Which Instrument Calibration Records Should Accompany RECD Tests?

An RECD test report usually arrives as a few pages of results: particulate concentrations, a reduction percentage, perhaps some CO and NOx figures, and a signature. What it often leaves out is the evidence that the instruments producing those numbers were reading correctly on the day.

That evidence is the calibration record. Without it, a 72% particulate reduction is a claim. With it, the number becomes something an engineer, an auditor or a State Pollution Control Board officer can check.

Retrofit Emission Control Devices are judged against a sharp threshold. Under the CPCB framework, an RECD must show at least a 70% reduction in particulate matter. When results sit a few points either side of that line, a small instrument error can decide the outcome. This guide goes through the instruments used in a typical RECD test one at a time. For each, it explains what the calibration record should show, how often it should be checked, and the warning signs to look for.

Why Calibration Records Matter More for RECD Tests

Routine stack monitoring compares one measurement against a limit. An RECD test compares two measurements against each other, before and after the device, and expresses the result as a ratio. That changes the risk:

• The post-RECD readings are small. A good device leaves very little particulate on the filter, so small absolute errors become large relative errors.
• Errors don’t always cancel. If the same well-behaved instrument is used for both measurements, some errors cancel in the ratio. If instruments change between tests, or drift during the day, they don’t.
• The result often triggers money or legal consequences. Acceptance payments, performance guarantees and compliance status can all depend on it. Our guide to RECD performance guarantees and operating conditions explains how these link together.

A calibration record turns “trust us” into “check for yourself.”

Two Kinds of Calibration Evidence

Before going instrument by instrument, it helps to separate two types of record. A complete test pack needs both.

1. Periodic calibration certificates. Issued when an instrument is calibrated against reference standards, usually by an accredited calibration laboratory. They show the instrument’s error at specific points, the measurement uncertainty, the date, and the due date for the next calibration. In India, look for calibration by a laboratory accredited by NABL for the relevant parameter, with traceability to national standards maintained by CSIR-National Physical Laboratory.

2. On-the-day checks. Quick verifications done at the test site, such as zero and span checks on gas analysers, leak checks on the sampling train, and balance checks with reference weights. They show that an instrument that was calibrated months ago is still reading correctly today.

A certificate without on-the-day checks proves the instrument was correct when it left the calibration lab. On-the-day checks without a certificate prove only that the instrument agrees with whatever reference was used on site. You need both.

The general framework for laboratory competence, including calibration and traceability, is ISO/IEC 17025. That is the standard NABL accreditation is based on.

The Instrument-by-Instrument Register

The instruments below appear in most site RECD tests that use gravimetric particulate sampling. Not every test uses all of them, but the report should cover every instrument whose reading feeds into the result.

1. Dry gas meter (sample volume)

What it does: measures the volume of gas drawn through the particulate filter. Concentration is filter mass divided by this volume, so any error in volume passes straight into the result.

Calibration record to request:

• Certificate showing the meter’s calibration factor (often called Y or gamma) against a reference meter or critical orifice, at several flow rates
• Date and due date
• Uncertainty of the calibration

On-the-day checks:

• Pre-test and post-test leak check of the whole sampling train, with the leak rate recorded
• Post-test calibration check against a reference, if the method requires it

Warning signs: a calibration factor far from 1.0 with no explanation, a post-test check that differs noticeably from the pre-test factor, or a leak check recorded as “OK” with no number.

The sampling approach in US EPA Method 5 shows how much attention a rigorous particulate method gives to meter calibration and leak checks. Indian stack-monitoring practice follows the same principles.

2. Pitot tube and differential pressure gauge (gas velocity)

What it does: measures gas velocity in the duct. This sets the isokinetic sampling rate and, combined with duct area, gives the total exhaust flow.

Calibration record to request:

• Pitot tube coefficient, either from calibration or a documented standard design with an inspection record
• Calibration certificate for the manometer or differential pressure transducer

On-the-day checks:

• Pitot leak check
• Visual inspection of the tube tips for damage or blockage
• Zeroing of the pressure gauge

Warning signs: a damaged or bent pitot, missing coefficient, or a pressure gauge used outside its calibrated range. Low-velocity flows at light load can fall near the bottom of a gauge’s range, where errors grow. US EPA Method 2 is the widely used reference for this measurement.

