Three main types of digital measuring instruments----digital calipers, digital micrometers and digital indicators are more and more popular because of their high accuracy, multi-function and easy way of operating. Especially in recent years, with advent of digital instruments of new types, quality improved and prices dropped, Market demand for digital instruments is increasing rapidly.

Digital measuring instruments represented by digital calipers have been produced and used for more than ten years. At present, there are about ten factories manufacturing digital instruments in China.

The following is a brief guide on operation, calibration and maintenance for Digital Measuring Instruments represented by Digital Caliper and Digital Indicator.

A. Guide to purchase of Digital Calipers

At present, three-button digital caliper with manual on/off power switch is very popular in the market. Three buttons are respectively for Metric/Inch system interchange at any position, manual on/off power and zero setting at any position. Digital Caliper of this type can keep its origin unchanged when it's on /off at any position. LCD display screen is made of special quartz glass instead of ordinary organic glass and is anti-scratching and can undergo scratching of an ordinary knife. The modular structure makes it easy to maintain.

Digital calipers with many different functions are available.

Special functions of modules are as follows:
Data Hold: Press HOLD button to hold its momentary measured value when it's too dim to read.
Tolerance setting: Press TOL button to set tolerance range. The deviation value of limitation at the upper end and the lower end of the workpieces to be measured . It can show whether the measured value is up to grade or not. If the value is below grade, it can show the value is above or below the tolerance.
Tracing Maximum Value: With this function, it's easier to measure the inside diameter such as a hole.
Tracing Minimum Value: With this function, it's easy to measure the outside diameter such as an shaft.
Value presetting: Usually,when measuring the diameter of a hole with the caliper's upper jaws, the width of the closed upper jaws must be added to the measured value. The negligence in adding the width may cause rejects. The inside diameter of a hole can be obtained directly and conveniently by presetting the width of the closed upper jaws.

Kinds of digital calipers:
Digital Caliper with carbide tipped measuring faces: Its wear resistance is greatly enhanced.
Digital gear tooth caliper: It's designed to measure the thickness of a gear tooth.. The measuring face is tipped with carbide.
Screw thread digital caliper: It's used to measure the working and single pitch diameter of internal and external threads.
Inside groove digital caliper: It's mainly used to measure the diameter of inside groove.
Inside groove digital caliper: It's used to measure the width and position of groove in a hole.
Outside groove digital caliper: It's mainly used to measure the dimension of recess outside the workpiece or the thickness of an irregularly shaped board.
Digital thickness caliper: It's mainly use to measure the thickness of a soft object such as rubber and sponge etc. It's with a low measuring force device sometimes.
Offset digital caliper: It makes it easier to measure the stepped sections with a main scale jaw sliding up and down.
Center distance digital caliper: It's used to measure the center distance between two holes.
Universal groove digital caliper: It's designed mainly to measure the inside width of a universal groove and its relative position.
Plastic digital caliper: It's made of carbon-fiber plastic or other engineering plastic.
Digital calipers of various types mentioned above bring great convenience in many situations.
To buy or order special measuring tools, a sketch map of with specifications should be provided to the manufacturer for suggesting a proper tool.

B. Guide to choice and purchase of digital indicators

By resolution classification, two types are grouped: digital indicator and micron digital indicator.
According to its main structure, digital indicators can be grouped into round digital indicator and rectangular digital indicator. Most of the rectangular digital indicators are horizontal. You can make a selection at your option. More attention must be paid to the accuracy, because the accuracy is quite different if the manufacturer is different, especially for micron digital indicators.

C. Calibration

Standards for calibration:
GB/T14899-94 China National Standard for digital calipers
ISO/DIS13385-96 Geometrical Metrology-Length Measuring Instrument:
The design of Calipers (including dial calipers and digital calipers) and International Standard for measurement technology.
JB 6079-92: China National Standard for Digital Outside Micrometers:
GB/T 18761-2002 : China National Standard for Digital Indicator

D. Operating Instructions:

1) Before using, clean the surface of the protective sticker of digital measuring instruments for several times with dry, clean cloth to remove water that might congeal. Because water is weak electrolyte, the ionization of pH indicator and hydroxyl ion can be made. The ions will weaken or block capacitive signals to such extent that display will be in a state of disorder. So much attention should be paid to it in wet condition. At present, waterproof digital calipers have been put on the market, but it's high in price.

2) Working Environment: Temperature: 5-40 degrees; Relative humility: below 80%. Prevent liquid containing eletrolyte from wetting the protective sticker.

3) Never apply any voltage on a digital caliper or scribing on it with a electrical pen for fear of damaging its circuit

4) Zero Adjusting: Clean the measuring faces (by rubbing a clean piece of paper between measuring faces for several times) and then close the measuring faces. The measuring force is about 3-5N. Press Zero button to set zero, then begin the measurement. The pushing force for zero setting should be equivalent as far as possible to the measuring force in order to reduce measuring error.

5) Measuring Instructions:
Usually, Absolute measurement is taken. "Zero" is set as mentioned above.
Differential measurement is usually taken to improve the accuracy of measurement. First, place a gauge block which is near the dimension of workpiece to be measured (or other precision part ) between measuring faces and push with proper measuring force (3-5N). Press Zero button to set relative zero. Then measure the workpiece and read differential value ( the differential value is usually very small, that's why it is called differential measurement ). Add this differential value to the dimension of gauge block and you'll get the actual dimension (or called absolute dimension ) of the workpiece.

6) Data Outputting and Processing
At present, most digital calipers in China have a data output port for data outputting. It can be seen by taking off a small cap on the case of module . Meanwhile, it can be connected with a PC or a printer for data processing or for the printing of analytic chart of measured figures via special cable and interface (usually called adaptor ).
Interface format: Synchronous serial.
Data: Binary code, 24 bits. Each datum will be transferred twice. The cycle is 300ms (slow response ) or 20ms ( fast response). Transmitting time: 0.5ms.
Four wires (from left to right): Negative power(-),Data D ,Clock pulse (CP), Positive Power(+).
Pulse range of data: 0 Level=0.2V, 1 Level =1.3V.
Clock Pulse (CP): 90KHz, effective for high electrical level.

7) Replacement of battery
Abnormal display (digits flashing or even no display) shows a flat battery. A 1.55V silver oxide button cell (G13-A or SR44) should be replaced.
The reason that a battery bought from a market doesn't work satisfactorily lies in the automatic discharges of the battery and the wearing-down of its power. Attention must be paid to the production date when buying batteries.

E. Trouble shooting

The current digital calipers seldom go wrong. The problems which clients meet with usually result from its wearing-down of batteries or poor contact of battery seat. The following problems might accidentally happen:
1) "Frozen digits" ---- resulting from accidental trouble in circuit, the digits are frozen and locked , sometimes some irrelative signs are displayed. Solution: Take out the battery and put it back after over 30 seconds. It'll become normal after one or several trials.
2) Flashing digits---- resulting from low voltage. To get it solved, you need to replace the battery as soon as possible.
3) Figures displayed in disorder-----resulting from its unstable display. It results from the wet protective sticker. To get it solved, please refer to the above-mentioned operating instructions.
4) Mechanical wear:
The method of repair is about the same as that of a vernier caliper. But see to it that it must avoid forceful striking. Take off the electronic module in case striking is a must. Repair can also be done for more times than vernier calipers, because its origin can be set at any position. It's worth paying attention to the repair of upper jaws. As the measuring faces for step measurement, the left end of caliper is hardened, therefore the root of upper jaws is hard and fragile,which makes it difficult for the repair.