How to Prepare Instruments for Calibration: A Practical Checklist

Why Preparation Affects Calibration Quality

Most organisations think about calibration as something that happens at the laboratory. The instrument goes in; a certificate comes out. But what happens between the moment an instrument is removed from service and the moment it arrives at the laboratory (or the moment a calibration technician arrives on-site), directly affects the calibration result and the speed of the process.

The calibration laboratory's job is to compare your instrument against a traceable reference standard and document the result accurately. Their job becomes harder (and your calibration less useful), when: the instrument arrives without identification (which physical unit was calibrated?), is missing its accessories (incomplete calibration), is contaminated (refusing or cleaning charge), is physically damaged and the damage is not declared (ambiguous as-found result), or arrives with dead batteries or in a non-functional state (delayed calibration).

Each of these failures adds time, cost, or ambiguity to the calibration process. This guide covers eight steps that eliminate the most common preparation failures and ensure your instruments arrive in a state that allows the laboratory to produce accurate, complete, and timely calibration certificates.

Step 1: Identify the Instrument and Check the Calibration Register

Before removing any instrument from service, check the calibration register. Confirm:

• The Asset ID or tag number matches the physical instrument. Asset tags can fall off; serial numbers can be confused. Send the wrong instrument and you receive a certificate for one asset linked to another.
• The calibration parameters required are documented. If the register says "DC voltage only" and the engineer in the field says "calibrate DC and AC," the discrepancy needs to be resolved before the instrument is sent.
• The instrument is actually due for calibration. It is not uncommon for instruments to be sent "early" by well-meaning technicians, but sending instruments before their due date uses calibration budget unnecessarily and shortens the effective calibration interval for the next cycle.
• The instrument has not already been sent (especially in larger facilities with multiple people submitting instruments). Duplicate submissions waste laboratory time and calibration budget.

Record the submission in the calibration register: date removed from service, submitted by, submitted to (laboratory name), expected return date. This creates an audit trail and prevents the instrument from showing as "overdue" while it is legitimately at the laboratory.

Step 2: Inspect the Instrument Before Sending

Physical inspection before submission serves two purposes: it identifies damage that would affect the calibration result, and it protects you from receiving a damaged instrument back with a "damaged on arrival" notation that you cannot dispute.

Conduct a pre-submission inspection and check for:

• Physical damage: cracked, bent, or broken probes; damaged test leads; cracked display; loose housing; bent pins on connectors; corroded contacts; damaged cable insulation.
• Contamination: chemical residue on probes or housings; biological contamination; excessive dust or particulate inside the housing.
• Missing parts: compare the instrument against its accessories list. Missing test leads, adapters, or cases should be noted.
• Known functional issues: does the instrument power on normally? Are all range switches and functions operating? Are there any intermittent faults, unusual readings, or error codes?

Document the pre-submission condition. Even if just on the calibration request form: "Probe tip has minor surface oxidation; otherwise in good condition." This creates a baseline that protects both you and the laboratory.

If the instrument is damaged, do not try to hide or minimise it. Send it with a clear declaration: "Instrument sent with known damage, bent probe tip. Please calibrate as-received and record as-found condition." A SAC-SINGLAS accredited laboratory will record the as-found condition accurately regardless; your declaration simply ensures the calibration request form and the certificate are consistent.

Step 3: Clean the Instrument

Calibration laboratories are required to maintain controlled environments (calibrated temperature, humidity, and cleanliness), to achieve the measurement uncertainties stated in their Scope of Accreditation. Instruments that arrive heavily contaminated disrupt these environments and can contaminate the laboratory's own reference standards.

Most accredited laboratories will refuse to calibrate heavily contaminated instruments, or will charge a cleaning fee before proceeding. Either outcome delays your calibration and adds cost.

Clean the instrument before submission:

• Probes and contacts: use isopropyl alcohol (IPA) on a lint-free cloth or swab. Do not use abrasive cleaners. For thermometer probes, clean along the length of the probe and allow to dry fully before packaging.
• Housings: wipe down with a damp cloth. Do not allow moisture inside the housing. For instruments with vents or fan grilles, use compressed air to remove dust.
• Test leads: clean connectors with IPA. Check insulation for cuts or deterioration.
• Pressure gauge faces: clean the face and the process connection thread. For instruments used with process fluids, flush the connection port before sending.

Cleaning is also the right time to inspect for damage. You are looking closely at the instrument, so any physical issues will be apparent.

Step 4: Include All Necessary Accessories

An instrument without its accessories is often an incomplete calibration. Common omissions:

• Thermometers and temperature probes: the probe and the connecting cable must travel together. If the calibration is for the probe-cable-instrument combination (as it should be for RTD and thermocouple systems), all three components are needed. Sending the probe without the cable means the laboratory can only calibrate the probe against a reference. They cannot assess the combined system error.
• Pressure gauges and transducers: include the process adapters and any pressure snubbers or manifolds that are used in service. The configuration affects the calibration.
• Multimeters: include all test leads. If the instrument is calibrated using specific leads (e.g. kelvin clips for resistance measurement, high-voltage probes), include those.
• Environmental data loggers: include the USB or RS-485 communications cable and any mounting brackets if calibration includes installed functionality.
• Torque wrenches: include the relevant sockets or drive adapters.

