezoresistor, the voltage changes. Current is about 4 to 8 mA and voltage is 8 to 24 V. Typical sensitivity is about 100 mV/g over the range 0 to 50 g. This type of sensor responds to frequencies up to 10 kHz.
Piezoelectric
A piezoelectric sensor generates charge when it is accelerated: typically 50 pC per g. It is necessary to integrate the charge to give a voltage which is related to the acceleration: this means that it is not suitable for low-frequency work, but piezoelectric sensors respond to frequencies up to 30 kHz.
Silicon bridge
A silicon bridge sensor is a piece of silicon that has been etched to leave a block of silicon at the end of a beam. When subjected to acceleration, the block exerts a force on the beam and the resistance of the beam changes. Maximum frequency is about 5 kHz. The sensor is a bridge, and so it requires an excitation signal of 5 to 10 V. Temperature compensation is required.
Micromachined silicon
Micromachined silicon accelerometers are a form of differential capacitor. One of the advantages of this type of sensor is the ability to measure DC acceleration (and consequently tilt). The maximum frequency is about 1 kHz. The popular Analog Devices ADXLxxx range of single and dual-axis sensors have built-in signal conditioning circuits that produce a voltage output suitable for use with our data loggers and oscilloscopes.
Voice coil
Voice coils work on the same principle as microphones, hence the name.
Audio signals
Pico products for measuring audio signals
For measuring high-quality audio signals and for audio spectrum analysis the PicoScope 4000 Series precision oscilloscopes are ideal. For less demanding applications, the lower cost PicoScope 3000 Series can also be considered.
Other information
The PicoScope software includes common audio measurements such as THD, SINAD and SFDR. It is included with all our oscilloscopes and data loggers.
We also have the following application notes on audio measurement:
Battery discharge
Pico has several products suitable for recording battery discharge. They all connect to a USB port on the computer.
- PicoLog 1012: This 12 analog input channels. The input voltage range is 0 to 2.5 V and the resolution is 2.5 mV. This device is suitable for measuring multiple channels at higher speed.
- PicoLog 1216: This has 16 analog input channels. The input voltage range is 0 to 2.5 V and the resolution is 625 µV.
- ADC-20: This has 8 single-ended or 4 differential high-resolution analog inputs. The input voltage range is –2.5 V to +2.5 V and the resolution is about 5 µV. This device should be used if more precise measurements are required at a slower speed.
Accessories
Terminal connector boards are available for the PicoLog 1000 Series and ADC-20 data loggers.
Application notes
Using an ADC-16 to monitor battery discharge
Current
The Pico range of current clamps allow current to be measured without having to break into the circuit. All of them can be used with any of our data logging or oscilloscope products:
- The MI077 and TA019 current clamps can measure AC or DC current up to 600 A.
- The TA009 and TA018 current clamps can measure AC or DC current from 10 mA to 60 A.
- The TA015 current clamp can measure AC current up to 1500 A and DC current up to 2000 A.
The Pico Current Monitoring Kit contains current clamps, power monitor, data logger and everything else you need to start logging currents from up to three separate circuits. It is ideal for measuring and balancing three-phase power supplies as well as machine monitoring and energy efficiency studies.
The PicoLog CM3 Current Data Logger is a three-channel data logger that can measure alternating currents of up to 200 A. The PicoLog CM3s features both USB and Ethernet interfaces and multiple units can be used on a single PC.
For small currents, a simple shunt resistor can be used to convert the current into a voltage, which the ADC can then measure. This can be done providing the signal can be grounded.
The resistor value can be calculated using the formula below:
Rb = Vmax / Imax
where Vmax is the maximum input voltage of the ADC, Imax is the maximum measured current and Rb ≪ Rin.
WARNING: This method is NOT suitable for monitoring mains currents. To monitor mains currents with data acquisition or oscilloscope products, use a current clamp.
Pico has four products where this resistor can easily be placed on a terminal board:
Flow
Flow is commonly sensed by measuring differential pressure across two points in a pipe. This can be done using the Venturi effect (by placing a restriction in the flow). An alternative approach is to use a Pitot tube. The main advantage of this type of approach is that disturbance of the flow can be kept to a minimum. One disadvantage is that two holes are usually required in the pipe, making cleaning difficult. Also be aware that many differential pressure sensors are intolerant to aggressive gases and chemicals. The method for measuring these sensors is described in the section on pressure sensors.
