A Fully Automatic Multimeter Calibration System

NIMTSittakul/Jutarat Tanarom/Narat Rujirat and Ajchara Charoensook
Vitawat

A Fully Automatic Multimeter
Calibration System
Using Programmable Switch
ABSTRACT The obtained results confirm that the system
This paper presents a fully can automatically operate without any
automatic multimeter calibration system significant negative effects and the
using a programmable switch. The switch calibration time is reduced from 35 minutes
was specially designed to be programmable to 25 minutes.
as well as portable and it also was designed
16 to be low insertion loss and low thermal Keywords: meter, automatic system,
electromotive force (EMF). The switch can calibration
be programmed via a built-in micro-controller
chipset and can be directly controlled by 1. INTRODUCTION
May-June 2012/Vol.14, No.67

a personal computer via RS232 interface. Nowadays many measurement
Then the switch was inserted in the multimeter instruments have been used in all laboratories
calibration system. Finally, the results of fully throughout the world. Unfortunately, their
automatic calibration system were compared accuracies are mostly proportional to the
with that of the conventional manual system. time period. As time passes, they may
function incorrectly and generate some
errors. The mistaken results from such
instruments can cause serious problems in
economic system and life safety since they
will be used for validating product standards
in the importing and exporting industries. In
order to ensure that they work perfectly, the
calibration process is required. In the past,
the calibration has to be performed manually
and this process usually takes long time.
Presently, fully automatic calibration
systems have been used worldwide and they
play an important role in the calibration of
measurement instruments [1-5]. They can
improve measurement accuracy,
repeatability and minimize routine jobs. Also
they are ease of use, provide faster and
convenient operation. Moreover, total
uncertainties of such systems are basically
improved due to the fact that one of

uncertainty components caused by statistical
analysis (type A uncertainty) can be reduced
[6-8]. This uncertainty source is based upon
normal distribution and often results from
random contribution such as human errors
and repeated measurements. The human
errors may come from several factors where
human interface occurs, for example
connecting loose cable or reading wrong
data [2]. These considerations require
human experience and correct instruction
to avoid subjective judgment in cable
handling and meter reading.
Unfortunately, not all calibration
systems can run automatically. Based on
author’s knowledge, one of the problems is
that the calibration measurement instruments
themselves may not have computer interface 17

outputs to directly transmit measured data
to Personal Computers (PC). This may due could be operated fully automatic. To date,
to cost reduction or it was not initially similar devices known as multiple channel
designed by manufactures but this issue scanners may take this job but they have
currently can be solved using computer some limitations [12-15]. Most of them are
vision technology where the display screen not designed for particular jobs,
is captured on a digital camera in image unprogrammable, bulky and very expensive.
format and then performing image to data For example, they may not endure high
processing to obtain the measured data current, may not be portable, programmable
[1-5, 9-11]. Another problem is that there is and may contain high insertion loss and
necessity of cable re-arrangement even thermal EMF. These properties are very
though some measurement instruments can important for calibrating accurate
be fully controlled via either Recommended measurement instruments such as accurate
Standard 232 (RS232) interface or General digital multimeters [16].
Purpose Interface Bus (GPIB) interface In this work, the significant
standardized by IEEE-488. This is because contribution is the demonstration of automatic
the system configuration might have to be calibration system using implemented
changed during calibration processes. In programmable switch. After programmed,
these cases, a manual standard test method the switch functions as a cable router which
has to be applied and metrologists have to can re-configurate cables per manufacture
be standby entire calibration processes as calibration procedure and it can be controlled
a result labor cost is prohibitive. These by a PC via RS232 interface. The RS232
processes are very time consuming and as interface was chosen to reduce necessity of
mentioned earlier they increases human expensive GPIB chipset usage and
errors. Therefore, it is interesting if one could consequently the production cost was
design a low-cost programmable switch considerably reduced. The measurement
which can perform as a cable router for cable instruments were controlled by a PC via GPIB
re-arrangement. With the switch, a system interface. The programmable switch was

or industry companies could perform the
same test. However, in this work MET/CAL
7.2 software was used for the test due to its
availability, friendly interface and ease of
use. This paper is organized as follows.
Section 2 gives a basic concept of the
designed automatic calibration system.
Section 3 explains the designed components
of the programmable automatic switch.
Section 4 demonstrates the fully automatic
calibration system using the programmable
switch and the measurement results. Section
5 provides the summary.
Fig.1: Block Diagram of Automatic Calibration System
2. MULTIMETER AUTOMATIC
specifically designed to be low-cost as well CALIBRATION SYSTEM
as low in un-preferred negative effects due The principle of automatic system
to resistive, mutual coupling and potential can be seen in figure 1. The PC controller
effects. Therefore, the components inside equipped with GPIB and RS232 built-in
the switch had to be carefully selected to boards. The GPIB board is used to control
18 minimize these effects [16]. Finally, the the multimeter and the instrument standard
switch was introduced to the calibration (calibrator); a standard device to source
system designed for multimeter calibration. constant and accurate values of DC/AC
The commercial software (MET/ voltage, current and resistance.

