1. Scope

This specification applies to the calibration of laboratory vibratory liquid density meters (including benchtop and portable vibratory density meters) under normal pressure.

2 Referenced Documents

This standard references the following documents:

JJF 1001 General Metrological Terms and Definitions

JJF 1005 General Terms and Definitions for Standard Reference Materials

JJF 1059.1 Evaluation and Expression of Measurement Uncertainty

JJF 1071 Rules for Drafting National Metrological Calibration Standards

ISO 12185 Crude petroleum and petroleum products—Determination of density—Oscillating U-tube method

ISO 15212-1 Oscillation-type density meters—Part 1: Laboratory instruments

For dated references, only the edition cited applies; for undated references, the latest edition of the referenced document (including any amendments) applies.

3. Terminology and Units of Measurement

3.1 Terminology

3.1.1 Certified Reference Material for Liquid Density

A liquid density reference material with a certificate of conformity. Its density value is determined using a traceable procedure, traceable to a solid density standard or sub-standard. Its density value carries an uncertainty with a given probability of inclusion.

Note: Commonly used for density value transfer.

3.2 Units of Measurement

The unit of measurement for density is kg/m³ or g/cm³.

4. Overview

A laboratory vibrating liquid density meter (hereinafter referred to as a density meter) mainly consists of a vibration sensor, a temperature sensor, a data processing unit, and a display unit. When the liquid density within the sensor changes, its vibration period changes accordingly. The density of the liquid under the corresponding measurement conditions is calculated using the instrument coefficients and equations. The relationship between the density of the liquid inside the sensor and its vibration period and temperature can be simplified to formula (1):

p=f(k,t,T) (1)

Where:

p—— Liquid density inside the sensor;

k—— Density meter coefficient;

t—— Liquid temperature;

T—— Vibration period.

5. Metrological Characteristics

This calibration specification focuses on the density meter's indication error and repeatability.

The relationship between the density meter's resolution and the maximum permissible error of indication is shown in Table 1.

Table 1 Maximum Permissible Error of Resolution and Indication Error

Density meter nominal parameters		The maximum permissible error of the density meter reading is: kg/m³
Resolution (kg/m³)	Calibration coefficient
1	2	士2
0.1	10	士1.0
0.1	5	士0.5
0.01	20	士0.20
0.01	10	±0.10
0.01	5	±0.05
0.001	100	士0.100
0.001	50	±0.050
Note: All the above indicators are not applicable to qualification and acceptance criteria and are for reference only.

 

The maximum permissible error in the table can be used as the basis for selecting the liquid density standard material for calibrating the densitometer.

6. Calibration Conditions

6.1 Environmental Conditions

The laboratory environment should meet the installation requirements of the densitometer and should be free from strong mechanical vibration and electromagnetic interference. The laboratory temperature should be controlled at (20±5)℃, and the relative humidity should be 20%～70%. The environment should be well-ventilated and equipped with fire-fighting facilities and a water source.

6.2 Calibration Standard Materials and Calibration Equipment

6.2.1 Standard Materials

The liquid density standard material used for calibrating the densitometer should be a certified standard material. Generally, the measurement uncertainty U(pref) of the selected standard material should not exceed 1/3 of the absolute value of the maximum permissible error of the densitometer being calibrated, i.e.: U(pref) ≤

For a densitometer with a maximum permissible error of ±0.05 kg/m³, the maximum permissible uncertainty of the liquid density standard material is 1/2 of the absolute value of the maximum permissible error.

The liquid density standard material used for calibration should be clear, transparent, free of visible particles and other substances, and possess good homogeneity. The viscosity of the liquid density standard material should be below 50 mPa·s.

6.2.2 Main Supporting Equipment

The main supporting equipment and technical requirements are shown in Table 2.

Table 2 Main Supporting Equipment

Equipment Name	Technical Requirements
Thermohygrometer	Temperature measurement range: 5–50℃, maximum permissible error ±0.5℃; Relative humidity measurement range: 10%–85%, maximum permissible error ±3%.
pressure gauge	Pressure measurement range: (800～1060) hPa, maximum permissible error: ±5.0 hPa
Syringe and accessories	The syringe volume is (10-20) mL
Note: The selected syringe and accessories must not react with the density standard liquid.