For applications where pipes regularly need cleaning, consider using a bending vane type of sensor. As the name suggests, this consists of a vertical vane that deflects as flow increases. This deflection is measured using a strain gauge. The method for measuring such sensors is covered in the section on strain.
‘Paddle wheel’ sensors rotate in proportion to flow. The rotation is detected by either optical or magnetic means. These sensors produce a pulsed output. The main advantage of such sensors is low cost, and some are also suitable for measuring aggressive gases and liquids. The main disadvantage is disruption to the flow. For information on interfacing to such sensors, see measuring frequency.
Ultrasonic and magnetic flow sensors allow flow to be measured with no moving parts. This minimizes (or eliminates) disturbance to flow and provides for increased reliability. The main disadvantage is cost. These sensors tend to have built-in signal conditioning with either voltage or 4–20 mA current loop outputs.
Food temperature
The USB TC-08 is our dedicated thermocouple unit. It accepts up to 8 thermocouples using standard miniature thermocouple connectors. It can accept all the standard thermocouple types: B, E, J, K, N, R, S and T. The channels can also be used as voltage inputs with a range of ±70 mV. It connects to the PC via a USB port and up to 20 USB TC-08 units can be used on a PC at once.
Application notes
Frequency
Many Pico products can be used to measure frequency. The choice of device is dependent on the frequency range, the voltage input range and the number of channels required.
There are four possible measurement requirements:
- Logging frequency variations over time: PicoLog can be used to record fluctuations in frequency over time.
- Measuring spot frequencies: PicoScope’s automatic measurements can be used to take spot frequency measurements and to show variation statistics.
- Investigating the frequency components of a signal: PicoScope’s spectrum analyzer shows the frequency spectrum of a signal, allowing the measurement of any frequency component within the signal.
- Viewing frequency variations over time: When used with the PicoScope 4000 Series 12-bit scopes, PicoScope can be used to graph fluctuations in frequency over time.
Light level
The DrDAQ Data Logger has a built-in light sensor, so can be used as a low-cost light meter or light-level data logger. In addition, the light sensor has a fast response time so can be used to investigate fast-changing light signals such as those produced by tube lights.
The circuit below can be used with any of our oscilloscope or data logging products to measure light levels. The circuit uses an LDR to sense light level and converts this to a voltage using a potential divider network.
Ra should be around 1 MΩ.
Oxygen in air
The DD103 oxygen sensor can be connected to the external sensor sockets on the DrDAQ Data Logger to measure oxygen in air.
Unlike previously available oxygen sensors, the DD103 oxygen-in-air sensor can measure the full 0 to 100% range. This makes it ideal for many chemistry, biology and physics experiments.
pH
The DrDAQ Data Logger has a dedicated pH input. Optional pH electrodes are also available. DrDAQ measures pH over the full 0 to 14 range with a resolution of 0.02 pH.
Despite the low cost of DrDAQ, options are provided for calibration and temperature compensation, allowing very accurate pH measurements.
The circuit below allows any of our oscilloscope and data logging products to monitor signals from pH probes. The op-amp needs to have a very high input impedance — an LT1114 is suitable.
Pressure
Most pressure sensors are ‘10 V bridge’ type that require a 10 V excitation voltage and produce millivolt outputs. An additional precision 10 V power supply is required to provide this excitation voltage when using this type of pressure sensor with any of our products.
Note that some pressure sensors have signal conditioning built in. These sensors usually have a voltage output or a 4–20 mA output. See the appropriate sections in this guide for information on measuring these signals.
Redox
Redox can be measured using the USB DrDAQ Data Logger together with a DrDAQ redox sensor.
The redox sensor (also known as an ORP sensor) can measure redox potential in the range of –1500 mV to +1500 mV. Positive readings indicate an oxidizing agent (addition of oxygen), whilst negative readings indicate a reducing agent (reduction of oxygen).