CAL7.2) from FLUKE Company was used to These output values from the
automatically control the switch together with calibrator are generally used to compare
the measurement instruments via both GPIB with the reading values of the multimeter
and RS232 interfaces and the data were [Unit under Calibrator (UUC)] to ensure that
automatically recorded in the database. The they are still within the specification of the
measured data are the electrical quantities multimeter of the manufacturer. If the output
of multimeter which are electrical voltage, of the multimeter is out of range, the
current and resistance. Then, the results instrument adjustment according to its
were compared with that of a conventional manufacturer has to be performed. The
manual calibration system. It has to be noted RS232 interface is used to control the
that the system can be operated by the other automatic switch which functions to re-route
control software and thus other laboratories cable configuration wired between the
instrument standards, multimeter and the
automatic switch. After the calibration
procedure is end, the reading results are
sent back to the PC controller to compare
with the specification issued by the
manufacturer.

3. PROGRAMMABLE
AUTOMATIC SWITCH DESIGN
As mentioned earlier, the switch
has to be carefully designed to reduce un-
Fig.2: Block Diagram of Automatic Switch preferred effects. The block diagram of the
designed switch can be seen in figure 2.

Figure 2 shows the main parts of
automatic switch. The switch was designed
to consist of 8 input ports and 7 output ports.
The numbers of ports of the switch were
defined from the minimum port requirement
of calibration system. These input and output
ports could be modified in future to fit with
any different measurement system being
used. The switch can be divided into two
main parts; relay circuit and the
microcontroller circuit board. They will be
detailed in the following sections.
3.1 Relay Circuit Design
The relay circuit here was designed
as shown in figure 3. Here, eleven relays with
the same part number of FTR-H1CA012V
from Fujitsu component limited company
were used. They all have maximum insertion Fig.3: Block Diagram of Relay Circuit
losses of 100mΩ (measured at 1A and
6VDC) and endure the maximum current of
14A. The normal operation is at 12V with the 19

operation current of 10A. It can be seen that
the connection of circuit within the automatic Table 1: Commands for Switch Control
switch can be re-configurated by turning
operation of each relay ON and OFF. No. Commands Status Input to Output
For example, if the relay 1 is on, the 1. 1W1E LED 1 ON CH 1 to CH 1
input 3 will be connected to the output 1. If CH 2 to CH 2
the relays 5 and 11 are on, the input 4 will
be directly connected to the input 7 and thus 2. 1W2E LED 2 ON CH 1 to CH 1
the input short circuit between inputs 4 and CH 2 to CH 2
7 is achieved. It has to be noted here that CH 3 to CH 3
the number of relays has to be minimized to
reduce the insertion loss as much as 3. 1W3E LED 3 ON CH 1 to CH 1
possible. The material of the relay contact is CH 2 to CH 2
made of gold plate silver alloy which is the CH 3 to CH 1
same as that of the cables. This is to reduce CH 4 to CH 2
the thermal EMF effects which are caused 4. 1W4E LED 4 ON CH 1 to CH 5
by the use of different materials. Theoretically, CH 2 to CH 2
the thermal EMF is zero and independent to
temperature if the same materials are used. 5. 1W5E LED 5 ON CH 5 to CH 5
3.2 Microcontroller Design CH 2 to CH 2
The microcontroller circuit of the
switch was designed using an 8 bit flash 6. 1W6E LED 6 ON CH 6 to CH 5
microcontroller (P89LV51RB2) with 80C51 CH 7 to CH 2
CPU core. This allows specific commands 7. 1W7E LED 7 ON CH 6 to CH 7
to be programmed on the switch for different CH 7 to CH 2
operations. The switch operation commands
can be seen in the Table 1. 8. 1W8E LED 8 ON No connected

Fig.4: Block Diagram of Power Supply

Each command represents a with smoothing capacitors to supply voltages
specific set of cable arrangement. For of 5 and 12 V to the microcontroller board
20 instance, if the command <1W7E> is sent and the relay circuit respectively. The dual
to the switch, the input channels 6 and 7 will transformers were used to down convert the
be automatically connected to output home voltage from 220V. The full-wave
channels 7 and 2 respectively. These bridge rectifier has been used here since it

commands can be re-programmed in the provides good output power stability which
microcontroller regarding what the is necessary for calibration.
measurement system performed. In the 3.3 Switch Assembly and Testing
microcontroller board, Light Emitting Diodes All components in the block
(LEDs) were included so that the operation diagram in figure 2 were brought together
status of the switch could be monitored. into a switch module as shown in figure 5. It
The power supply was designed can be seen that all components were
as can be seen in figure 4 and it was assembled in a plastic block orderly to
implemented to be full- wave bridge rectifier reduce the coupling effects from metallic
materials.