3. Temperature sensors (stack, meter, probe and filter)

What it does: stack temperature converts measured gas volumes to normal conditions. Meter temperature corrects the dry gas meter reading. Probe and filter temperatures confirm the sampling train stayed at the specified temperature, which affects how much condensable material is collected.

Calibration record to request:

• Certificates for each thermocouple and its readout, or a calibration of the complete loop
• The temperature range covered, which should include the actual stack temperatures (often 150–500 °C for DG exhaust)

On-the-day checks:

• A quick comparison against a reference thermometer, or an ambient-temperature cross-check of all sensors before the test

Warning signs: a calibration that only covers 0–100 °C used to measure exhaust at 400 °C, or probe and filter temperature records missing from the report.

4. Barometer (ambient pressure)

What it does: converts measured volumes to normal (standard) conditions. A small error in pressure moves every concentration result by the same proportion.

Calibration record to request: a current calibration certificate for the barometer used, or a documented source for the pressure value (such as a calibrated station barometer nearby, adjusted for elevation).

Warning signs: a “standard” pressure of 1013 hPa assumed for a test site at significant altitude. For hill sites, see RECD selection for diesel generators at high altitude.

5. Analytical balance (filter weighing)

What it does: weighs the particulate filters before and after sampling. The mass gain is the particulate collected.

This is the most sensitive instrument in an RECD test, because post-RECD filter masses can be very small.

Calibration record to request:

• Balance calibration certificate, with readability and uncertainty at the relevant mass range
• Certificates for the reference weights used for routine checks

On-the-day (or weighing-day) checks:

• Balance check with reference weights before each weighing session, recorded
• Filter conditioning records: temperature and humidity of the weighing room, and conditioning time
• Weights of control (reference) filters weighed alongside the samples, to detect drift between pre- and post-weighing
• Field blank results: filters that went to site and were handled like samples but weren’t exposed to exhaust

Warning signs: no conditioning records, no control filters, no field blanks, or post-RECD filter mass gains that are close to the balance’s practical detection limit.

6. Gas analysers (O₂, CO₂, CO, NOx, HC)

What they do: O₂ and CO₂ are used to correct concentrations to a reference basis and to detect dilution. CO, NOx and HC are needed to show that the RECD hasn’t adversely affected other emissions.

Calibration record to request:

• Periodic calibration or service record for each analyser
• Certificates for the calibration gas cylinders, showing concentration, uncertainty, traceability and expiry date

On-the-day checks:

• Zero and span check for each gas before and after testing, with the readings recorded
• Drift between pre- and post-test checks, compared with the method’s acceptance limit
• Sample line leak or bias check where the method calls for it

Warning signs: expired gas cylinders, span checks done only before the test, or span gas concentrations far from the levels actually measured. For example, spanning a CO analyser at 1,000 ppm and then reading 15 ppm downstream of a catalyst puts the measurement at the bottom of the range, where accuracy is weakest.

The importance of traceable calibration gases is well recognised in stack-emission practice. The UK accreditation body’s policy on traceability of calibration gas standards is one published example of the requirements. The same logic applies to labs working in India.

7. Dilution system flow controllers (if used)

What they do: set the ratio between exhaust and dilution air in a partial-flow dilution system. The raw concentration is calculated by multiplying the diluted reading by this ratio.

Calibration record to request:

• Calibration certificates for each mass flow controller, stating the gas used for calibration
• Records of any tracer-gas checks of the dilution ratio

Warning signs: a nominal dilution ratio quoted with no measured confirmation. We explain why this matters in how dilution can mislead RECD before-and-after measurements.

8. Opacity meter (if used)

What it does: gives a quick indication of smoke density. It can’t prove a particulate mass reduction, but it is often used for screening and for ongoing checks.

Calibration record to request: a calibration certificate and on-the-day zero and neutral-density filter checks.

Warning signs: an opacity result presented as the main evidence for a 70% reduction. See smoke opacity vs particulate mass.

9. Load measurement (power analyser or DG controller)

What it does: records the electrical load during each sample, so load matching can be verified and results can be expressed in g/kWh.

