The short answer
An inch-to-millimeter conversion is exact, but a nominal bag-size label is not necessarily an exact manufactured dimension. Use the existing filter bag size chart for identification, then place controlling dimensions, reference points and agreed tolerances on the order drawing.
Separate the name from the measured feature
NIST defines 1 in as exactly 25.4 mm. Thus a mathematical length of 7 in is 177.8 mm. This calculation does not establish that a bag marketed with a 7 in label has that exact body diameter, ring outside diameter or sealing diameter. A nominal label can describe a product family rather than an inspection requirement.
Keep three fields in the comparison: catalog designation, controlled drawing value and observed measurement. Do not replace one with another because the values are close. The JINLV size chart remains the catalog reference; a purchase-specific drawing defines the accepted article and its interfaces.
Declare which unit and document control acceptance
Choose a controlling unit for the drawing. If a second unit is shown for convenience, label it as reference unless the responsible parties have explicitly defined how both apply. Repeatedly converting and rounding an already rounded value can move the apparent acceptance boundary.
Also state which drawing revision governs if a quotation, sample label and purchase order disagree. A supplier's nominal inch description can remain in the commercial description while the agreed millimeter dimensions control inspection. This is a document-control choice, not permission to change the product geometry without technical review.
Define the measurement before comparing results
Length may be measured from the top of the ring, its seating plane or the fabric attachment to the bottom seam or lowest point. These are different features. A ring outside diameter is not a substitute for the bag body's width or for the contact diameter of a sealing lip. Mark the reference points on a sketch.
For flexible fabric, state whether the sample is new or used, dry or conditioned, relaxed or under a specified measurement load. Select a method appropriate to the tolerance and record its limitations. NIST explains that uncertainty belongs with the measurement result; extra decimal places on a display do not remove ambiguity in the feature being measured.
Use a dimensional-discrepancy matrix
Resolve disagreements by their cause rather than imposing a standard tolerance that no party approved.
| Discrepancy | Likely question | Required clarification |
|---|---|---|
| Nominal inches differ from chart millimeters | Are these product labels or controlled dimensions? | Identify the authoritative drawing and feature |
| Two inspectors report different lengths | Did they use the same start and end references? | Align the datum sketch and sample condition |
| Converted values imply different pass limits | Which unit and rounding rule control? | Use the agreed controlling limits, not rounded reference values |
| Measured result is close to a limit | Is measurement capability adequate for this decision? | Apply an agreed decision rule and review uncertainty |
Agree tolerances from function and inspection capability
Do not infer a tolerance from the spread of several used bags or the number of decimal places in a catalog. Discuss what variation the mating seat, basket and bottom support can accept, and what the supplier can manufacture and demonstrate. Ring and textile dimensions may require different methods and acceptance criteria.
As a hypothetical dispute, one inspector could include the ring height in bag length while another starts at the seating plane. Converting both readings perfectly will not make them comparable. Re-measure the agreed feature, preserve the original observations and document the resolution. If the specified tolerance cannot be verified reliably, revise the measurement plan or seek responsible technical review before releasing the order.
Your review checklist
- Keep nominal label, drawing value and measurement in separate fields.
- Set controlling units and reference-only conversions.
- Identify the governing document and revision.
- Sketch each measurement's start, end and contact references.
- State sample condition and the agreed measurement method.
- Agree tolerances, measurement capability and the acceptance decision rule.
Limits of this guidance
General dimensional communication and metrology guidance. No universal filter-bag tolerance or automatic compatibility is established. Functional limits and acceptance methods require agreement for the exact bag, housing and basket; this is not engineering approval.
Questions buyers ask
Is 180 mm simply an incorrect conversion of 7 in?
Not necessarily: 7 in converts exactly to 177.8 mm, but 180 mm may be a separately specified catalog dimension. Determine whether the two entries describe the same controlled feature before calling either an error.
Can we use one tolerance for every part of the bag?
Only if the responsible parties have justified it. Flexible body dimensions and a sealing-ring profile have different functional and measurement considerations; a blanket tolerance may leave the critical interface uncontrolled.
Technical references
References support general principles or the named supplier's own products; they do not certify or validate a JINLV configuration.
- SI Units – Length
- Metrological Traceability: Frequently Asked Questions and NIST Policy
- Standard Liquid Filter Bag Size Chart (#1–#5) | JINLV
AI-assisted technical synthesis of the references below, with editorial checks. This is general procurement guidance, not an independently signed engineering review or a product validation report.
English is the indexing version. Chinese and Indonesian are reading translations of the same article.