Tech tip1 min read
Using a Hydrometer to Control Processor Chemistry
How to measure developer and fixer specific gravity with a hydrometer in deep tank processors, with EPM target values and a recommended hydrometer.
A hydrometer is an instrument for measuring Specific Gravity (the ratio of water to concentrated chemistry). Deep tank processors such as Allen or Houston Fearless use concentrated developer and fixer, and regular specific gravity checks ensure correct mixing and image quality.
A hydrometer is a hollow tube with a weighted bottom section and a graduated stem. In a denser liquid, less of the stem sinks below the surface. The reading is taken where the top of the meniscus meets the stem.
Highlights
- Check specific gravity at least weekly
- The test can be done in a graduated cylinder or directly in the chemical tank
- The value must fall between the upper and lower limits set by the manufacturer
- Too much water (weak mix) = low background density
- Too much chemistry (strong mix) = high background density
Specific gravity specifications
| Solution | Mix ratio | Specific Gravity |
|---|---|---|
| EPM Developer Concentrate | 1:7 | 1.045 to 1.055 |
| EPM Fixer Concentrate | 1:3 | 1.080 to 1.105 |
Recommended hydrometer
| Parameter | Value |
|---|---|
| Supplier | VWR International |
| Catalog Number | 34610-002 |
| Range | 1.000 to 1.220 |
| Approximate length | 300mm |
Source:Eastman Park Micrographics(epminc.com)(opens in a new tab)
Frequently asked questions
How is a hydrometer used to measure specific gravity in deep tank processors?
The hydrometer is placed in a graduated cylinder or directly in the chemical tank, and the reading is taken where the top of the meniscus meets the graduated stem.
What is the allowed specific gravity range for EPM Developer Concentrate at a 1:7 mix ratio?
The allowed range is 1.045 to 1.055.
What is the allowed specific gravity range for EPM Fixer Concentrate at a 1:3 mix ratio?
The allowed range is 1.080 to 1.105.
How does excess water affect background density in deep tank processor chemistry?
Too much water creates a weak mix and results in low background density.
Which hydrometer is recommended for measuring specific gravity in deep tank processors?
The recommended hydrometer is VWR International Catalog Number 34610-002, with a range of 1.000 to 1.220 and an approximate length of 300mm.
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