Calibration record to request:

• For an independent power analyser: a current calibration certificate
• For DG controller readings: at minimum, a cross-check against a calibrated clamp meter or analyser, recorded on the day

Warning signs: load recorded as “approximately 50%” or only in kVA. See why generator load must be matched in RECD comparison tests.

10. Differential pressure across the RECD and engine back-pressure

What they do: show the soot load in the device, detect regeneration, and confirm that back-pressure stays within the engine maker’s limit.

Calibration record to request: calibration certificates for the pressure transmitters or gauges, covering the range actually used.

Warning signs: a back-pressure claim with no instrument identified. See how exhaust back-pressure affects a DG set and RECD testing during regeneration.

The Register at a Glance

Instrument	Feeds into	Periodic certificate	On-the-day check	Most common gap
Dry gas meter	Sample volume → concentration	Yes	Leak checks, post-test check	Leak check with no number
Pitot + gauge	Velocity, flow, isokinetic rate	Yes	Leak check, inspection	Gauge used at bottom of range
Temperature sensors	Normalisation, train temperature	Yes	Cross-check	Range doesn’t cover exhaust
Barometer	Normalisation	Yes	Compare with reference	Assumed standard pressure
Analytical balance	Filter mass	Yes, plus weight certificates	Weight checks, control filters	No blanks or controls
Gas analysers	O₂/CO₂ correction, other gases	Yes, plus gas certificates	Zero/span before and after	Expired or unsuitable span gas
Dilution flow controllers	Dilution ratio	Yes	Tracer check	Nominal DR only
Opacity meter	Screening	Yes	Zero, filter check	Used as main evidence
Power measurement	Load matching, g/kWh	Yes (analyser)	Cross-check	Load in kVA only
Pressure transmitters	ΔP, back-pressure	Yes	Zero	No instrument named

Worked Example: How Small Errors Move a Ratio

These figures are illustrative. A site test gives a baseline PM concentration of 80 mg/Nm³ and a post-RECD concentration of 22 mg/Nm³, for a reduction of 72.5%.

Now consider two instrument problems that affect only the post-RECD test.

Problem 1: balance drift. The post-RECD filter gained 0.40 mg. Between pre-weighing and post-weighing, the balance drifted by +0.03 mg, and no control filters were weighed to catch it. The recorded mass gain becomes 0.43 mg, which is 7.5% higher than the truth.

• Post-RECD concentration becomes 22 × 1.075 = 23.65 mg/Nm³
• Reduction becomes (80 − 23.65) ÷ 80 = 70.4%

The baseline filter gained several milligrams, so the same 0.03 mg drift there would be under 1%.

Problem 2: meter calibration factor. The dry gas meter used for the post-RECD test reads 4% high, but the certificate shows it was calibrated a year and a half ago and is overdue. The sample volume is overstated, so the concentration is understated by about 3.8%.

• Post-RECD concentration becomes 22 ÷ 1.04 ≈ 21.15 mg/Nm³
• Reduction becomes (80 − 21.15) ÷ 80 = 73.6%

Neither error is dramatic on its own. But one pushes the result towards failure and the other towards a comfortable pass, and when results sit near 70%, that’s enough to change the decision. The only way to know which way an error went, or whether there was one, is through the calibration and check records.

For more on how measurement uncertainty should be handled in the reduction calculation, see how to calculate RECD particulate reduction.

What Traceability Means in Practice

You’ll see the word “traceable” on many certificates. It means there is an unbroken chain of comparisons linking the instrument to a national or international measurement standard, each step with a stated uncertainty.

A typical chain for a gas analyser looks like this:

1. National measurement standard (maintained by a national metrology institute such as CSIR-NPL)
2. Reference gas or reference instrument at an accredited calibration laboratory
3. Certified calibration gas cylinder supplied to the testing lab
4. Span check of the analyser on site using that cylinder
5. Measurement of the DG exhaust

If any link is missing, for example an uncertified cylinder or an expired certificate, the chain breaks and the result can’t be traced back to a recognised standard.