If an accessory is missing, note it on the calibration request form. The laboratory will advise whether calibration can proceed for the available parameters, or whether the missing accessory must be sourced before calibration.

Step 5: Charge Batteries and Verify Functionality

Instruments should arrive at the laboratory in working condition. A flat battery, a blown fuse, or a display that does not power on means the laboratory technician cannot begin calibration until the issue is resolved, which means delays and potentially additional charges for time spent troubleshooting.

Before submitting:

• Charge the instrument fully (for rechargeable battery instruments).
• Replace batteries if below 50% capacity (for disposable battery instruments).
• Replace any blown fuses (carry spare fuses in the instrument case).
• Power the instrument on, cycle through all functions, and confirm normal operation.
• If you notice any fault, abnormal reading, or error code. Document it on the calibration request form. Do not attempt to repair the instrument before sending if the fault is internal; the calibration laboratory or the manufacturer's service centre is the appropriate resource.

For instruments with internal memory (data loggers, calibrators with stored programmes), consider whether the stored data needs to be downloaded before sending. The calibration process may reset or overwrite stored data.

Step 6: Complete the Calibration Request Form Accurately

The calibration request form is your primary communication with the laboratory. A well-completed form eliminates the most common causes of delay and prevents incorrect calibrations.

Required information:

• Asset ID / tag number: the identifier from your calibration register.
• Make, model, and serial number: confirm these match the physical instrument.
• Calibration parameters required: specify exactly which parameters need to be calibrated. A multimeter calibration may include DC voltage, AC voltage, DC current, AC current, resistance, frequency, and continuity, but you may only need DC voltage and resistance if the other functions are not used in your process. Calibrating only what you need reduces cost and turnaround time.
• Calibration range required: if the instrument has a wider range than your application requires, you may only need calibration over the portion of the range you use. This is particularly relevant for pressure gauges and temperature instruments.
• Accuracy or tolerance required: this is critical input for the laboratory. If your process requires ±0.5°C accuracy, state it. If your internal specification is different from the manufacturer's specification, state it.
• Any known faults or anomalies: document everything noted in Steps 2 and 5.
• Special instructions: if you require the certificate in a specific format (e.g. for GMP the certificate may need specific language), state it. If you require the instrument calibrated without adjustment (some OOT investigations require as-found only, no adjustment), state it.
• Contact name and return address: ensure the correct return address is clearly stated.

Step 7: Package Properly for Transport

Vibration and shock during transport are among the most common causes of instrument damage between the facility and the laboratory. Proper packaging prevents damage and eliminates the "damaged in transit" ambiguity.

Packaging guidelines:

• Use original packaging where available. Instrument manufacturers design packaging specifically for their instruments' handling characteristics. Original foam inserts hold the instrument in a fixed position that prevents movement-induced damage.
• If original packaging is not available: wrap the instrument in bubble wrap (minimum 50 mm on all sides), place in a rigid outer box with void fill to prevent movement, and seal the box securely.
• Fragile instruments (glass thermometers, optical instruments, precision gauges with fine adjustments): double-box. Place the wrapped instrument in an inner box, then seal the inner box inside a larger outer box with additional padding between them.
• Probes and cables: coil cables loosely and secure with a cable tie. Do not coil cables tightly, this stresses the insulation. Place probes in protective sheaths or tubes.
• Mark the outer box: write "FRAGILE. CALIBRATION INSTRUMENTS" and "THIS SIDE UP" on all sides of the outer box. Include the laboratory's address label and your return address.
• Include the calibration request form inside the package, not just on the outside where it can be detached.

Step 8: Preparing for an On-Site Calibration Visit

On-site calibration requires a different set of preparations. The calibration technician is coming to your facility with portable reference equipment, so the environment and the instrument's setup must be ready.

Before the technician arrives:

• Confirm instrument accessibility: the instrument must be reachable, and if it is installed in a panel or rack, the relevant sections must be safely accessible. The technician should not need to disassemble other equipment to reach the instrument.
• Confirm operating condition: the instrument should be at its normal operating temperature and configuration. Environmental sensors and temperature monitors should have been running for at least 30–60 minutes before calibration to allow thermal stabilisation.
• Prepare the environment: the calibration area should be free from excessive vibration, electromagnetic interference, direct sunlight, and temperature drafts that could affect the reference standards.
• Provide power: ensure appropriate power outlets are available. Portable calibration equipment typically requires standard 230 V / 50 Hz supply.
• Arrange a stable work surface: the technician needs space to set up the reference equipment, the calibration adapter, and the documentation. A stable, level surface (a workbench or table) adjacent to the instrument being calibrated is required.
• Have the calibration request documentation ready: the same information required for in-lab calibration applies. Have the Asset IDs, required parameters, and any special instructions ready when the technician arrives.

Common Mistakes That Delay Calibration

The following table summarises the most common preparation failures and their consequences. Most of them are avoidable in under five minutes, and every one of them has a real-world cost.