How to Do Routine Maintenance for Instruments

The routine maintenance of instruments is a very important task, which is an indispensable part of ensuring production safety, stable operation and other aspects. It reflects the concept of Total Quality Management and Prevention First. Instrument maintenance staff should do a good job in the routine maintenance of instruments to ensure their normal operation.

The routine maintenance of instruments generally includes the following four tasks.

How to Do Routine Maintenance for Instruments

Routine Maintenance Steps for Instruments

Patrol Inspection

The maintenance staff select the best patrol inspection route based on the distribution of instruments in their responsible area, and conduct at least one patrol inspection every day. During the patrol inspection, they should be consult the operator in the control room on duty to understand the operation of the instruments and keep a record of the inspection.

  1. Check whether the instrument indications and records are normal, whether the on-site primary instrument (transmitter) indication is consistent with the control room display instrument and regulating instrument indication, and whether the output indication of the regulator is consistent with the valve position of the regulating valve;
  2. Check whether the instrument power supply voltage is within the specified range and whether the air source (0.14MPa) reaches the rated value;
  3. Check the insulation and heat tracing conditions of the instruments;
  4. Check the damage and corrosion of the instrument body and connecting parts;
  5. Check the condition of the instrument, that is, the nameplate should be clear and correct, the components should be intact and complete, the fasteners should have good contact, the terminal wiring should be firm, the adjustable parts should be in the adjustable position, and the sealing parts should be leak free.
  6. Check the equipment and environment to ensure they are neat, clean, and meet the work requirements. The entire machine should be clean, free of rust, the paint layer should be flat, bright, and free of peeling. The instrument pipelines and circuits should be laid neatly, and the pipeline and circuit markings should be complete, clear, and accurate.

Periodical Blowdown

Pollutant Discharge

Pollutant discharge mainly targets instruments such as pressure transmitters and differential pressure transmitters. Due to the presence of dust, oil scale, small particles, etc. in the measuring medium, which deposit in the pressure pipe (or in the pressure valve), it directly or indirectly affects the measurement.

Blowing and Washing

Blowing is a method of using air or liquid to prevent direct contact between the measured medium and instrument components or measuring pipelines, in order to protect the measuring instrument and implement measurement.

The following precautions should be taken when blowing:

  1. The blowing gas or liquid must be a flow medium allowed by the tested process object, not undergo chemical reactions with the process medium, clean, free of solid particles, non corrosive, and with good fluidity;
  2. The supply source of cleaning liquid should be sufficient and reliable, not affected by process operations;
  3. The pressure of the blowing fluid should be higher than the maximum pressure that can be reached at the measurement point during the process, ensuring that the blowing fluid is continuously and stably blowing according to the designed flow rate;
  4. Use a flow limiting orifice or a rotor flow meter with adjustable resistance to measure and control the flow rate of the blowing liquid or gas;
  5. The inlet point of the flushing fluid should be as close as possible to the instrument source component, in order to maintain the minimum pressure drop generated by the flushing fluid in the measurement pipeline;
  6. In order to minimize measurement errors as much as possible, it is required that the flow rate of the blowing fluid must be constant.

Insulation and Heat Tracing

Checking the insulation and heat tracing of instruments is one of the routine maintenance tasks of instruments. It is related to energy conservation, preventing instrument damage from freezing, and ensuring the normal operation of the instrument measurement system. It is an important task that cannot be ignored in instrument maintenance.

In winter, during patrol inspections, instruments installed on process equipment and pipelines should be checked, such as electromagnetic flow meters, flange differential pressure transmitters, and control valves. They should be checked to see if the insulation material has fallen off and if it has been dampened by rainwater, causing the insulation material to become ineffective.

Regular Verification

Our factory’s instruments are managed in categories A, B, and C.

Class A is a mandatory inspection instrument, which is calibrated by external units;

Class B instruments are important process control points, and periodic calibration can be divided into two categories: maintenance calibration and periodic calibration. During the maintenance period, instruments that are difficult to disassemble during operation can be arranged for disassembly and inspection. Regular calibration should be conducted within the validity period for inspection;

Class C is a general indicating instrument. Most of the instruments in our factory are disposable confirmation meters, and under normal circumstances, they do not need to be calibrated.

Routine Maintenance Procedures for Instruments

Pressure Transmitter


The pressure transmitter should be periodical blowdown:

  1.  For transmitters used to measure liquid pressure, the number of discharge cycles shall not be less than once 2 days;
  2. For transmitters used to measure gas pressure, compressed air should be used to regularly blow the pressure pipe, with a frequency of no less than 1 time/2 days.