When you review a calibration certificate, check that:

• It names the specific instrument by serial number, matching the instrument used in the test
• It is issued by a laboratory accredited for that type of calibration, and carries the accreditation mark or reference
• It states measurement results and uncertainty, not just “passed”
• It was valid on the test date
• It covers the range actually used in the test

Building the Calibration Pack: What to Ask the Lab For

Write these requirements into your purchase order or test scope before testing. A clause like this works well:

The testing laboratory shall provide, with the test report, a calibration annex containing: (a) a list of every instrument used, with make, model and serial number; (b) a valid calibration certificate for each, issued by an accredited calibration laboratory and covering the range used; (c) certificates for all calibration gases and reference weights; (d) records of all on-site checks, including leak checks, zero and span checks with pre- and post-test drift, and balance checks; (e) filter conditioning, control filter and field blank records.

Our guides on writing a technical RFQ for an RECD project and RECD tender specifications with measurable performance requirements show where to place clauses like this.

How to Review the Calibration Pack in 15 Minutes

You don’t need to be a metrologist to do a useful first review. Work through these questions:

1. Is every instrument listed? Compare the instrument list against the measurements in the report. Every number in the results should trace to a listed instrument.
2. Do the serial numbers match? A certificate for a different meter than the one used doesn’t count.
3. Were certificates valid on the test date? Check issue and due dates.
4. Do the ranges fit? Look especially at temperature, low-velocity pressure and low-concentration gas ranges.
5. Are on-site checks recorded with numbers? “Leak check OK” isn’t enough. Look for the measured leak rate and the acceptance limit.
6. Was drift checked after the test, as well as before? Post-test checks catch problems that developed during the day.
7. Are blanks and controls present for filter weighing?
8. Were gas cylinders in date?

If any answer is “no” or “can’t tell,” ask the lab before accepting the result. A competent lab should be able to provide the missing records quickly.

Calibration for Periodic and Ongoing Checks

The same logic applies to tests after acceptance. If you plan periodic checks of RECD performance, as described in our RECD maintenance guide, keep calibration annexes from every test in the same file. That way you can tell whether a change in results reflects the device or a change in instruments.

Simple site instruments you use yourself, such as a handheld opacity meter or a differential pressure gauge on the RECD, also need calibration if their readings are going to support any decision. A gauge that has read 5 mbar low for two years can hide steadily rising back-pressure.

Keep these records alongside your other compliance documents. Our environmental compliance checklist for generator owners shows where they fit, and they are worth having ready for a Pollution Control Board inspection.

Related Reading on RECD Testing

• Planning an RECD emissions test before and after installation
• How to compare RECD test results from different laboratories
• How fuel and engine condition can confound RECD performance tests
• How to set up a witnessed RECD performance demonstration
• Estimating kilograms of PM avoided after an RECD retrofit
• How to read an RECD test certificate

Frequently Asked Questions

Is the lab’s NABL accreditation enough, or do I still need the calibration certificates?

Accreditation shows that the lab has a quality system for calibration and testing within a defined scope. It doesn’t show that every instrument used on your test day was in calibration. Ask for the instrument-specific certificates and on-site check records as well.

How often should stack sampling equipment be calibrated?

Intervals depend on the instrument, the method and the lab’s quality system. Many items are calibrated annually, with more frequent checks for heavily used equipment. What matters most for you is that each certificate was valid on the test date, and that on-site checks confirm the instrument was still reading correctly.

What is a span gas, and why does its certificate matter?

A span gas is a cylinder of known gas concentration used to check and adjust an analyser’s reading. If the cylinder’s concentration is wrong or uncertified, every reading from that analyser is shifted by the same proportion. The certificate states the concentration, its uncertainty, its traceability and its expiry date.

Can a test be valid if one calibration certificate had expired?

It may still be technically sound, but it’s no longer fully traceable for that measurement. The lab should explain why, show any interim checks it did, and assess how the instrument affects the result. For a borderline result or a contractual decision, consider a retest.

Do I need calibration records for the RECD’s own pressure gauge?

If its readings are used in the test report or in decisions about maintenance and back-pressure, yes. An uncalibrated gauge is fine for a rough indication, but not as evidence.

Ask for the Evidence, Not Just the Answer

A test report that comes with a complete calibration pack is worth far more than one that doesn’t, because anyone can check it. Aceget supplies CPCB type-approved Retrofit Emission Control Devices for DG sets from 25 kVA to 2500 kVA, and we help clients specify the documentation that should accompany acceptance tests. You can view our own certifications and type approvals, read about DG set emission regulations, or contact our team for help planning your test scope.