4. EXPERIMENT AND
EXPERIMENTAL RESULTS
In this section, the programmable
switch was employed in the multimeter
calibration system in the laboratory of
National Institute of Metrology (Thailand)
[NIMT] under controlled conditions
(Temperature 23+/- 2 degree and Humidity
50+/- 15%). Figure 6 shows the configuration
of automatic calibration system using the
programmable switch.
The configuration was set
Fig.5: Automatic Programmable Switch according to the block diagram in figure 1

and Unit under Calibration (UUC) service Table 2: Measurement Results
manual where the instrument standard in this
Range Applied Input UUC Reading Manual Automatic switch
test is the calibrator (Fluke 5720/5725) and
DC Voltage
the UUC is the multimeter (Fluke 45). The
calibration procedures was written and 0 mV 0 mV -0.003 mV -0.003 mV
programmed to the commercial calibration /90 mV 90 mV 89.990 mV 89.991 mV
software (MET/CAL 7.2). The calibrator /900 mV 900 mV 899.94 mV 899.94 mV
sourced AC/DC Voltage, AC/DC Current and /3 V 3V 2.9997 V 2.9997 V
resistance and the reading data from the /30 V 30 V 29.995 V 29.995 V
multimeter were recorded and saved to the /300 V 300 V 299.95 V 299.95 V
PC. Each measurement data point was
/1000 V 1000 V 999.88 V 999.88 V
averaged from five raw data points. To
AC Voltage
check the effects of the use of automatic
switch in the system, the comparison test /30 mV 15 mV @ 1 kHz 14.994 mV 14.993 mV
between the manual and automatic systems 15 mV @ 100 kHz 13.985 mV 13.984 mV
were performed. The results can be seen in /300 mV 300 mV @ 1 kHz 299.78 mV 299.77 mV
the Table 2. The results reveal that both 300 mV @ 100 kHz 295.06 mV 295.10 mV
measured reading values are close to the /3 V 3 V @ 1 kHz 2.9977 V 2.9977 V
applied inputs and they are within the /30 V 30 V @ 1 kHz 29.976 V 29.976 V
mulitmeter specification [17]. Moreover, the
system with the switch can operate without
/300 V 300 V @ 1 kHz 299.84 V 299.84 V 21
/1000 V 750 V @ 1 kHz 749.82 V 749.81 V

any negative effects. Both measurements
are very similar and the variation of the Resistance
measured data comes from the standard 0Ω 0Ω 0.00 Ω 0.00 Ω
deviation of statistic. It can be seen that the /300 Ω 190 Ω 190.05 Ω 190.04 Ω
resistance reading values for 0Ω is exactly /3 kΩ 1.9 kΩ 1.9001 kΩ 1.9000 kΩ
zero since the cable and relays insertion /30 kΩ 19 kΩ 18.997 kΩ 18.996 kΩ
losses were zeroed by the meter. The /300 kΩ 190 kΩ 189.97 kΩ 189.97 kΩ
calibration time were reduced from 35
/3 MΩ 1.9 MΩ 1.8999 MΩ 1.8998 MΩ
minutes to 25 minutes since the time of
/30 MΩ 19 MΩ 19.0 MΩ 19.0 MΩ
manual cable arrangement was removed.
DC Current
0 mA 0 mA -0.0003 mA -0.0003 mA
/3 mA 3 mA 2.9996 mA 2.9995 mA
/30 mA 30 mA 29.997 mA 29.997 mA
/100 mA 100 mA 99.982 mA 99.981 mA
/1A 1A 0.9990 A 0.9991 A
/10 A 9A 8.9995 A 8.9997 A
AC Current
/3 mA 3 mA @ 1 kHz 2.9982 mA 2.9981 mA
/30 mA 30 mA @ 1 kHz 29.986 mA 29.985 mA
/100 mA 100 mA @ 1 kHz 99.953 mA 99.951 mA
/1A 1 A @ 1 kHz 1.0004 A 1.0004 A
/10 A 9 A @ 1 kHz 9.0021 A 9.0023 A

Calibration Time 35 minutes 25 minutes
Fig.6: Automatic Calibration System

5. SUMMARY for Evaluating and Expressing the uncertainty
A fully automatic calibration system of NIST Measurement Results, NIST
using a programmable switch has been Technical note, pp.1297, 1994.
successfully demonstrated. The switch can [7] NIS-81, The Treatment of Uncertainty in
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negative effects for calibrating the multimeter Laboratory, 1994
(Fluke 45) using the source Calibrator (Fluke [8] International Organization for Standardization /
5720/5725). The calibration time is reduced International Electrotechnical Commission,
from 35 minute to 25 minute. This is because ISO/IEC Guide 17025 General requirements
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6. ACKNOWLEDGEMENT Air-conditioning Control System”,
The work was co-funded by the Proceedings of CIBSE National Conference,
National Institute of Metrology (Thailand) and Computers in Construction Industry, pp.
Thai government. The authors would like to 345-355, 1993.
acknowledge and appreciate their help. In [10] W. L. Chan, L. S. L. Pang, C. F. Ma,
addition, the authors would like to thank for “Computer Vision Applications in
the best support of the members of the PowerSubstations”, Proceedings of the 2004
22 electrical working instrument laboratory; Mr. IEEE International Conference on Electric
Adithep Jang-on and Mr. Kongsak Tongboon. Utility Deregulation, pp. 383-388, 2004
[11] L. S. L. Pang, “The First Successful
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A Fully Automatic Multimeter Calibration System

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