When operating the differential pressure transmitter, it is necessary to strictly follow the on/off procedure of the three-valve manifold to avoid damage to the transmitter sensor due to one-way over-pressure. The operation of the three-valve manifold is as follows:

  • When transitioning from working state to debugging state, first close the low-pressure side globe valve, then open the balance valve, and finally close the low-pressure side globe valve;
  • When transitioning from debugging state to working state, first open the high-pressure side globe valve, then close the balance valve, and finally open the low-pressure side globe valve;
  1. For pressure transmitters measuring steam or high-temperature liquids, avoid discharging all the cooling liquid in the pressure pipe during discharge. Otherwise the measurement medium with high temperature will damage the sensor.
  2. For pressure transmitters measuring high-pressure medium, safety must be taken into account during discharge.

Electromagnetic Flow Meter

sanitary electromagnetic flow meter

  1. Check the moisture resistance inside the terminal boxes of the flow meter at least once a year. If the color of the silicone gel changes, it should be replaced. But if there is suspicion of water immersion, check the instrument;
  2. Tighten the pipe connection nuts twice a year;
  3. Regularly inspect the electrodes and lining to see if the tested liquid will adhere or cover them. Check if the lining is damaged due to corrosion,remove dirt and scale on and near the electrode, use a soft brush or cloth to remove dirt, and do not scratch the inner lining.

PH Meter

JCMIK-PH3.0 Online PH Meter

  1. The dissolved oxygen probe should be gently cleaned with water regularly every week. If any damage is found to the membrane head, it should be replaced in time. If the electrolyte is contaminated, it should also be replaced in a time.
  2. The method for testing the dissolved oxygen probe is very simple. After removing the membrane head, COS4 has two electrodes. The top gold electrode (measuring electrode) must turn yellow, while the silver electrode (counting electrode) must turn bright, otherwise it should be cleaned or regenerated. If the probe is good, the probe without a film head must display zero as much as possible. At this time, both the electrolyte and the new membrane head can be replaced. If the temperature signal is still not connected properly after inspection, the entire probe will be scrapped.

Temperature Instruments

Thermal Resistance Thermometer

  1. Check whether there is water inside the protective tube or whether there are metal shavings or dust inside the junction box;
  2. Check whether there is poor contact in the instrument system, whether the wires appear to be broken or not, and whether they are connected or not;
  3. Check for any partially grounded areas;

Thermocouple Thermometer

  1. Check whether there ismoisture or electrode leakage inside the thermocouple or not;
  2. Check whether there isany short circuits in the thermocouple junction box or not;
  3. Check if there is a short circuit or polarity reversal in the compensation wire;
  4. Check if the indicating instruments and thermocouples are compatible;
  5. Check if the thermocouple and compensation wire are compatible;
  6. Check the contact condition at the wiring point;
  7. Check if the thermocouple has deteriorated;
  8. Check for intermittent short circuits or grounding in thermocouples and compensating wires.

Ordinary Pressure Gauge

Stainless steel pressure gauge

  1. Check the pressure gauge attachment for leaks or damage;
  2. Check if the instrument is damaged;
  3. Check if the running instruments normallyindicate;
  4. Check if the instrument has exceeded its validity period;
  5. Wipe the dust and dirt off the instrument panel, keeping it clean and visible;
  6. When the equipment is shut down, the sampling valve should be closed. If the equipment needs to be started, the valve should be opened after the equipment is running normally to avoid damage to the instrument due to excessive pressure during equipment startup.

Pressure Type Thermometer, Bimetallic Thermometer

Bimetallic Thermometer

  1. Check for leaks at the instrument connections;
  2. Check if the instrument is damaged;
  3. Check if the running instruments normallyindicate;
  4. Check if the instrument has exceeded its validity period;
  5. Wipe off dust and dirt from the instrument panel, keeping it clean and visible.

Digital Display Meter

This type of instrument can be calibrated on site.

  1. It is strictly prohibited to press the instrument buttons indiscriminately andchange internal parameters;
  2. Check if the instrument indications are normal;
  3. Check if the instrument has exceeded its validity period;
  4. Check if the instrument wiring is firm;
  5. Regularly clean the dustof the instrument.

Floating Ball Instruments

  1. The float should be cleaned once every six months. If it cannot be cleaned during operation, maintenance can be carried out during the maintenance period;
  2. Check for damage and dirt on the floating ball attachment, and clean and maintain it;
  3. For those with sewage valves, they should be discharged at least once per shift.

Ammeter, Kilowatt-Hour Meter

  1. Check if the instrumentnormally operates;
  2. Check the current circuit for poor contact and open circuits to ensure the safe operation of the instrument, terminals, and current transformers;
  3. Check the instrument for any signs of burning damage;
  4. Check if the instrument has exceeded its validity period.


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