1.0    Routine servicing

Routine servicing or preventative maintenance of process equipment falls in to three areas:

Routine visual and mechanical inspection, typically every four or six months. 
#  Checking and adjustment of span and zero (recalibration) typically every six months. 
#  Troubleshooting on commissioned systems.

These recommended intervals should be monitored and governed by engineering institutions, taking into account the following factors:

#  Accuracy required for the application 
#  Frequency and manner of use 
#  Environmental influence 
#  Cost and risk

The intervals should achieve a balance between cost and risk. Where large errors are found in the performance and accuracy, the intervals should be shortened so as to achieve the optimum operation of the weighing equipment, if however there are no errors the intervals can be increased.

1.1   Routine Visual Mechanical Inspection

This should include visual inspection of all items of a process weighing system. Points to look out for are:

# Loose transducers fittings 
# Chafed or damaged cables 
# Rusty transducers fittings, especially built-in ball and cup and self aligning types 
# Loose junction box or indicator mounts 
# Rubbing or jamming lift-off preventers and jacking bolts 
# Any process material or objects, jammed in between weighed and non-weighed structure, transducer and base plate 
# Mechanical check on correct torque setting for all mechanical fittings should be made.

1.2   Checking And Adjustment Of Zero And Span (Recalibration)

The vessel or hopper should be emptied and the zero point noted. Apply known test weights or fill the vessel or hopper with known amounts of material. The reference value should be noted and compared with the accuracy statement or the initial installation performance of the system. Zero and span settings should be interactively adjusted to obtain true zero reading and accurate reference value( test weights etc.)

If feasible after any adjustments have been carried out, an evaluation test should be carried out to obtain reassurance that the weighing system is back to 'normal' performance.

Please note that many obvious and major faults together with some extremely obscure ones can be caused by the condition of the mechanical interface assemblies between input and transducers. As such, interfaces vary widely between different weighing systems. The following guide presumes a high degree of confidence in the correct functioning of the mechanical interface.

1.4   Equipment Required for Servicing and Trouble Shooting

Since the accuracy's of many transducers applications can be better than 0.1%, you will require good quality test equipment to establish potential transducer problems.

Assuming that a visual inspection has inferred a problem with the electrical side of a transducer application, then the next step is to isolate the problem to one of the following:

(a) The electronics of the application (amplifier, indicator or controllers 
(b) The cabling and junction box system. 
(c) The transducers

With the transducers still installed in the weighing application and using the accompanying troubleshooting   which illustrates a step by step procedure to identify a transducer problem. Item (a) of the above will not be covered in great detail as this procedure is directed at establishing the correct operation of the transducers system. 

1.6   Physical Inspection

A physical inspection of the transducers installation can give an indication to where the problems may be. If the transducer is covered with rust, corroded or badly oxidized and abrasions the action would be to start by monitoring the transducer circuit resistance. If however the physical inspection fails to identify any damage, a more detailed evaluation will be required.

All areas of the transducer are sealed to protect the internal circuitry of the transducer from contamination by water or chemicals, check that the sealing has not been degraded. Concentrate on the welded covers and cable gland. 

1.7   Monitor Transducers Excitation

Most transducers are powered by a 10V or 12V D.C. or an AC voltage (for transmitter it is 12V to 36V DC). check that the magnitude is correct within say ± 5%(Or against commissioning data if available). For electronics using +V and –V (as specified) check each value with respect to earth.

1.8    Transducers Cable Resistance

By disconnecting the transducers cable at the indicator or controller, you are able to perform core to core resistance checks looking for:

#  Positive excitation to positive sense = 5 Ohms per 100 meters for 16/0.2 cable. 
#  Negative excitation to negative sense = 5 Ohms per 100 meters for 16/0.2 cable. 
#  Positive output to negative excitation = 350 Ohms* + 5 Ohms per 100 meters of cable, divided by the number of transducers. 
#  Positive excitation to negative excitation = 350 Ohms*(changes as per TCR value)+5 Ohms per 100 meters of cable, divided by the number of transducers. 
#  Positive excitation to positive output, positive excitation to negative output, negative excitation to positive output and negative excitation to negative output all of these resistance should be of equal value within 2 to 3 Ohms

 * May vary dependent upon model

1.9   Monitor Transducer Resistance

Disconnect in turn the transducer cable at the local junction box and perform the following tests.

1.10   Impedance Check

Connect all the four or six transducers cable cores together, and monitor the impedance between the combination of the cores and the body of the transducers. The maximum voltage used for this test should be no greater than 50V DC ( minimum acceptable resistance is 500 mega ohms )

1.11   Bridge Resistance

With a digital mutimeter ( Not an Analog Meter) accurate to better than ± 0.5 Ohms, perform core to core resistance checks. With reference to the load cell manufacturers data sheet or calibration data.

Check the input and output resistance of each individual transducers, with reference to the manufacturer data sheet, and if the readings are outside the data sheet by 1% tolerance this usually indicates a damaged cell. If the overall input and output resistance tests are within normal specifications, you could still have a damaged cell. Often when a transducers is damaged by overload or shock weight, opposite pairs of resistors will be deformed by stress, but in opposite directions. The only way to determine this is to test each leg of the bridge to track down the exact location of the fault.

If any transducers fails these tests then it should be replaced with a new one.

1.12   Monitor transducers mV Output At Zero

Reconnect the transducers positive / negative excitation and sense cores to the electronics leaving the signal cores free to be connected to a Multimeter. Commissioning data is invaluable as a reference, if not available calculate the expected mV output value from the transducers. Check the zero load mili volt output by comparison either with commissioning data or transducers specification sheet. Typical industrial transducers should be better than +/-5% of full scale output at true zero load.

If the transducers under test displays a shift greater than 10%, it has probably been physically distorted, and should be replaced. 

1.13   Monitor Transducers mV Output With a Known Weight

Return the load cell to a loaded position, monitoring the mV output. This should be noted, then place a known weight onto the transducers under investigation. An increase in output should be observed, this change should be noted.

The mV change can then be calculated to give the sensitivity of the transducers, which can be compared with the manufacturers data sheet, or the commissioning data. A value of 2mV/V ± 0.1% is commonly used, but it can be any one of the following 0.5, 1, 2, 2.5 & 3 mV/V.

This will often only be a guide to the transducers sensitivity which can change when installed into a weighing application.

The above tests are carried out on each transducers in the application. 

1.14   Indicator / Controlled Checks

This is best performed with either a dummy transducers or variable milli volt source. Determine the correct and full operation of the indicator/controller with reference to the troubleshooting section in the manufacturer's handbook.