Resistance
Pico has two products that can be used for measuring and recording resistance:
Other products
Other Pico products can also be used to monitor resistance. This is achieved using a precision voltage reference and a known resistance. The two resistances are connected in series and fed by the precision voltage source. The voltage developed across the unknown resistor can then be measured and used to infer the resistance.
Pico has two products where the resistors and voltage source can easily be placed on a terminal board:
- ADC-20 with terminal board: can monitor 8 channels with high accuracy.
- PicoLog 1000 Series with terminal board: can monitor 12 or 16 channels at higher speed.
Sound level
- The DrDAQ Data Logger has a built-in microphone that can directly measure sound level over the 55 to 100 dB range. The low cost of DrDAQ makes it ideal as either a sound-level meter or sound-level data logger.
- PicoScope 4224 IEPE Oscilloscope: The ideal instrument for use with a phantom-powered, calibrated microphone, as it has a built-in IEPE power output. Just plug in the microphone and use PicoScope like a normal scope.
Speed of a car
One of our series of educational technical notes, this experiment looks at measuring the speed of a car. (Unfortunately due to budget restrictions a rather small car had to be used!)
Strain, force and load
The strain gauge is perhaps the most popular sensor for measuring force and deflection. As a strain gauge is stretched or compressed, its resistance changes. By mounting the strain gauge on a calibrated carrier, force can be indirectly measured. Such a sensor is commonly referred to as a load cell. Load cells consist of one or more strain gauges configured in an industry-standard ‘10 V bridge’ arrangement. Sensitive load cells are used in weighing scales, while at the other extreme heavy industrial load cells can be used to measure loads of several tonnes.
As mentioned, most load cells are ‘10 V bridge’ types that require a 10 V excitation voltage and produce millivolt outputs. An additional precision 10 V power supply is required to provide this excitation voltage when using this type of pressure sensor with any of our products.
For accurate measurement of up to 8 slowly changing signals, use the ADC-20 or ADC-24 data logger, or the TC-08 thermocouple and voltage data logger.
For rapidly changing signals, use one of our precision oscilloscopes such as the 12-bit PicoScope 4224.
Temperature
Temperature is the most commonly measured real-world signal. We have several products dedicated to measuring temperature. In addition, if you wish to monitor a mix of temperatures and other parameters, our data logging products provide a simple plug-and-play solution.
Application notes
See also
Voltage
The majority of Pico products can be used for measuring voltage. To ensure you choose the correct product you must consider the following:
- How many voltages (channels) need to be measured
- How big (or small) are the voltages
- How fast the signals change
- How long you wish to record the voltage for
How many voltages (channels) need to be measured
If your requirement is to measure a large number of channels, consider the PicoLog 1012 (12 channels) or the PicoLog 1216 (16 channels). If more channels are required then it is possible to use multiple ADC units on the same PC to give very high channel counts. If you have a number of voltages to record over a wide area, then the EnviroMon networked data logging system can measure up to 30 channels per logger.
How big (or small) are the voltages?
Most of our data logging products have fixed input ranges (2.5 V or 5 V). These can be easily increased through the use of simple potential divider circuits. Our oscilloscope products have software selectable ranges (10 mV to 100 V).
If you wish to measure high voltages then the range of our oscilloscope products can be extended to 1000 V using suitably rated x10 scope probes. For higher voltages, and high-current supplies such as mains (house current), we recommend the use of one of our oscilloscope products with an isolating x100 differential scope probe.
If you wish to measure small voltages, you need to consider the input range of the device and also the resolution:
ProductInput rangeResolutionSampling rateLSB voltage*PicoLog 10120 to 2.5 V10 bits1 MS/s2.5 mVPicoLog 12160 to 2.5 V12 bits1 MS/s625 µVUSB TC-08±70 mV20 bits10 S/s0.067 µVADC-20±2.5 V20 bits16 S/s4.8 µVADC-24±2.5 V24 bits16 S/s0.298 µVUSB PT-1040 to 2.5 V24 bits1 S/s0.156 µV
How fast the signals change
If your signals have frequency components above 1 kHz then consider our oscilloscope products. If all your signals are lower than 1 kHz you can use either our data logging or oscilloscope products.
How long you wish to record the voltage for
If you wish to record voltages for long periods of time (more than say 5 minutes), use one of our data loggers. For shorter periods of time, use a PicoScope.
