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Training

Training. Making accurate Electrical Measurements. Introduction. Sources of errors in electrical measurements Understanding instrument specification Getting the best from a Digital multimeter and Calibrator. Making good connection.

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Training

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  1. Training Making accurate Electrical Measurements

  2. Introduction • Sources of errors in electrical measurements • Understanding instrument specification • Getting the best from a Digital multimeter and Calibrator

  3. Making good connection • Large errors in measurements can be easily caused by using the incorrect leads. • The key measurements areas where leads effect accuracy are:- Low DC voltage High Value Resistance Low AC current Low resistance 4 wire connections

  4. Measuring Low DC Voltage • When measuring DC voltage below 1volt the main source’s of error are thermal generated voltages (emf) in the lead connections. These voltages which can easily be tens of micro volts constantly change as temperature gradients change. • Thermo electric voltages occur when ever 2 different metals are in contact with a temperature gradient across the contact. This is how a thermocouple works.

  5. Reducing Thermal EMF 1 • There are three ways to reduce EMF:- Firstly and most important Use connectors made with metals which produce low thermal voltages, like gold, silver. Avoid connectors made, or plated with materials like nickel, which is often use to plate ‘banana’ type plugs. Note nickel is used in ‘K type’ thermocouple and can produce up to 33uV/C. The terminals in precision instruments are usually made with oxygen free copper, and gold plated and if connected to a similar material the EMF voltage will be less than 1uV/C. The Transmille precision lead set contains one pair of safety 4mm to 4mm low thermal leads.

  6. Reducing Thermal EMF - 2 • There are three ways to reduce EMF:- Second Avoid large temperature gradients across the measurement area. These can be caused by strong drafts of cold or warm air coming from air condition units, the air from such can be several degrees different. Also take care with sun light through a window or a instrument or soldering iron causing local heating. Try to obtain a stable environment and leave connection to thermally stabilise before zeroing the system. A calibrator or DMM which was last used on high current may take many minutes to settle down.

  7. Reducing Thermal EMF - 3 • There are three ways to reduce EMF:- Thirdly Make sure there is the minimum number of connection between source and measurement instrument. Do not extended leads by plugging together. Try to keep leads short and close so they will be at same temperature.

  8. Reducing Thermal EMF - 4 • After you have done all you can to reduce the EMF repeating the measurement several times can give a better measurement as the effects of thermals are usually random. With some measurements it is possible to reverse leads and take the difference, but note that holding the connectors will warm them up, even the friction caused by plugging in causes heat. • A practical limit is around 0.5uV and this is a common value to used in uncertainty calculations. Some manufactures to help when calculating uncertainties including Transmille try to give a figure for this as in part it is the interaction between the instrument terminal and the lead and in practice it will always be present to some degree.

  9. Making Accurate High Value Resistance Measurements - 1 • There are 3 problems measuring resistance above 100kohm. Firstly is lead insulation resistance. Most test leads are made out of PVC type plastic which is not a good insulator. A typical figure for insulation for this type of test leads could be between 50 and 100Gohms. Even dirt on the leads will reduce insulation. Test leads with an insulation resistance of 100Gohm will give an error of 1% when measuring 1Gohm, and 0.1% on 100Mohm. Note that Insulation resistance is not connected with voltage/current rating of the lead. Screened leads made from PTFE are the best, however this is brittle and brakes very easily. Almost as good is 50 Ohm Coax with a polythene insulating material.

  10. Making Accurate High Value Resistance Measurements - 2 Second problem is noise pick – up in the test leads and the resistor itself. There is always present a strong field from the Power line and this can easily swamp out the true signal. As this pick up is AC it presence may not be noticed, all that may be seen is a more noisy reading. This pick up will not be a nice sine wave but a distorted signal, which when averaged out by the DMM’s filter will give an offset which will also result in a reading error A 1Gohm resistor measured with a DMM at 10 volts will only have a current of 10nA flowing in the test leads, a noise pick up of just 10 Pico amps will be 0.1% noise on the reading. It is therefore very important that screened leads are used, and the case of the resistor is earthed.

  11. Making Accurate High Value Resistance Measurements - 3 To overcome, or at least reduce the lead and pick up problem it is very common to measure high value resistors at higher voltages to obtain higher currents to measure. The voltage is generated by a calibrator and the current is measured by a Pico amp meter or electrometer. Transmille 2000 series can also be used for this measurement. This however introduces the effect of ‘voltage coefficient’ in the resistor where the actual value of the resistor changes with the applied voltage, this is not due to self heating as in a power resistor but due to ‘voltage stress’ and is an instantaneous effect. It may be necessary to characterise the resistor for its voltage coefficient.

  12. Low AC Current Measurements • The main problem with this is current leakage to earth through capacitive coupling through mains transformers, capacitance to case etc, in fact anywhere there is an un guarded path to earth. Note this is not a DC path but an AC path which is frequency dependant. Again Pick – up is also a problem, use screened leads but keep the distance short as the capacitance of screened cable is high and this will again leak some current away.

  13. Low value Resistance • To measure low ohms a 4wire Kelvin connection must be used, otherwise the resistance of the connections & leads used will also be measured. • Kelvin connection work by using separate leads for the measurement current and for measuring the voltage drop across the resistor. Any voltage dropped along the leads carrying the current is not measured, and there is no current flowing in the voltage sense leads the resistance of the leads will make no difference. • Thermal EMF in the voltage sense leads will make a difference and the same precautions as used for any low DC voltage measurement must be taken. Note some DMM’s can automatically compensate for thermal EMFs by turning the measurement current on and off and measuring the voltage difference.

  14. Instrument Specifications. • Different manufacturers present specifications in many different ways. Often the different word is term is used for the same thing. • Basically however all specification come down to the same 3 terms and a temperature coefficient 1: A percentage of the actual Value Sometime called % or ppm of reading or output 2: A percentage of the Full scale of that range Sometime called % or ppm of range or full scale 3: A fixed allowance – or Floor Often expressed as a number of digits or counts, Or directly in units, example 3uV

  15. Instrument Specifications - 1 • To work out the allowance at any point simple work out each value and add them up. Note that the ‘counts’ or ‘Digits’ refers to the last digit in the display – the resolution • Example Measuring 1 volt on a DMM with a spec of .05% of reading +3counts • 1 Volt at 0.05% = 5mV • 3 counts on 1.999V display = 3mV • Total error allowed = 8mV

  16. Instrument Specifications - 2 • PPM and Percentage • These terms work in the same way, but percentage is based on 100, ppm is based on one million, • (ppm = parts per million) • 0.01% = 100ppm

  17. Instrument Specifications - 3 An instrument accuracy specification is based on 3 factors • The instruments change with time • The Design which may limit initial accuracy • The uncertainties to which the laboratory calibration can be performed The specification normally provided by the manufacturer are 1 and 2 combined, this of course requires that the manufacture supplies also the time, often one year that the specification applies for. This specification is relative to calibration standards, which means no account has been taken of the uncertainties to which the instrument has been calibrated to. Absolute accuracy can be calculated by adding this figure to the uncertainty to which the calibrator is certified. Transmille belive it is unhelpful for the manufacture to give an absolute figure based on the manufactures own laboratory, as this limits the choice of the customer to select the calibration source.

  18. Getting the best out of modern high performance DMMs and calibrators • Leave on in a stable environment • Run any auto-cal / self cal before use • Connect using the best leads for the measurement • Select the best range and settings • Null / zero any offset errors first • Allow time to stabilise after use on high power ranges

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