6.2.3 Auxiliary Equipment and Materials

6.2.3.1 Cleaning Solution: Distilled water, anhydrous ethanol, petroleum ether, detergent, etc., can effectively remove residues of liquid density standards from the sensor without corroding or causing adhesion to the sensor and accessories.

6.2.3.2 Air Pump: For densitometers without a built-in air pump, an air source is required. The air pressure provided by the source is typically no greater than 0.1 MPa.

6.2.3.3 Constant Temperature Water Bath: Display resolution no less than ±0.1℃, stability equal to or better than ±0.2℃.

7 Calibration Items and Methods

7.1 Preparations Before Calibration

7.1.1 The densitometer to be calibrated should have a nameplate or product identification; it should include model number, serial number, resolution, and manufacturer information; the densitometer display should provide clear and complete readings; the sensor interior should be clean and dry; buttons should have good contact and function normally; and all connecting fasteners should be secure.

7.1.2 The density meter needs to be powered on and preheated before calibration. The preheating time can be based on the time specified in the instrument's instruction manual or a user-defined time. If no such specification is provided, the preheating time should be no less than 30 minutes.

7.1.3 For density meters with self-calibration functions, self-calibration should be performed according to the instructions in the instrument's instruction manual before calibration.

7.2 Calibration Method

a) For density meters with temperature control, the temperature must first be set to the calibration temperature. After selecting the liquid density standard for a specific density point to be calibrated, use the liquid injection device provided with the density meter or manually inject the liquid density standard into the instrument sensor, completely expelling any air bubbles. After the instrument temperature stabilizes, the density meter measurement result pi is obtained.

For density meters without temperature control, a constant temperature water bath is needed to control the liquid density standard to the calibration temperature. After the conditions stabilize, the density meter measurement result pi is obtained.

b) Rinse the density meter sensor using a cleaning agent that easily dissolves the liquid density standard, as specified in the instruction manual. Dry the sensor with clean, dry air to ensure no liquid residue remains.

c) Repeat the liquid density measurement, completing at least five measurements at the calibration temperature.

Note: Fresh liquid must be injected for each repeated measurement.

d) Repeat steps a) through c) to complete the measurement for all density points requiring calibration.

8 Data Processing and Calibration Result Expression

8.1 Indication Error

The indication error E for each calibration point is calculated as follows:

E=p—Pref                                                      (2)

    

(3)

Where:

E — density indication error, kg/m³;

p — average density meter reading, kg/m³;

Pref — mass value of liquid density standard, kg/m³;

n — number of measurements, n ≥ 5;

pi — density meter reading in a single measurement, kg/m³.

8.2 Repeatability

The repeatability of the density meter calibration points is expressed by the experimental standard deviation s:

                           (4)

In the formula:

pi— Single measurement density meter reading, kg/m³;

p— Average density meter reading, kg/m³;

n— Number of measurements, n≥5.

8.3 Expression of Calibration Results

The calibration certificate should include at least the following information:

a) Title: "Calibration Certificate";

b) Laboratory name and address;

c) Location of calibration (if different from the laboratory address);

d) Unique identifier of the certificate (e.g., number), identification of each page and total number of pages;

e) Customer's name and address;

f) Description and clear identification of the instrument being calibrated;

g) Identification of the technical specification on which the calibration was based, including its name and code;

h) Identification of the liquid density standard used in this calibration;

i) Description of the calibration environment;

j) Explanation of the calibration results and their measurement uncertainties;

k) Recommended recalibration interval;

m) Signature or equivalent identification of the certificate issuer;

m) Declaration that the calibration results are valid only for the calibrated object.

9 Recalibration Interval

The recommended recalibration interval for density meters is no more than one year. Since the recalibration interval is determined by factors such as instrument usage (frequency of use, environmental conditions) and instrument quality, the user can decide the recalibration interval independently based on actual usage.

Appendix A

(0～40)℃ Pure Water Density Table (excluding air)

Unit: kg/m³